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Entrainment of the Human Circadian Clock to the Natural Light-Dark Cycle
Abstract
The electric light is one of the most important human inventions. Sleep and other daily rhythms in physiology and behavior, however, evolved in the natural light-dark cycle [1], and electrical lighting is thought to have disrupted these rhythms. Yet how much the age of electrical lighting has altered the human circadian clock is unknown. Here we show that electrical lighting and the constructed environment is associated with reduced exposure to sunlight during the day, increased light exposure after sunset, and a delayed timing of the circadian clock as compared to a summer natural 14 hr 40 min:9 hr 20 min light-dark cycle camping. Furthermore, we find that after exposure to only natural light, the internal circadian clock synchronizes to solar time such that the beginning of the internal biological night occurs at sunset and the end of the internal biological night occurs before wake time just after sunrise. In addition, we find that later chronotypes show larger circadian advances when exposed to only natural light, making the timing of their internal clocks in relation to the light-dark cycle more similar to earlier chronotypes. These findings have important implications for understanding how modern light exposure patterns contribute to late sleep schedules and may disrupt sleep and circadian clocks.
... onset, peak overnight, and decline after habitual sleep offset. 1,2 Thus, high levels of melatonin represent the biological night, 3,4 though there are large individual differences in peak melatonin levels. 5 Melatonin levels are also highly responsive to light exposure, such that light exposure during biological night acutely suppresses melatonin levels, 6 whereas exposure to the natural light-dark cycle shifts the melatonin rhythm earlier and reduces individual differences in circadian phase. 2 The rapid rise in melatonin levels, known as the dim light melatonin onset (DLMO), marks the beginning of biological night and is the primary indicator of circadian phase in humans. ...
... 5 Melatonin levels are also highly responsive to light exposure, such that light exposure during biological night acutely suppresses melatonin levels, 6 whereas exposure to the natural light-dark cycle shifts the melatonin rhythm earlier and reduces individual differences in circadian phase. 2 The rapid rise in melatonin levels, known as the dim light melatonin onset (DLMO), marks the beginning of biological night and is the primary indicator of circadian phase in humans. 7 As such, DLMO and the phase relationship between DLMO and bedtime have been wellcharacterized in healthy adults 5,8,1,9 and in delayed sleep-wake phase disorder, a circadian rhythm disorder marked by delayed sleep timing and circadian phase. ...
... [10][11][12] Recent findings suggest that DLMO timing is stable across 3 months when sleep timing is also stable, 13 suggesting DLMO measured in a laboratory on a single night may be representative of typical circadian phase when consistent sleep schedules are maintained. DLMO timing is also associated with individual differences in chronotype indicators, including morningness-eveningness 14 and mid-sleep on free days, 2,15 such that later chronotype is associated with later DLMO and vice versa. Similarly, DLMO timing is associated with sleep onset, sleep midpoint, and sleep offset times, with sleep midpoint being most strongly associated with DLMO timing relative to sleep onset and offset in free-living adults. ...
... Light is the body's strongest zeitgieber, or environmental cue about time. Natural daylight is usually 100 to 1000 times brighter than artificial light, and a lack of exposure to natural sunlight, even with the use of electrical lighting, has been shown to alter circadian physiology and sleep behavior [18]. The time of awakening is additionally correlated with sunrise and tends to be later in the winter [19]. ...
... The time of awakening is additionally correlated with sunrise and tends to be later in the winter [19]. Establishing year-round DST could, therefore, result in population-level sleep disruption and fatigue, particularly during winter months [4,6,18,19]. ...
... This increase in darkness around the time of morning awakening is a strong argument against permanent DST [4][5][6]. In the absence of schedule constraints or artificial light, humans naturally awake around or after sunrise [18]. A mismatch between the timing of sleep due to schedule constraints and a human's natural circadian rhythmicity can result in recurrent symptoms of fatigue known as "social jet lag" [55,56]. ...
Background: Permanent Daylight Savings Time (DST) may improve road safety by providing more daylight in the evening but could merely shift risk to morning commutes or increase risk due to fatigue and circadian misalignment. Methods: To identify how potential daylight exposure and fatigue risk could differ between permanent DST versus permanent Standard Time (ST) or current time arrangements (CTA), generic work and school schedules in five United States cities were modeled in SAFTE-FAST biomathematical modeling software. Commute data were categorized by morning (0700–0900) and evening (1600–1800) rush hours. Results: Percent darkness was greater under DST compared with ST for the total waking day (t = 2.59, p = 0.03) and sleep periods (t = 2.46, p = 0.045). Waketimes occurred before sunrise 63 ± 41% percent of the time under DST compared with CTA (42 ± 37%) or ST (33 ± 38%; F(2,74) = 76.37; p < 0.001). Percent darkness was greater during morning (16 ± 31%) and lower during evening rush hour (0 ± 0%) in DST compared with either CTA (morning: 7 ± 23%; evening: 7 ± 14%) or ST (morning: 7 ± 23%; evening: 7 ± 15%). Discussion: Morning rush hour overlaps with students’ commutes and shift workers’ reverse commutes, which may increase traffic congestion and risk compared with evening rush hour. Switching to permanent DST may be more disruptive than either switching to ST or keeping CTA without noticeable benefit to fatigue or potential daylight exposure.
... In addition, to this remarkable plasticity of the circadian system to adjust to short-term changes, recent studies have focused on the differences in individual responses to similar light exposure profiles [22][23][24] . Overall, the effects of light on sleep and circadian phase depend on a wide range of individual traits such as age [25][26][27] , race/ethnicity [28][29][30] , sex 31,32 , chronotype 21,33,34 , photosensitivity 11,27 , and photic history [35][36][37] . The current understanding of the influence of light history on circadian physiology and behavior, especially in real world settings, is limited and a challenging research gap for chronobiology 24 . ...
... The current understanding of the influence of light history on circadian physiology and behavior, especially in real world settings, is limited and a challenging research gap for chronobiology 24 . While strong evidence supports that a recent change in light history over days can affect the sensitivity to light of the circadian system 21,36 , less is known about the influence of long-term photic history over months and seasons [22][23][24] . Among the few studies that have addressed the effect of long-term light history on circadian system and sleep in real-world settings two of them have taken advantage of Antarctica as a natural laboratory 38,39 . ...
... Photic history may influence the individual physiological response to light and underlies plastic changes of the circadian phase across seasons (long-term light history) 69 , and less gradual transitions such as trans-latitudinal trips or between urban and natural environments (short-term light history) 17,21,24,70 . Although it is clear that photic history will affect subsequent response to light, only a few studies have focused on light exposure differences to assess phase shifts in real-life conditions 15,39,71 . ...
The effect of light, main zeitgeber of the circadian system, depends on the time of day it is received. A brief trip to the Antarctic summer (ANT) allowed us to explore the impact of a sudden and synchronized increase in light exposure on activity-rest rhythms and sleep patterns of 11 Uruguayan university students, and to assess the significance of light history in determining individual circadian phase shift. Measurements collected in the peri-equinox in Montevideo, Uruguay (baseline situation, MVD) and in ANT, included sleep logs, actigraphy, and salivary melatonin to determine dim-light melatonin onset (DLMO), the most reliable marker of circadian phase. The increase in light exposure in ANT with respect to MVD (affecting both light-sensitive windows with opposite effects on the circadian phase) resulted in no net change in DLMO among participants as some participants advanced their DLMO and some others delayed it. The ultimate cause of each participant’s distinctive circadian phase shift relied on the unique change in light exposure each individual was subjected to between their MVD and ANT. This study shows an association between the individual light history and the circadian phase shift.
... Light is the most important zeitgeber for both nonhuman mammals (Refinetti, 2015) and humans (Duffy and Wright Jr., 2005). Therefore, the daily alternation between the environmental day and night caused by the Earth's rotation around its axis has a powerful impact on the timing of human circadian rhythms-in other words, the daily light-dark cycle synchronizes the internal, or biological, day and night with the external day and night (Stothard et al., 2017;Wright Jr. et al., 2013). However, social activities can also affect circadian timing by influencing when people are awake (i.e., with their eyes open) and are exposed to visible light, not only from the Sun, but also from artificial sources (Mistlberger and Skene, 2004). ...
... Notably, people who prefer to be awake and active during evening hours, i.e., those with evening chronotypes, are at a higher risk for depression (Levandovski et al., 2011), cardiovascular disease, diabetes, and all-cause mortality (Knutson and von Schantz, 2018). While chronotype is often thought of as an inherent trait-like characteristic, "eveningness" could be in part perpetuated by decreased exposure to morning light and increased exposure to evening light (Skeldon et al., 2017;Wright Jr. et al., 2013), which delays the timing of circadian rhythms relative to the light-dark cycle (Casiraghi et al., 2020). Eveningness is associated with suicidal ideation in adolescents, as well as with specific risk factors for suicidal behavior, such as substance use disorders, impulsivity, and aggression (Gau et al., 2007). ...
Background:
Increasing evidence suggests that conditions with decreased morning and increased evening light exposure, including shift work, daylight-saving time, and eveningness, are associated with elevated mortality and suicide risk. Given that the alignment between the astronomical, biological, and social time varies across a time zone, with later-shifted daylight exposure in the western partition, we hypothesized that western time zone partitions would have higher suicide rates than eastern partitions.
Methods:
United States (U.S.) county-level suicide and demographic data, from 2010 to 2018, were obtained from a Centers for Disease Control database. Using longitude and latitude, counties were sorted into the western, middle, or eastern partition of their respective time zones, as well as the northern and southern halves of the U.S. Linear regressions were used to estimate the associations between suicide rates and time zone partitions, adjusting for gender, race, ethnicity, age group, and unemployment rates.
Results:
Data were available for 2872 counties. Across the U.S., western partitions had statistically significantly higher rates of suicide compared to eastern partitions and averaged up to two additional yearly deaths per 100,000 people (p < .001).
Limitations:
Ecological design and limited adjustment for socioeconomic factors.
Conclusions:
To our knowledge, this is the first study of the relationship between longitude-based time zone partitions and suicide. The results were consistent with the hypothesized elevated suicide rates in the western partitions, and concordant with previous reports on cancer mortality and transportation fatalities. The next step is to retest the hypothesis with individual-level data, accounting for latitude, photoperiodic changes, daylight-saving time, geoclimatic variables, physical and mental health indicators, as well socioeconomic adversity and protection.
... Light is the body's strongest zeitgieber, or environmental cue about time. Natural daylight is usually 100 to 1000 times brighter than artificial light and a lack of exposure to natural sunlight, even with the use of electrical lighting, has been shown to alter circadian physiology and sleep behavior [21]. Time of awakening is additionally correlated with sunrise and tends to be later in the winter [22]. ...
... This increase in darkness around the time of morning awakening is a strong argument against permanent DST [4][5][6]. In the absence of schedule constraints or artificial light, humans naturally awake around or after sunrise [21,70]. A mismatch between the timing of sleep due to schedule constraints and human's natural circadian rhythmicity can result in recurrent symptoms of fatigue known as "social jet-lag" [70][71][72]. ...
Background: Permanent Daylight Savings Time (DST) may improve road safety by providing more daylight in the evening but could merely shift risk to morning commutes or increase risk due to fatigue and circadian misalignment. Methods: To identify how potential daylight exposure and fatigue risk could differ between permanent DST versus permanent Standard Time (ST) or current time arrangements (CTA), generic work and school schedules in five United States cities were modeled in SAFTE-FAST biomathematical modeling software. Commute data were categorized by morning (0700-0900) and evening (1600-1800) rush hours. Results: Percent darkness was greater under DST compared to ST for the total waking day (t=2.59, p=0.03) and sleep periods (t=2.46, p=0.045). Waketimes occurred before sunrise 63%±41% percent of the time under DST compared to CTA (42%±37%) or ST (33%±38%; F(2, 74)=76.37; p<0.001). Percent darkness was greater during morning (16%±31%) and lower during evening rush hour (0%±0%) in DST compare to either CTA (morning:7%±23%; evening:7%±14%) or ST (morning:7%±23%; evening:7%±15%). Discussion: Morning rush hour overlapped with students’ commutes and shift worker reverse commutes, which may increase traffic congestion and risk compared to evening rush hour. Switching to permanent DST may be more disruptive than either switching to ST or keeping CTA without noticeable benefit to fatigue or potential daylight exposure.
... It has been shown that bright light at night, such as artificial lighting, causes a delay and disruption of circadian rhythms and sleep [1] in both rural areas [2][3][4] and urban areas [5,6], and is associated with various health problems [7]. It has been found that circadian rhythm phases are advanced by camp life without or with less access to artificial lighting in adults [8] and children [9]. These results suggest that artificial lighting at night causes delay in circadian rhythms in humans in modern society. ...
... However, a field study has shown that camping life with much sunlight in the morning and little access to artificial lighting at night advances circadian phases in children (age range, 9-14 years) [9]. The results of this study were similar to those of Wright et al. 's study of the circadian phase shifts in adults (30.3 ± 8.5 years old) during camping life [8]. However, it is not easy to discuss age-related differences in light induced circadian phase shifts because these studies are not strictly environmentally controlled like laboratory experiments. ...
Physiological effects of light exposure in humans are diverse. Among them, the circadian rhythm phase shift effect in order to maintain a 24-h cycle of the biological clock is referred to as non-visual effects of light collectively with melatonin suppression and pupillary light reflex. The non-visual effects of light may differ depending on age, and clarifying age-related differences in the non-visual effects of light is important for providing appropriate light environments for people of different ages. Therefore, in various research fields, including physiological anthropology, many studies on the effects of age on non-visual functions have been carried out in older people, children and adolescents by comparing the effects with young adults. However, whether the non-visual effects of light vary depending on age and, if so, what factors contribute to the differences have remained unclear. In this review, results of past and recent studies on age-related differences in the non-visual effects of light are presented and discussed in order to provide clues for answering the question of whether non-visual effects of light actually vary depending on age. Some studies, especially studies focusing on older people, have shown age-related differences in non-visual functions including differences in melatonin suppression, circadian phase shift and pupillary light reflex, while other studies have shown no differences. Studies showing age-related differences in the non-visual effects of light have suspected senile constriction and crystalline lens opacity as factors contributing to the differences, while studies showing no age-related differences have suspected the presence of a compensatory mechanism. Some studies in children and adolescents have shown that children’s non-visual functions may be highly sensitive to light, but the studies comparing with other age groups seem to have been limited. In order to study age-related differences in non-visual effects in detail, comparative studies should be conducted using subjects having a wide range of ages and with as much control as possible for intensity, wavelength component, duration, circadian timing, illumination method of light exposure, and other factors (mydriasis or non-mydriasis, cataracts or not in the older adults, etc.).
... The human clock actively synchronizes to specific zeitgebers, of which light is the predominant one for most organisms, including humans (Duffy and Wright, 2005;Wright et al., 2013). The phase-relationship between the circadian clock and the zeitgeber is called phase of entrainment (PoE). ...
... Modern lifestyle deprives people of both natural light during the day (e.g. by mostly living indoors) and darkness during the night (use of artificial light) (Roenneberg et al., 2003). The strength of light as a zeitgeber has therefore drastically decreased, thereby delaying most chronotypes (with the exception of extreme larks) and increasing the gap between larks and owls (Wright et al., 2013). The increasing mismatch between the internal and external clock has become a stressor for the majority of the population (Roenneberg et al., 2015), leads to so-called social jetlag (SJL) and consequently to sleep debt over the course of the workweek, which is often compensated for on weekends (Wittmann et al., 2006). ...
Late chronotype, which often leads to higher social jetlag (SJL), is strongly associated with the prevalence of smoking. Any circadian disruption, strain, or misalignment, results in people not being able to live according to their biological time as is described by SJL, which we will therefore use as umbrella term. We hypothesized two scenarios potentially explaining the association between smoking and SJL: (A) If smoking delays the clock, circadian phase should advance upon quitting. (B) If people smoke more to compensate the consequences of SJL, circadian phase should not change upon quitting. To distinguish between these two hypotheses, we accompanied participants of a smoking cessation program (not involving nicotine replacement products) across the cessation intervention (3 weeks prior and 6 weeks after) by monitoring their circadian behavior, sleep quality, and daytime sleepiness via questionnaires and actimetry. Our results show no effects of cessation on SJL, chronotype, sleep quality, or daytime sleepiness, thereby favoring scenario (B). Thus, smoking may be a consequence of rather than a cause for SJL. Daytime sleepiness was a significant predictor for the outcome in our model but did not improve with cessation.
... First, we consider the light stimulus, motivated by real world light profiles [62,144], given by namely, it gives a mechanism for sleep/wake timing to turn the forcing of process L off and on. ...
... The light profiles we considered are consistent with [140] and motivated by real world light profiles [62,144] however, these profiles will vary drastically for each individual depending on many factors such as work schedule, amount of travel and chronotype. Since at the boundary the circadian phase has increased sensitivity to light the results here imply that teenagers have the highest risk of being unable to synchronize to the 24 hour day. ...
Sleep is essential for most living things to function. Many features of sleep are not yet understood however, mathematical models are playing an important role in developing our understanding of many of the physiological properties of sleep. We introduce the most well-known model of sleep regulation, the two process model which proposes that sleep-wake cycles can be modelled by the interaction between two oscillators. This ostensibly simple model is an interesting example of a nonsmooth dynamical system whose rich dynamical structure has been relatively unexplored. A key aim of this work is to further understand how transitions between monophasic (one sleep a day) and polyphasic (many sleeps a day) sleep occur in the two process model. The two process model can be framed as a one-dimensional map of the circle which, for some parameter regimes, has gaps. As is a feature of continuous circle maps the bifurcation set consists of saddle-node Arnold tongues. We show that border collision bifurcations that arise naturally in maps with gaps extend and supplement these tongues. We see how the periodic solutions that are created by saddle-node bifurcations in continuous maps transition to periodic solutions created by period-adding bifurcations as seen in maps with gaps. With this deeper understanding of the dynamics and bifurcation structure of the two process model we use modified versions of the model to explain two experimental data sets. An ultradian rhythm is a recurrent period or cycle which repeats multiple times across the day. We consider the sleep wake patterns of a the common vole, Microtus Arvalis, which has ultradian rest activity and feeding patterns. By deriving parameters for the two process model from EEG data and sleep/ wake onset times we are able to simulate with high accuracy the key features of spontaneous sleep-wake patterns in the voles. However, to explain phenomena seen in sleep deprivation experiments we include a high amplitude ultradian oscillation alongside the circadian, the results allow us to give some physiological insight into the internal mechanisms which drive sleep/wake onset times in the common vole. Across the human lifespan there are many changes in the physiological properties of sleep, sleep timing and sleep duration. In adolescence sleep timing is delayed and there is a reduction in slow wave sleep which continues into old age as sleep timing gradually becomes earlier. Using a modified two process model which incorporates a van der Pol oscillator driven by external light signals into the circadian process we show that changes in sleep timing and duration across the lifespan can be explained by varying parameters. Model simulation show that these changes can be understood by a simultaneous reduction in the amplitude of the circadian oscillator and the upper asymptote of the homeostatic sleep pressure.
... Changes to light exposure patterns directly affect alertness [20,21], alter circadian entrainment and phase [22,23], and affect mood [24][25][26]. Large-scale Biobank work suggests increased light at night is particularly associated with poorer psychiatric outcomes [27]. ...
People with a diagnosis of schizophrenia often have poor sleep, even when their psychotic symptoms are relatively well managed. This includes insomnia, sleep apnoea, hypersomnia, and irregular or non-24 h sleep-wake timing. Improving sleep would better support recovery, yet few evidence-based sleep treatments are offered to this group. This paper presents a mixed methods feasibility and acceptability study of Light-Dark and Activity Rhythm Therapy (L-DART). L-DART is delivered by an occupational therapist over 12 weeks. It is highly personalisable to sleep phenotypes and circumstances. Ten participants with schizophrenia spectrum diagnoses and sleep problems received L-DART; their sleep problems and therapy goals were diverse. We measured recruitment, attrition, session attendance, and adverse effects, and qualitatively explored acceptability, engagement, component delivery, adherence, activity patterns, dynamic light exposure, self-reported sleep, wellbeing, and functioning. Recruitment was ahead of target, there was no attrition, and all participants received the minimum 'dose' of sessions. Acceptability assessed via qualitative reports and satisfaction ratings was good. Adherence to individual intervention components varied, despite high participant motivation. All made some potentially helpful behaviour changes. Positive sleep and functioning outcomes were reported qualitatively as well as in outcome measures. The findings above support testing the intervention in a larger randomised trial ISRCTN11998005.
... Research carried out to date has focused mainly on the biological and social bases of the relationship between chronotype and well-being as well as between chronotype and depressive symptoms (Etain et al., 2011;Wittmann et al., 2006), with specific reference to clock gene expression (Jones et al., 2019), natural light exposure (Wright et al., 2013), and misalignment between circadian and social clock (Wittmann et al., 2006) (Dietch et al., 2023;. The latter study also shows that M-types are generally perceived as more competent and assertive, while E-types are more often labelled lazy, less conscientious and less reliable. ...
Recent research provides evidence for the negative social perceptions of eveningchronotypes and their consequences on mental health. However, there is a lack ofstudies indicating whether these negative, socially shared beliefs may become inter-nalized in negative self-perceptions of evening-types (E-types). The present articleprovides a seminal empirical analysis of the role of self-liking and self-competence inthe associations between chronotype and both depressiveness and well-being. In thefirst part of the study, the participants completed the Composite Scale of Morning-ness. On the basis of the chronotype cut-off criteria for Composite Scale of Morning-ness distribution, 100 individuals were classified as morning-types (M-types) and66 individuals as E-types. Therefore, 166 participants (80 women and 86 men) aged18–36 years (M ± SD: 29.27 ± 4.81 years) took part in the second part of the study,and completed questionnaires measuring self-liking, self-competence, life satisfac-tion, positive and negative affect, and depressiveness. Results show that E-typesscored lower in self-liking, self-competence and subjective well-being, and higher indepressive symptoms than M-types. Controlling for age and gender, we obtained sig-nificant mediation effects, showing that the relationship between chronotype andsubjective well-being might stem from the lower levels of self-liking and self-competence among E-types, and that the relationship between chronotype anddepressive symptoms might stem from the lower level of self-liking among E-types.Our results suggest that self-liking and self-competence are important antecedentsof lower well-being and higher depressiveness reported by E-types. Socially sharedstereotypes of M-types and E-types can be internalized by the extreme chronotypes,which may significantly affect their psychological health.
... The circadian rhythm acts as an internal pacemaker for a range of physiologic functions and systems, including sleep, metabolism, the endocrine system, and the cardiovascular system [9]. Natural light exposure during the day and reduced light exposure (darkness) at night play a crucial role in synchronizing the internal circadian rhythm with the external 24-hour daynight cycle [7,10]. Artificial light exposure also impacts this synchronization [11]. ...
Objective
To investigate behavioral sleep habits, self-perceived quality of sleep, and chronotype, and to examine their association with clinically relevant levels of depression in Swedish adolescents.
Method
Questionnaire data were obtained from a representative sample of Swedish adolescents (n = 8449; 50.8% girls; aged 12–16). Depression was defined as >13 BDI-II scores. Logistic regression modelling estimated the effects of sleep duration, sleep quality, and chronotype on depression, adjusted for socio-demographic factors.
Results
On weekdays, approximately 46% of adolescents slept less than the recommended length of eight hours per night (depressed: 68%, non-depressed: 40%). On weekends, however, only 17% slept shorter than recommended. Short weekday sleep duration was more common among girls than boys (53% vs. 38%) and girls reported worse sleep quality. The regression model showed that depression was predicted by weekday sleep duration (OR = 0.773, p < .0001), sleep quality (OR = 0.327, p < .0001), and late chronotype (OR = 1.126, p = .0017), but not by weekend sleep duration. A 30-minute increase in weekday sleep duration was associated with about 10% lower odds of depression.
Conclusions
A substantial proportion of Swedish adolescents do not seem to meet the sleep recommendations of eight hours per night. Short sleep duration on weekdays, poor sleep quality, and late chronotype were associated with increased risk of depression. Interventions promoting longer weekday sleep duration (e.g., later school start times) seem relevant in this context, but further research is needed to investigate the directionality and underlying mechanisms of these associations.
... Environmental light surrounding humans also contributes to individual differences in M-E preference, because the mammalian circadian clock is adjusted mainly by light input via the retinohypothalamic tract. Importantly, the average circadian phase of melatonin secretion (closely related to M-E preference) in modern humans under artificial light conditions is significantly delayed in comparison with that under natural light conditions [37]. Environmental light is now controllable by modern humans, dependent on the individual's preferences in the use of room illumination and light-emitting devices. ...
The circadian clock is adjusted by light inputs via the retinohypothalamic tract. Because environmental light is controllable for modern humans at the individual’s preference although under social schedules, individual differences in time-related psychology and behavior may be associated with morningness-eveningness preference (M-E preference). To examine this hypothesis, we used the Time Management Scale and Time Anxiety Scale to quantify time-related psychology and behavior. These scales aim to evaluate “awareness of effective time management and utilization” and “anxiety about uncontrollable time schedule and unexpected time-related outcome”, respectively. According to our correlation analysis using mid-sleep time as a marker for M-E preference, we obtained results supporting our hypothesis in the correlation between the M-E preference values and the Time Management Scale scores, with larger “time estimation” and “taking each moment as it comes” scores associated with more morningness and eveningness, respectively. Considering that modern humans likely become night owls under artificial light conditions, it appears plausible that lower awareness of time management leads to more eveningness.
... Exposure to sunlight plays a vital physiological role in humans and many animals by promoting the synthesis of vitamin D [1], regulating circadian rhythms [2], and enhancing mood [3]. However, extended exposure to sunlight can lead to detrimental overexposure to ultraviolet (UV) rays, elevating the risk of sunburn and skin cancer [4]. ...
In this study, diethylamino hydroxybenzoyl hexyl benzoate (DHHB)-encapsulated cationic polystyrene particles (PS-DHHB) were synthesized and dispersed in water using miniemulsion polymerization. Low-molecular-weight poly(vinyl alcohol) was employed as an emulsifier, facilitating both the formation of monomer droplets and the subsequent polymerization process. Importantly, the inclusion of polystyrene (PS) in the monomer droplet increased the solid content and acted as a stabilizing agent, thereby enhancing the encapsulation efficiency of DHHB. The synthesized PS-DHHB particles exhibited ultraviolet (UV) absorption properties closely akin to those of DHHB. Additionally, the introduction of aqueous alcohol, used as an antibacterial agent, did not lead to the leaching of DHHB from the PS-DHHB particles. The study further revealed that PS-DHHB adhered more effectively to hair compared to anionic PS-DHHB and was superior in mitigating UV-induced hair protein loss in the 320–400 nm range compared to cationic PS particles without DHHB. In summary, the study resulted in the development of water-dispersible PS-DHHB particles with significant applications in polymer science and cosmetics, owing to their efficient UV absorption and effective hair adhesion capabilities.
... Patterns of bright daytime light and low night-time light serve to enhance the amplitude and stability of the circadian clock as well as align its timing appropriately with daily activities [7][8][9]22 . As modern humans spend ~90% of the day indoors 10 , our light-exposure patterns are typically less bright in the day and more bright at night than naturalistic patterns across our evolutionary history 11 . Addressing this deviation from our natural light/dark cycles may improve the general mental health of people in industrialized societies. ...
Circadian rhythm disturbance is a common feature of many psychiatric disorders. Light is the primary input to the circadian clock, with daytime light strengthening rhythms and night-time light disrupting them. Therefore, habitual light exposure may represent an environmental risk factor for susceptibility to psychiatric disorders. We performed the largest to date cross-sectional analysis of light, sleep, physical activity, and mental health (n = 86,772 adults; aged 62.4 ± 7.4 years; 57% women). We examined the independent association of day and night-time light exposure with covariate-adjusted risk for psychiatric disorders and self-harm. Greater night-time light exposure was associated with increased risk for major depressive disorder, generalized anxiety disorder, PTSD, psychosis, bipolar disorder, and self-harm behavior. Independent of night-time light exposure, greater daytime light exposure was associated with reduced risk for major depressive disorder, PTSD, psychosis, and self-harm behavior. These findings were robust to adjustment for sociodemographics, photoperiod, physical activity, sleep quality, and cardiometabolic health. Avoiding light at night and seeking light during the day may be a simple and effective, non-pharmacological means of broadly improving mental health.
... Results from Model 1 supported our hypothesis that the absence of natural light during winter induces a delay in chronotype and an increase in social jetlag. The influence of the photoperiod on sleep and circadian patterns has already been established 28 . Our findings concerning the delayed chronotypes are in line with those that showed more than a half-hour delay in mid-sleep during winter among people living within extreme photoperiods 20 and during a winter Antarctic campaign 22 . ...
Chronotype is a reliable biomarker for studying the influence of external zeitgebers on circadian entrainment. Assessment of chronotype variation in participants exposed to extreme photoperiods may be useful to investigate how changes in light–dark cycle modulate the circadian system. This study aimed to examine chronotype and sleep changes during a winter campaign at the Argentine Antarctic station Belgrano II. A sample of 82 men who overwintered in Antarctica completed the Munich Chronotype Questionnaire during March (daylight length: 18.6 h), May (daylight length: 2.8 h), July (daylight length: 0 h), September (daylight length: 14.5 h), November (daylight length: 24 h). The main results showed a decrease in sleep duration and a delay in chronotype and social jetlag during the polar night, highlighting the influence of social cues and the impact of the lack of natural light on circadian rhythms.
... This showed the potential for great benefits from light, as the melatonin offset had been associated with the lowest point of human performance in multiple past studies (e.g. Wright et al., 2013). ...
https://cie.co.at/publications/second-international-workshop-circadian-and-neurophysiological-photoreception
This Technical Note reports on the proceedings and consensus of the invited experts of The Second International Workshop on Circadian and Neurophysiological Photometry, 2019, Manchester (UK). This workshop acted on the basis of a consensus of the participants, who are also the advisers to this report, and led to the first international expert consensus recommendations on light exposure for health.
... The natural day-night cycle is the primary synchronizer of circadian rhythms in humans. 1 The advent of electricity has led to 24/7 work practices including night shift and rotating shift schedules, increasing nighttime light exposure in modern society. Unfortunately, these modern work habits have been associated with an elevated risk of various health conditions, including cancer, cardiovascular diseases, depression, diabetes, gastrointestinal problems, and metabolic syndrome. ...
Background: The natural day-night cycle synchronizes our circadian rhythms, but modern work practices like night shifts disrupt this pattern, leading to increased exposure to nighttime light. This exposure is linked to various health issues. While some studies have explored the effects of night shifts on human circadian rhythms, there is limited research on the consequences of long-term exposure to shift-work light conditions. Rodents can provide valuable insights into these effects. This study aimed to examine how short- or long-term exposure to rotating shifts and chronic jetlag affects the core circadian oscillators in the liver and skin of mammals.
Methods: C57BL/6J male mice were subjected to simulated shift-work light conditions, including short-term or long-term rotating shifts and chronic jet-lag conditions. Liver and skin samples were collected every four hours over a 24-hour period on the second day of constant darkness. RNA was extracted and qRT-PCR analysis was conducted to measure the circadian gene expression in liver and skin tissues. Circadian rhythm analysis using CircaCompare compared the control group to mice exposed to shift-work light conditions.
Results: The liver's circadian clock is significantly altered in mice under long-term rotating shift conditions, with a lesser but still noticeable impact in mice experiencing chronic jetlag. However, short-term rotating shift conditions do not significantly affect the liver's circadian clock. Conversely, all three simulated shift conditions affect the skin's circadian clock, indicating that the skin clock is more sensitive to shift-work light conditions than the liver clock. Compared to the liver, the skin's circadian clock is greatly affected by long-term rotating shift conditions.
Conclusions: The study findings indicate more pronounced disturbances in the canonical clock genes of the skin compared to the liver under simulated shift-work light conditions. These results suggest that the skin clock is more vulnerable to the effects of shift-work.
... Hence, sleep patterns and chronotype depend on zeitgebers and also vary with age and sex (Roenneberg and Zerbini 2019). Acute changes in light exposure (Wright et al. 2013), social pressures (Zerbini et al. 2021), and programmed exercise schedules (Thomas et al. 2020) affect sleep patterns and chronotype. On the other hand, people adapted to living conditions with differences in light exposure (de la Iglesia HO et al. 2015;Moreno et al. 2015), social pressures (Razavi et al. 2019), and programmed exercise schedules (Coirolo et al. 2022) also exhibit changes in their sleep patterns and chronotypes. ...
Variations in circadian phase are expected after extreme changes of
regular schedules. We took advantage of a real-life model of dance
students to address the influence of social pressures on chronotype
and sleep patterns in a paired longitudinal study. Seventeen dancers (18–27 years old) being trained at the Uruguayan national
dance school ENFA-SODRE switched from training in the night
shift in 2019 to the morning shift in 2021 and were evaluated
using the Munich Chronotype Questionnaire. Dancers showed
a significant advance in their chronotype (07:08 ± 01:39 to
05:12 ± 00:56, p = 0.043) and a significant increase in social jetlag
(1.71 ± 1.05 to 2.46 ± 0.80, p = 0.027), while sleep duration fell
within the recommended range for the age in both the night shift
and the morning shift. The change in chronotype is correlated with
the change in social disruption (n = 12, p = 0.038, R2 = 0.36). Dancers
alternately used the strategy of oversleeping on free days (associated with higher SJL) or advancing their chronotype (with slight or
no change in SJL). Analyzing the effects at the individual level, we
were able to highlight the plasticity of the circadian system to cope
with the impact of the morning training schedule within nonsedentary late young adults who abruptly, but permanently, changed the timing of their dance training.
... The natural day-night cycle is the primary synchronizer of circadian rhythms in humans. 1 The advent of electricity has led to 24/7 work practices including night shift and rotating shift schedules, increasing nighttime light exposure in modern society. Unfortunately, these modern work habits have been associated with an elevated risk of various health conditions, including cancer, cardiovascular diseases, depression, diabetes, gastrointestinal problems, and metabolic syndrome. ...
Background: The natural day-night cycle synchronizes our circadian rhythms, but modern work practices like night shifts disrupt this pattern, leading to increased exposure to nighttime light. This exposure is linked to various health issues. While some studies have explored the effects of night shifts on human circadian rhythms, there is limited research on the consequences of long-term exposure to shift-work light conditions. Rodents can provide valuable insights into these effects. This study aimed to examine how short- or long-term exposure to rotating shifts and chronic jetlag affects the core circadian oscillators in the liver and skin of mammals.
Methods: C57BL/6J male mice were subjected to simulated shift-work light conditions, including short-term or long-term rotating shifts and chronic jet-lag conditions. Liver and skin samples were collected every four hours over a 24-hour period on the second day of constant darkness. RNA was extracted and qRT-PCR analysis was conducted to measure the circadian gene expression in liver and skin tissues. Circadian rhythm analysis using CircaCompare compared the control group to mice exposed to shift-work light conditions.
Results: The liver's circadian clock is significantly altered in mice under long-term rotating shift conditions, with a lesser but still noticeable impact in mice experiencing chronic jetlag. However, short-term rotating shift conditions do not significantly affect the liver's circadian clock. Conversely, all three simulated shift conditions affect the skin's circadian clock, indicating that the skin clock is more sensitive to shift-work light conditions than the liver clock. Compared to the liver, the skin's circadian clock is greatly affected by long-term rotating shift conditions.
Conclusions: The study findings indicate more pronounced disturbances in the canonical clock genes of the skin compared to the liver under simulated shift-work light conditions. These results suggest that the skin clock is more vulnerable to the effects of shift-work.
... Sunlight has an indispensable importance for living things in nature [1][2][3] . However, the direct absorption of UV will lead to the formation of pyrimidine dimers between adjacent pyrimidines in DNA strands usually in the form of cyclobutene pyrimidine dimers (CPDs) and pyrimidine (6-4) pyrimidone photoproducts (6-4PPs) which causes great damage [4][5][6] . ...
Sunlight has an indispensable importance for living things in nature [1-3]. However, the direct absorption of UV will lead to the formation of pyrimidine dimers between adjacent pyrimidines in DNA strands usually in the form of cyclobutene pyrimidine dimers (CPDs) and pyrimidine (6-4) pyrimidone photoproducts (6-4PPs) which causes great damage [4-6]. A DNA repair system, known as photoreactivation, can effectively repair the dimers using photolyase [7-9], which has currently been found in plants, prokaryotic and eukaryotic cells [10-12]. This study was carried out to determine whether photolyase DNA repair can be observed in yeast. Several yeast Petri dishes were treated with ultraviolet radiation, different treatments were then added to them, and the colonies were counted after culturing, hence verifying that yeasts can use the photoreactivation process.
... Given the severe and highly prevalent comorbidities associated with the E-type, it is relevant to explore specific treatments targeting BD patients with evening preference and/or delayed sleep-wake schedules. Despite the widespread belief that chronotype is an unmodifiable trait, promising studies have demonstrated successful interventions in advancing circadian preference in healthy subjects through manipulating light exposure (Wright et al., 2013) or by using a Transdiagnostic Sleep and Circadian Intervention (Harvey et al., 2018). ...
The present study evaluates the effect of exogenous melatonin (exo-MEL) on sleep and circadian parameters in patients with bipolar disorder (BD) and delayed sleep-wake phase disorder (DSWPD). BD euthymic patients (n = 83, mean age = 45.13 ± 13.68, males 56%) were evaluated for chronotype (reduced Morningness-Eveningness Questionnaire [rMEQ]), sleep quality (Pittsburgh Sleep Quality Index), sleep and circadian parameters (actigraphic monitoring). Patients that fulfilled criteria for DSWPD (n = 25) were treated for three months with exo-MEL 2 mg administered approximately 4 h before the sleep onset time (SOT) actigraphically-determined at baseline. Sleep and circadian parameters at baseline (T0) and after the exo-MEL treatment (T1) were compared using paired Wilcoxon test. In patients that completed the treatment (n = 19), the rMEQ score increased between T0 (median = 8.0 [IQR = 7.0, 11.0]) and T1 (median = 13.5 [IQR = 9.3, 15.0], p-value = 0.006), the SOT was advanced between T0 (median = 00:55 [IQR = 00:25, 01:39] and T1 (median = 00:09 [IQR = 23:41, 01:04], p-value = 0.039), the sleep efficiency and total sleep time increased (T0: median = 84.4 [IQR = 81.3, 89.4]; T1 (median = 90.3 [IQR = 85.5, 92.9] %, p-value = 0.01, and T0: median = 7.20 [IQR = 6.15, 8.15]; T1: median = 7.7 [IQR = 7.0, 9.3] hours, p-value = 0.04, respectively). These results indicate that in BD with comorbid DSWPD, the self-reported chronotype, the sleep onset time, and sleep efficiency and duration were modified after a personalized treatment with exo-MEL, suggesting its potential efficacy in improving sleep patterns in BD. The absence of proper control groups and of treatment randomization constitute limitations of our study and further randomized controlled trials are required to confirm our results.
... The natural day-night cycle is the primary synchronizer of circadian rhythms in humans. 1 The advent of electricity has led to 24/7 work practices including night shift and rotating shift schedules, increasing nighttime light exposure in modern society. Unfortunately, these modern work habits have been associated with an elevated risk of various health conditions, including cancer, cardiovascular diseases, depression, diabetes, gastrointestinal problems, and metabolic syndrome. ...
Background: The natural day-night cycle synchronizes our circadian rhythms, but modern work practices like night shifts disrupt this pattern, leading to increased exposure to nighttime light. This exposure is linked to various health issues. While some studies have explored the effects of night shifts on human circadian rhythms, there is limited research on the consequences of long-term exposure to shift-work light conditions. Rodents can provide valuable insights into these effects. This study aimed to examine how short- or long-term exposure to rotating shifts and chronic jetlag affects the core circadian oscillators in the liver and skin of mammals.
Methods: C57BL/6J male mice were subjected to simulated shift-work light conditions, including short-term or long-term rotating shifts and chronic jet-lag conditions. Liver and skin samples were collected every four hours over a 24-hour period on the second day of constant darkness. RNA was extracted and qRT-PCR analysis was conducted to measure the circadian gene expression in liver and skin tissues. Circadian rhythm analysis using CircaCompare compared the control group to mice exposed to shift-work light conditions.
Results: The liver's circadian clock is significantly altered in mice under long-term rotating shift conditions, with a lesser but still noticeable impact in mice experiencing chronic jetlag. However, short-term rotating shift conditions do not significantly affect the liver's circadian clock. Conversely, all three simulated shift conditions affect the skin's circadian clock, indicating that the skin clock is more sensitive to shift-work light conditions than the liver clock. Compared to the liver, the skin's circadian clock is greatly affected by long-term rotating shift conditions.
Conclusions: The study findings indicate more pronounced disturbances in the canonical clock genes of the skin compared to the liver under simulated shift-work light conditions. These results suggest that the skin clock is more vulnerable to the effects of shift-work.
... The widespread availability of electrical lighting in recent times has increased nocturnal activities and granted humans the ability to personally select their light-dark cycles and prolong wakefulness activities well into the night (5). This ability to modify the timing of wakefulness can lead to a misalignment between the external (environmental) and the internal circadian physiology (6). ...
Achieving synchronization between the central and peripheral body clocks is essential for ensuring optimal metabolic function. Meal timing is an emerging field of research that investigates the influence of eating patterns on our circadian rhythm, metabolism, and overall health. This narrative review examines the relationship between meal timing, circadian rhythm, clock genes, circadian hormones, and metabolic function. It analyzes existing literature and experimental data to explore the connection between mealtime, circadian rhythms, and metabolic processes. The available evidence highlights the importance of aligning mealtime with the body’s natural rhythms to promote metabolic health and prevent metabolic disorders. Specifically, studies show that consuming meals later in the day is associated with a elevated prevalence of metabolic disorders, while early time-restricted eating, such as having an early breakfast and an earlier dinner, improves levels of glucose in the blood and substrate oxidation. Circadian hormones, including cortisol and melatonin, interact with mealtimes and play vital roles in regulating metabolic processes. Cortisol, aligned with dawn in diurnal mammals, activates energy reserves, stimulates appetite, influences clock gene expression, and synchronizes peripheral clocks. Consuming meals during periods of elevated melatonin levels, specifically during the circadian night, has been correlated with potential implications for glucose tolerance. Understanding the mechanisms of central and peripheral clock synchronization, including genetics, interactions with chronotype, sleep duration, and hormonal changes, provides valuable insights for optimizing dietary strategies and timing. This knowledge contributes to improved overall health and well-being by aligning mealtime with the body’s natural circadian rhythm.
... Due to the increasing popularity of electric lighting over the centuries, the functioning of the circadian rhythm may have been disrupted (Wright et al., 2001(Wright et al., , 2013. To measure the effects of a disrupted circadian rhythm, researchers use various circadian rhythm derivatives. ...
The association between the circadian rhythm and diseases has been well-established, while the association with mental health is less explored. Given the heritable nature of the circadian rhythm, this study aimed to investigate the relationship between genes underlying the circadian rhythm and mental health outcomes, as well as a possible gene-environment correlation for circadian rhythm. In a sample from the Netherlands Twin Register (N = 14,021), polygenic scores (PGSs) were calculated for two circadian rhythm measures: Morningess and Relative Amplitude. The PGSs were used to predict mental health outcomes such as subjective happiness, quality of life, and depressive symptoms In addition, we performed the same prediction analysis in a within-family design in a subset of dizygotic twins. The PGS for Morningness significantly predicted Morningness (R2 = 1.55%,) and Depressive Symptoms (R2= 0.22%,). The PGS for Relative Amplitude significantly predicted General Health (R2 = 0.12%,) and Depressive Symptoms (R2 = 0.20%,). Item analysis of the depressive symptoms showed that 4/14 items were significantly associated with the PGSs. The within-family results hinted at a gene-environment correlation for Morningness. Overall, the results showed that people with a genetic predisposition of being a morning person or a high relative amplitude are likely to have fewer depressive symptoms. Contrarily to our hypotheses, the four associated depressive symptoms described symptoms related to decision-making, energy, and feeling worthless, rather than sleep. Our findings plead for a substantial role for the circadian rhythm in depression research, and to further explore the gene-environment correlation in the circadian rhythm.
That disruptions of the body's internal clockwork can lead to negative health consequences, including cancer, is a plausible hypothesis. Yet, despite strong mechanistic and animal support, the International Agency for Research on Cancer (IARC) experts considered epidemiological evidence as limited regarding the carcinogenicity of "shift-work involving circadian disruption" (2007) and "night shift work" (2019). We use directed acyclic graphs (DAGs) to outline a concept of circadian causes that discloses challenges when choosing appropriate exposure variables. On this basis, we propose to move beyond shift-work alone as a direct cause of disease. Instead, quantifying chronodisruption as individual doses can lead to interpretable circadian epidemiology. The hypothesis is that doses of chronodisruption cause disrupted circadian organisation by leading to desynchronization of circadian rhythms. Chronodisruption can be conceptualized as the split physiological nexus of internal and external times. Biological (or internal) night - an individual's intrinsically favoured sleep time window - could be the backbone of circadian epidemiology. In practice, individual doses that cause disrupted circadian organisation are derived from the intersection of time intervals of being awake and an individual's biological night. After numerous studies counted work shifts, chronobiology may now advance circadian epidemiology with more specific dose estimation - albeit with greater challenges in measurement (time-dependent individual data) and analysis (time-dependent confounding).
This article deals with tracking a reference spectrum, enhancing the circadian entrainment of the occupants. The choice of the reference spectrum was made with an emphasis on maintaining the CCT, maintaining a CRI value of at least 80 for the general work area, and maintaining the luminous flux. Use of the Levenberg–Marquardt (LM) algorithm results in the optimal number of LEDs needed for spectrum synthesis. As a result, this work intends to offer a solution for making a luminaire. The percentage error in CCT for the simulated spectrums was less than 0.4%. The daylight spectrum is also selected as a reference spectrum so that close characteristics of sunlight are reached, meeting human comfort needs while also delivering the advantages of sunlight. This novel method of keeping a reference spectrum to design a LED luminaire for circadian entrainment with high luminous efficacy leads to the human-centric lighting system.
To investigate the impact of different exercise training schedules (following a fixed schedule or at random times of the day) on clock genes and myokine expression patterns in the skeletal muscle of tumor‐bearing mice. Mice were divided into three groups: tumor (LLC), tumor + exercise training (LLC + T) always performed at the same time of the day (ZT2) and exercise training at random times of the day (ZTAlt). Mice were inoculated subcutaneously with Lewis lung carcinoma cells. The gastrocnemius muscle was dissected and the clock gene expression (Clock/Per1/Per2/Per3/Rev‐Erbα/GAPDH) was investigated by quantitative reverse transcription polymerase chain reaction with SYBR® Green. Myokine content in muscle (tumour necrosis factor alpha/IL‐10/IL‐4) was assessed by enzyme‐linked immunosorbent assay. At the end of the protocol, the trained groups showed a reduction in total weight, when compared to Lewis lung carcinoma. Tumor weight was lower in the LLC + T (ZTAlt), when compared to LLC. Clock gene mRNA expression showed a significant increase for ZT20 in the groups that performed physical exercise at LLC + T (ZTAlt), when compared with LLC. The Per family showed increased mRNA expression in ZT4 in both trained mice groups, when compared with LLC. LLC + T (ZTAlt) presented reduction of the expression of anti‐inflammatory myokines (Il‐10/IL‐4) during the night, compared with LLC + T(ZT2). Exercise training is able to induce marked modification of clock gene expression and of the production of myokines, in a way that is dependent on schedule exercise training strategy. Taken together, the results show that exercise is a potent Zeitgeber and may thus contribute to change clock genes expression and myokines that are able to reduce the tumor weight.
Light has been shown to have a non-visual impact on the biological aspects of human health, particularly on circadian rhythms. Building windows are a potential means of regulating daylight conditions for circadian health and well-being. As a result of advancements in window and glazing technologies and variations in outdoor solar/ sky conditions, understanding daylight's spectral characteristics, which pass through building window systems, is complex. Therefore, a systematic review and summary of the knowledge and evidence available regarding windows' impact on human circadian health is necessary. This study provides an overview of research in this domain, compares approaches and evaluation metrics, and underscores the importance of window parameters' influence on circadian health. Published studies available on various online databases since 2012 were evaluated. The findings of this study define a holistic approach to the melanopic performance of windows and provide an overview of current knowledge regarding the effect of windows on circadian health. Additionally, this work identifies future research directions based on the studies reviewed. This research contributes to the growing body of knowledge on the impact of windows on circadian health, which has implications for the design and construction of buildings in ways that support indoor human health and well-being from the circadian light adequacy perspective.
Objective: Exposure to light at night (LAN) may influence sleep timing and regularity. Here, we test whether greater light exposure during sleep (LEDS) associates with greater irregularity in sleep onset timing in a large cohort of older adults.
Methods: Light exposure and activity patterns, measured via wrist-worn actigraphy (ActiWatch Spectrum), were analyzed in 1,933 participants with 6+ valid days of data in the Multi-Ethnic Study of Atherosclerosis (MESA) Exam 5 Sleep Study. Summary measures of LEDS averaged across nights were evaluated in linear and logistic regression analyses to test the association with standard deviation (SD) in sleep onset timing (continuous variable) and irregular sleep onset timing (SD≥1.36 hours, binary). Night-to-night associations between LEDS and absolute differences in nightly sleep onset timing were also evaluated with distributed lag non-linear models and mixed models.
Results: In between-individual linear and logistic models adjusted for demographic, health, and seasonal factors, every 5-lux unit increase in LEDS was associated with an increase of 7.8 minutes in sleep onset SD (β=0.13 hours, 95%CI:0.09-0.17) and 40% greater odds (OR=1.40, 95%CI:1.24-1.60) of irregular sleep onset. In within-individual night-to-night mixed model analyses, every 5-lux unit increase in LEDS the night prior (lag0) was associated with a 2.2-minute greater deviation of sleep onset the next night (β=0.036 hours, p<0.05). Conversely, every 1-hour increase in sleep deviation (lag0) was associated with a 0.35-lux increase in future LEDS (β=0.347 lux, p<0.05).
Conclusion: LEDS was associated with greater irregularity in sleep onset in between-individual analyses and subsequent deviation in sleep timing in within-individual analyses, supporting a role for LEDS in exacerbating irregular sleep onset timing. Greater deviation in sleep onset was also associated with greater future LEDS, suggesting a bidirectional relationship. Maintaining a dark sleeping environment and preventing LEDS may promote sleep regularity and following a regular sleep schedule may limit LEDS.
Life on earth has evolved under a consistent cycle of light and darkness caused by the earth's rotation around its axis. This has led to a 24-hour circadian system in most organisms, ranging all the way from fungi to humans. With the advent of electric light in the 19th century, cycles of light and darkness have drastically changed. Shift workers and others exposed to high levels of light at night are at increased risk of health problems, including metabolic syndrome, depression, sleep disorders, dementia, heart disease, and cancer. This book will describe how the circadian system regulates physiology and behavior and consider the important health repercussions of chronic disruption of the circadian system in our increasingly lit world. The research summarized here will interest students in psychology, biology, neuroscience, immunology, medicine, and ecology.
In 1976, Pittendrigh and Daan established a theoretical framework which has coordinated research on circadian clock entrainment and photoperiodism until today. The “wild clocks” approach, which concerns studying wild species in their natural habitats, has served to test their models, add new insights, and open new directions of research. Here, we review an integrated laboratory, field and modeling work conducted with subterranean rodents (Ctenomys sp.) living under an extreme pattern of natural daily light exposure. Tracking animal movement and light exposure with biologgers across seasons and performing laboratory experiments on running-wheel cages, we uncovered the mechanisms of day/night entrainment of the clock and of photoperiodic time measurement in this subterranean organism. We confirmed most of the features of Pittendrigh and Daan’s models but highlighted the importance of integrating them with ecophysiological techniques, methodologies, and theories to get a full picture of the clock in the wild. This integration is essential to fully establish the importance of the temporal dimension in ecological studies and tackling relevant questions such as the role of the clock for all seasons in a changing planet.
Experimental and interventional studies show that light can regulate sleep timing and sleepiness while awake by setting the phase of circadian rhythms and supporting alertness. The extent to which differences in light exposure explain variations in sleep and sleepiness within and between individuals in everyday life remains less clear. Here, we establish a method to address this deficit, incorporating an open-source wearable wrist-worn light logger (SpectraWear) and smartphone-based online data collection. We use it to simultaneously record longitudinal light exposure (in melanopic equivalent daylight illuminance), sleep timing, and subjective alertness over seven days in a convenience sample of 59 UK adults without externally imposed circadian challenge (e.g., shift work or jetlag). Participants reliably had strong daily rhythms in light exposure but frequently were exposed to less light during the daytime and more light in pre-bedtime and sleep episodes than recommended [T. M. Brown et al. , PLoS Biol. 20 , e3001571 (2022)]. Prior light exposure over several hours was associated with lower subjective sleepiness with, in particular, brighter light in the late sleep episode and after wake linked to reduced early morning sleepiness (sleep inertia). Higher pre-bedtime light exposure was associated with longer sleep onset latency. Early sleep timing was correlated with more reproducible and robust daily patterns of light exposure and higher daytime/lower night-time light exposure. Our study establishes a method for collecting longitudinal sleep and health/performance data in everyday life and provides evidence of associations between light exposure and important determinants of sleep health and performance.
Time-of-day differences in acute exercise performance in mice are well established with late active phase (afternoon) runners exhibiting significantly greater endurance performance compared to early active phase (morning) runners. In this study, we asked if performance adaptations would be different when training for 6 weeks at two different times of day, and if this corresponds to steady state changes in the phase of peripheral tissue clocks. To address these questions, we endurance trained female PER2::Luciferase mice, at the same relative workload, either in the morning, at ZT13, or in the afternoon, at ZT22. Then, after training, we recorded luminescence from tissues of PER2::Luciferase mice to report timing of tissue clocks in several peripheral tissues. After 6 weeks, we found that both groups exhibited significant improvements in maximal endurance capacity (total treadmill work)(p < 0.0001), but the morning runners exhibited an enhanced rate of adaptation as there was no detectable difference in maximal endurance capacity (p = 0.2182) between the morning and afternoon runners. In addition, morning and afternoon runners exhibited divergent clock phase shifts with a significant 5-hour phase advance in the EDL (p < 0.0001) and soleus (p < 0.0001) of morning runners, but a phase delay in the EDL (p < 0.0001) and Soleus (p < 0.0001) of afternoon runners. Therefore, our data demonstrate that morning training enhances endurance adaptations compared to afternoon training in mice, and we suggest this is due to phase advancement of muscle clocks to better align metabolism with exercise performance.
Achieving synchronization between the central and peripheral body clocks is essential for ensuring optimal metabolic function. Meal timing is an emerging field of research that investigates the influence of eating patterns on our circadian rhythm, metabolism, and overall health. This narrative review examines the relationship between meal timing, circadian rhythm, clock genes, circadian hormones, and metabolic function. It analyzes existing literature and experimental data to explore the connection between mealtime, circadian rhythms, and metabolic processes. The available evidence highlights the importance of aligning mealtime with the body’s natural rhythms to promote metabolic health and prevent metabolic disorders. Specifically, studies show that consuming meals later in the day is associated with an elevated prevalence of metabolic disorders, while early time-restricted eating, such as having an early breakfast and an earlier dinner, improves levels of glucose in the blood and substrate oxidation. Circadian hormones, including cortisol and melatonin, interact with mealtimes and play vital roles in regulating metabolic processes. Cortisol, aligned with dawn in diurnal mammals, activates energy reserves, stimulates appetite, influences clock gene expression, and synchronizes peripheral clocks. Consuming meals during periods of elevated melatonin levels, specifically during the circadian night, has been correlated with potential implications for glucose tolerance. Understanding the mechanisms of central and peripheral clock synchronization, including genetics, interactions with chronotype, sleep duration, and hormonal changes, provides valuable insights for optimizing dietary strategies and timing. This knowledge contributes to improved overall health and well-being by aligning mealtime with the body’s natural circadian rhythm.
Objective:
The objective of this study was to compare the time to discharge between daytime and nighttime ketamine administration to children undergoing primary facial repair in the emergency department (ED).
Methods:
This retrospective, cross-sectional study was performed in a sample of children aged younger than 18 years and requiring sedation for primary facial repair in 2019. Children who received ketamine for reasons other than facial repair were excluded. All patients were initially injected with 4 mg/kg of ketamine intramuscularly and additionally injected if sedation failed. The time of injection and awakening were recorded in the electronic medical record system by nurses in charge, and the level of wakefulness was determined with a postanesthesia discharge scoring system administered by physicians.
Results:
A total of 562 cases of ketamine administration were divided into 2 groups: daytime and nighttime. We defined daytime and nighttime as 8 a.m. and 8 p.m./sunrise and sunset, respectively. They found that there were no significant differences between 2 groups in each standard (95% confidence interval, -4.55-4.55; P = 0.877 and 95% confidence interval, -6.41-2.41; P = 0.487, respectively).
Conclusions:
The findings of the study suggest that the time of ketamine injection has no relationship to duration of sedation for primary facial repair in children.
Objective:
This study aims to investigate the associations of bedtime and its combination with sleep duration and sleep quality with all-cause mortality.
Methods:
We conducted a prospective cohort study using data collected from 2008 to 2018 in the Dongfeng-Tongji cohort. Among 40,097 participants aged 62.1 on average at baseline, we applied Cox regression models to assess hazard ratios and 95% confidence intervals for mortality risk.
Results:
During a mean follow-up of 8.2years, 4345 deaths were documented. U-shaped associations of bedtime and sleep duration with all-cause mortality were observed. Compared with bedtime between 10:01 PM and 11:00 PM, the hazard ratio (95% confidence interval) for all-cause mortality was 1.34 (1.20-1.49) for ≤9:00 PM, 1.18 (1.09-1.27) for 9:01-10:00 PM, and 1.50 (1.13-2.00) for >12:00 AM, respectively. Participants with sleep duration of <6, 6-<7, 8-<9, and ≥9 h/night had a respective 39%, 21%, 11%, and 25% higher all-cause mortality risk than those sleeping 7-<8 h/night. Additionally, participants with a healthy sleep score of 3, characterized as proper bedtime (10:01 PM-12:00 AM), moderate sleep duration (7-<8h/night), and good/fair sleep quality, had a significantly 36% (hazard ratio, 0.64; 95% confidence interval, 0.56-0.74) lower all-cause mortality risk than those with a score of 0.
Conclusions:
Individuals with early or late bedtimes and short or long sleep duration were at higher all-cause mortality risks. Having healthy sleep habits may significantly reduce death risk.
Purpose: To assess differences in the pupillary light responses (PLRs) to blue and red evening lights between children and adolescents.
Methods: Forty healthy participants (8-9 years, n=21; 15-16 years, n=19) completed a PLR assessment 1 h before their habitual bedtime. After a 1 h dim-light adaptation period (<1 lux), baseline pupil diameter was measured in darkness for 30 s, followed by a 10 s exposure to 3.0x1013 photons/cm2/s of either red (627 nm) or blue (459 nm) light, and a 40 s recovery in darkness to assess pupillary re-dilation. Subsequently, participants underwent 7 min of dim-light re-adaptation followed by an exposure to the other light condition. Lights were counterbalanced across participants.
Results: Across both age groups, maximum pupil constriction was significantly greater (p< 0.001, ηp2=0.48) and more sustained (p< 0.001, ηp2=0.41) during exposure to blue compared to red light. For adolescents, the post-illumination pupillary response (PIPR), a hallmark of melanopsin function, was larger after blue compared with red light (p= 0.02, d=0.60). This difference was not observed in children. Across light exposures, children had larger phasic (p< 0.01, ηp2=0.20) and maximal (p< 0.01, ηp2=0.22) pupil constrictions compared to adolescents.
Conclusions: Blue light elicited a greater and more sustained pupillary response than red light across participants. However, the overall amplitude of the rod/cone-driven phasic response was greater in children than in adolescents. Our findings using the PLR highlight a higher sensitivity to evening light in children compared to adolescents, and continued maturation of the human non-visual photoreception/system throughout development.
The theory that the people of the early modern period slept in well-defined segments of ‘first’ and ‘second’ sleeps has been highly influential in both scholarly literature and mainstream media over the past twenty years. Based on a combination of scientific, anthropological and textual evidence, the segmented sleep theory has been used to illuminate discussions regarding important aspects of early modern nocturnal culture; mainstream media reports, meanwhile, have proposed segmented sleep as a potentially ‘natural’ and healthier alternative to consolidated blocks of sleep. In this article, I re-examine the scientific, anthropological and early modern literary sources behind the segmented sleep theory and ask if the evidence might support other models of early modern sleep that are not characterised by segmentation, while acknowledging that construction of such models is by nature limited and uncertain. I propose a more diverse range of interpretations of early modern texts related to sleep, with important implications for medical and social history and literary scholarship.
La temporalidad de la actividad de todos los seres vivos está pautada por la existencia de ritmos biológicos que dependen de la expresión rítmica de un grupo de genes ancestrales, cuya dinámica se sincroniza diariamente con diversas variables cíclicas del ambiente. En los mamíferos en general, y en el ser humano en particular, el sistema circadiano está comandado por un verdadero marcapaso endógeno ubicado en los núcleos supraquiasmáticos del hipotálamo. Estos núcleos reciben información lumínica directa del ambiente que utilizan para anclar el período de su ritmo endógeno a las 24 hs y para coordinar diversos ritmos circadianos conductuales, fisiológicos y hormonales como por ejemplo el ciclo sueño vigilia, la temperatura corporal y los niveles de melatonina (hormona de la noche). La melatonina es la señal temporizadora por excelencia del sistema circadiano humano y se acepta que el momento de inicio de la secreción de melatonina nocturna en condiciones de luz tenue es el mejor indicador del ritmo circadiano endógeno. Existen diferencias individuales en las preferencias circadianas de la población, lo que permite identificar 3 cronotipos: matutinos, intermedios y vespertinos, que dependen de factores genéticos y ambientales. En este trabajo se evalúa el impacto de la vida urbana moderna sobre el reloj biológico humano que constituye un tema de debate actual en Cronobiología. Vivir en ciudades ha distorsionado el patrón de exposición a la luz y ha impuesto una agenda social demandante, que fuerza a ajustes crónicos del sistema circadiano, que a su vez dependen de la tipología circadiana individual.
Circadian rhythms are entrained by external factors such as sunlight and social cues, but also depend on internal factors such as age. Adolescents exhibit late chronotypes, but worldwide school starts early in the morning leading to unhealthy sleep habits. Several studies reported that adolescents benefit from later school start times. However, the effect of later school start time on different outcomes varies between studies, and most previous literature only takes into consideration the social clock (i.e. local time of school starting time) but not the solar clock (e.g. the distance between school start time and sunrise). Thus, there is an important gap in the literature: when assessing the effect of a school start time on chronotype and sleep of adolescents at different locations and/or seasons, the solar clock might differ and, consistently, the obtained results. For example, the earliest school start time for adolescents has been suggested to be 08:30 hours, but this school start time might correspond to different solar times at different times of the year, longitudes and latitudes. Here, we describe the available literature comparing different school start times, considering important factors such as geographic position, nationality, and the local school start time and its distance to sunrise. Then, we described and contrasted the relative role of both social and solar clocks on the chronotype and sleep of adolescents. As a whole, we point and discuss a gap in literature, suggesting that both clocks are relevant when addressing the effect of school start time on adolescents' chronotype and sleep.
Key differences exist between individuals in terms of certain circadian-related parameters, such as intrinsic period and sensitivity to light. These variations can differentially impact circadian timing, leading to challenges in accurately implementing time-sensitive interventions. In this work, we parse out these effects by investigating the impact of parameters from a macroscopic model of human circadian rhythms on phase and amplitude outputs. Using in silico light data designed to mimic commonly studied schedules, we assess the impact of parameter variations on model outputs to gain insight into the different effects of these schedules. We show that parameter sensitivity is heavily modulated by the lighting routine that a person follows, with darkness and shift work schedules being the most sensitive. We develop a framework to measure overall sensitivity levels of the given light schedule and furthermore decompose the overall sensitivity into individual parameter contributions. Finally, we measure the ability of the model to extract parameters given light schedules with noise and show that key parameters like the circadian period can typically be recovered given known light history. This can inform future work on determining the key parameters to consider when personalizing a model and the lighting protocols to use when assessing interindividual variability.
The endogenous melatonin onset in dim light (DLMO) is a marker of circadian phase
that can be used to appropriately time the administration of bright light or exogenous
melatonin in order to elicit a desired phase shift. Determining an individual’s circadian
phase can be costly and time-consuming.We examined the relationship between
theDLMOand sleep times in 16 young healthy individuals who slept at their habitual
times for a week. TheDLMOoccurred about 2 hours before bedtime and 14 hours after
wake. Wake time and midpoint of sleep were significantly associated with the
DLMO (r = 0.77, r = 0.68 respectively), but bedtime was not (r = 0.36). The possibility
of predicting young healthy normally entrained people’s DLMOs from their sleep
times is discussed.
Objective: This the second of two articles reviewing the scientific literature on the evaluation and treatment of circadian rhythm sleep disorders (CRSDs), employing the methodology of evidence-based medicine. We herein report on the accumulated evidence regarding the evaluation and treatment of Advamced Sleep Phase Disorder (ASPD), Delayed Sleep Phase Disorder (DSPD), Free-Running Disorder (FRD) and Irregular Sleep-Wake Rhythm ISWR). Methods: A set of specific questions relevant to clinical practice were formulated, a systematic literature search was performed, and relevant articles were abstracted and graded. Results: A substantial body of literature has accumulated that provides a rational basis the evaluation and treatment of CRSDs. Physiological assessment has involved determination of circadian phase using core body temperature and the timing of melatonin secretion. Behavioral assessment has involved sleep logs, actigraphy and the Morningness-Eveningness Questionnaire (MEQ). Treatment interventions fall into three broad categories: 1) prescribed sleep scheduling, 2) circadian phase shifting ("resetting the clock"), and 3) symptomatic treatment using hypnotic and stimulant medications. Conclusion: Circadian rhythm science has also pointed the way to rational interventions for CRSDs and these treatments have been introduced into the practice of sleep medicine with varying degrees of success. More translational research is needed using subjects who meet current diagnostic criteria.
1.1. Entrainment of circadian wheel running activity was monitored over a period of several months for northern grasshopper mice (Onychomys leucogaster) and golden mantled ground squirrels (Spermophilus lateralis) housed out-of-doors and exposed to direct sunlight (average light intensity = 55,000 lx).2.2. Sighted animals entrained their activity rhythms to the natural photoperiod; the mice were nocturnal and the squirrels diurnal in their locomotor activity.3.3. The activity rhythms of all blind animals from each species failed to entrain to the light-dark cycle and free-ran with endogenous periods > 24 hr for squirrels and < 24 hr for grasshopper mice.4.4. Failure of entrainment in blind ground squirrels and grasshopper mice indicates that these species lack functional extraocular photoreceptors; they thus differ from non-mammalian vertebrates in which extraocular photoreception is well documented.5.5. The significance of exclusive reliance by mammals on ocular photoreceptors for entrainment of circadian rhythms is discussed.
Insufficient sleep is associated with obesity, yet little is known about how repeated nights of insufficient sleep influence energy expenditure and balance. We studied 16 adults in a 14- to 15-d-long inpatient study and quantified effects of 5 d of insufficient sleep, equivalent to a work week, on energy expenditure and energy intake compared with adequate sleep. We found that insufficient sleep increased total daily energy expenditure by ∼5%; however, energy intake-especially at night after dinner-was in excess of energy needed to maintain energy balance. Insufficient sleep led to 0.82 ± 0.47 kg (±SD) weight gain despite changes in hunger and satiety hormones ghrelin and leptin, and peptide YY, which signaled excess energy stores. Insufficient sleep delayed circadian melatonin phase and also led to an earlier circadian phase of wake time. Sex differences showed women, not men, maintained weight during adequate sleep, whereas insufficient sleep reduced dietary restraint and led to weight gain in women. Our findings suggest that increased food intake during insufficient sleep is a physiological adaptation to provide energy needed to sustain additional wakefulness; yet when food is easily accessible, intake surpasses that needed. We also found that transitioning from an insufficient to adequate/recovery sleep schedule decreased energy intake, especially of fats and carbohydrates, and led to -0.03 ± 0.50 kg weight loss. These findings provide evidence that sleep plays a key role in energy metabolism. Importantly, they demonstrate physiological and behavioral mechanisms by which insufficient sleep may contribute to overweight and obesity.
Cognitive and affective processes vary over the course of the 24 h day. Time of day dependent changes in human cognition are modulated by an internal circadian timekeeping system with a near-24 h period. The human circadian timekeeping system interacts with sleep-wakefulness regulatory processes to modulate brain arousal, neurocognitive and affective function. Brain arousal is regulated by ascending brain stem, basal forebrain (BF) and hypothalamic arousal systems and inhibition or disruption of these systems reduces brain arousal, impairs cognition, and promotes sleep. The internal circadian timekeeping system modulates cognition and affective function by projections from the master circadian clock, located in the hypothalamic suprachiasmatic nuclei (SCN), to arousal and sleep systems and via clock gene oscillations in brain tissues. Understanding the basic principles of circadian and wakefulness-sleep physiology can help to recognize how the circadian system modulates human cognition and influences learning, memory and emotion. Developmental changes in sleep and circadian processes and circadian misalignment in circadian rhythm sleep disorders have important implications for learning, memory and emotion. Overall, when wakefulness occurs at appropriate internal biological times, circadian clockwork benefits human cognitive and emotion function throughout the lifespan. Yet, when wakefulness occurs at inappropriate biological times because of environmental pressures (e.g., early school start times, long work hours that include work at night, shift work, jet lag) or because of circadian rhythm sleep disorders, the resulting misalignment between circadian and wakefulness-sleep physiology leads to impaired cognitive performance, learning, emotion, and safety.
Intrinsically photosensitive retinal ganglion cells (ipRGCs) express the photopigment melanopsin and regulate a wide array of light-dependent physiological processes. Genetic ablation of ipRGCs eliminates circadian photoentrainment and severely disrupts the pupillary light reflex (PLR). Here we show that ipRGCs consist of distinct subpopulations that differentially express the Brn3b transcription factor, and can be functionally distinguished. Brn3b-negative M1 ipRGCs innervate the suprachiasmatic nucleus (SCN) of the hypothalamus, whereas Brn3b-positive ipRGCs innervate all other known brain targets, including the olivary pretectal nucleus. Consistent with these innervation patterns, selective ablation of Brn3b-positive ipRGCs severely disrupts the PLR, but does not impair circadian photoentrainment. Thus, we find that molecularly distinct subpopulations of M1 ipRGCs, which are morphologically and electrophysiologically similar, innervate different brain regions to execute specific light-induced functions.
While much research has investigated the effects of exogenous melatonin on sleep, less is known about the relationship between the timing of the endogenous melatonin rhythm and the sleep-wake cycle. Significant inter-individual variability in the phase relationship between sleep and melatonin rhythms has been reported although the extent to which the variability reflects intrinsic and/or environmental differences is unknown. We examined the effects of different sleeping schedules on the time of dim light melatonin onset (DLMO) in 28 young, healthy adults. Participants chose to maintain either an early (22:30-06:30 h) or a late (00:30-08:30 h) sleep schedule for at least 3 weeks prior to an overnight laboratory visit. Saliva samples were collected under dim light (<2 lux) and controlled posture conditions to determine salivary DLMO. The 2-h difference between groups in the enforced sleep-wake schedule was associated with a concomitant 1.75-h delay in DLMO. The mean phase relationship between sleep onset and DLMO remained constant (~2 h). The variance in DLMO time, however, was greater in the late group (range 4.5 h) compared to the early group (range 2.4 h) perhaps due to greater effect of environmental influences in delayed sleep types or greater intrinsic instability in their circadian system. The findings contribute to our understanding of individual differences in the human circadian clock and have important implications for the diagnosis and treatment of circadian rhythm sleep disorders, in particular if a greater normative range for phase angle of entrainment occurs in individuals with later sleep-wake schedules.
The traditional approaches to predict entrainment of circadian clocks by light are based on 2 concepts that were never successfully unified: the non-parametric approach assumes that entrainment occurs via discrete daily phase shifts while the parametric approach assumes that entrainment involves changes of the clock's velocity. Here the authors suggest a new approach to predict and model entrainment. Unlike the traditional approaches, it does not assume a priori the mechanism of how the internal and external cycle lengths are matched (via phase shifts, velocity changes, or even other mechanisms). It is based on a circadian integrated response characteristic (CIRC) that describes how the circadian system integrates light signals at different circadian phases, without specifying exactly when and how fast its progression is affected. Light around subjective dawn compresses the internal cycle; light around subjective dusk expands it. While the phase response curve (PRC) describes the results of experiments using light stimuli (of specified duration and intensity), the CIRC reflects how the system integrates any given light profile, be it single pulses or any form of light-dark cycle (from skeleton photoperiods to natural light profiles). CIRCs are characterized by their shape (determining the extent of their dead zone) and their asymmetry (the ratio of its compressing and expanding portions). They are dimensionless (time/time), and their maximum is by definition 1. To make predictions about entrainment, the CIRC is multiplied with the light intensity/sensitivity at any given time point. Unlike the PRC and the velocity response curve, the CIRC can be assessed on the basis of entrained steady states, by modeling experimental results. The CIRC approach makes several predictions that can be tested experimentally.
Circadian timing affects sleep onset. Delayed sleep onset can reduce sleep duration in adolescents required to awake early for a fixed school schedule. The absence of short-wavelength ("blue") morning light, which helps entrain the circadian system, can hypothetically delay sleep onset and decrease sleep duration in adolescents. The goal of this study was to investigate whether removal of short-wavelength light during the morning hours delayed the onset of melatonin in young adults.
Dim light melatonin onset (DLMO) was measured in eleven 8th-grade students before and after wearing orange glasses, which removed short-wavelength light, for a five-day school week.
DLMO was significantly delayed (30 minutes) after the five-day intervention, demonstrating that short-wavelength light exposure during the day can be important for advancing circadian rhythms in students.
Lack of short-wavelength light in the morning has been shown to delay the circadian clock in controlled laboratory conditions. The results presented here are the first to show, outside laboratory conditions, that removal of short-wavelength light in the morning hours can delay DLMO in 8th-grade students. These field data, consistent with results from controlled laboratory studies, are directly relevant to lighting practice in schools.
Circadian rhythms are endogenous 24 h cycles that persist in the absence of external time cues. These rhythms provide an internal representation of day length and optimize physiology and behaviour to the varying demands of the solar cycle. These clocks require daily adjustment to local time and the primary time cue (zeitgeber) used by most vertebrates is the daily change in the amount of environmental light (irradiance) at dawn and dusk, a process termed photoentrainment. Attempts to understand the photoreceptor mechanisms mediating non-image-forming responses to light, such as photoentrainment, have resulted in the discovery of a remarkable array of different photoreceptors and photopigment families, all of which appear to use a basic opsin/vitamin A-based photopigment biochemistry. In non-mammalian vertebrates, specialized photoreceptors are located within the pineal complex, deep brain and dermal melanophores. There is also strong evidence in fish and amphibians for the direct photic regulation of circadian clocks in multiple tissues. By contrast, mammals possess only ocular photoreceptors. However, in addition to the image-forming rods and cones of the retina, there exists a third photoreceptor system based on a subset of melanopsin-expressing photosensitive retinal ganglion cells (pRGCs). In this review, we discuss the range of vertebrate photoreceptors and their opsin photopigments, describe the melanopsin/pRGC system in some detail and then finally consider the molecular evolution and sensory ecology of these non-image-forming photoreceptor systems.
Light is considered the most potent synchronizer of the human circadian system and exerts many other non-image-forming effects, including those that affect brain function. These effects are mediated in part by intrinsically photosensitive retinal ganglion cells that express the photopigment melanopsin. The spectral sensitivity of melanopsin is greatest for blue light at approximately 480 nm. At present, there is little information on how the spectral composition of light to which people are exposed varies over the 24 h period and across seasons. Twenty-two subjects, aged 22+/-4 yrs (mean+/-SD) participated during the winter months (November-February), and 12 subjects aged 25+/-3 yrs participated during the summer months (April-August). Subjects wore Actiwatch-RGB monitors, as well as Actiwatch-L monitors, for seven consecutive days while living in England. These monitors measured activity and light exposure in the red, green, and blue spectral regions, in addition to broad-spectrum white light, with a 2 min resolution. Light exposure during the day was analyzed for the interval between 09:00 and 21:00 h. The time course of white-light exposure differed significantly between seasons (p = 0.0022), with light exposure increasing in the morning hours and declining in the afternoon hours, and with a more prominent decline in the winter. Overall light exposure was significantly higher in summer than winter (p = 0.0002). Seasonal differences in the relative contribution of blue-light exposure to overall light exposure were also observed (p = 0.0006), in particular during the evening hours. During the summer evenings (17:00-21:00 h), the relative contribution of blue light was significantly higher (p < 0.0001) (40.2+/-1.1%) than during winter evenings (26.6+/-0.9%). The present data show that in addition to overall light exposure, the spectral composition of light exposure varies over the day and with season.
Differences in morningness-eveningness among humans are commonly ascribed to circadian parameters, such as circadian period and responsivity to environmental time cues, as well as homeostatic sleep drive. Light is the primary synchronizer of the human biological clock, and if circadian differences exist between morning and evening types, they should have different phase angles of entrainment to the light/dark cycle; that is, morning and evening types should have different patterns of light exposure relative to endogenous circadian phase (ECP). When phase angle of entrainment is strictly defined as the relationship between a marker of ECP and the timing of light exposure, such differences have been demonstrated in the laboratory under controlled light/dark cycles and have recently been shown under conditions of spring and summer light exposure outside the laboratory, taking into account the variable intensity of light. Here, we report similar results from a large (n=66), diverse cohort of morning and evening types across the age span studied at all different times of the year. Differences between morning and evening types in light exposure relative to ECP, indicative of a difference in the phase angle of entrainment to the external light/dark cycle, were found. Specifically, evening types, compared to morning types, had a higher ratio of phase advancing to phase delaying by light. We interpret this as indicating a longer circadian period (tau) in evening types.
An English language self-assessment Morningness-Eveningness questionnaire is presented and evaluated against individual differences in the circadian vatiation of oral temperature. 48 subjects falling into Morning, Evening and Intermediate type categories regularly took their temperature. Circadian peak time were identified from the smoothed temperature curves of each subject. Results showed that Morning types and a significantly earlier peak time than Evening types and tended to have a higher daytime temperature and lower post peak temperature. The Intermediate type had temperatures between those of the other groups. Although no significant differences in sleep lengths were found between the three types, Morning types retired and arose significantly earlier than Evening types. Whilst these time significatly correlated with peak time, the questionnaire showed a higher peak time correlation. Although sleep habits are an important déterminant of peak time there are other contibutory factors, and these appear to be partly covered by the questionnaire. Although the questionnaire appears to be valid, further evaluation using a wider subject population is required.
The authors treated winter depression in 13 patients with typical seasonal affective disorder by extending the length of winter days with bright and dim light in the morning and evening in a balanced-order crossover study. Bright light had a marked antidepressant effect, whereas the dim light did not. This response could not be attributed to sleep deprivation. Subsequent pilot studies indicated that bright evening light alone is probably also effective. Several patients were able to maintain the antidepressant response throughout the winter months by continuing daily light treatments.
Two- to threefold variations in sleep length were observed in 12 subjects living on self-selected schedules in an environment free of time cues. The duration of polygraphically recorded sleep episodes was highly correlated with the circadian phase of the body temperature rhythm at bedtime and not with the length of prior wakefulness. Furthermore, the rate of REM (rapid eye movement) sleep accumulation , REM latency, bedtime selection, and self-rated alertness assessments were also correlated with the body temperature rhythm.
The accuracy with which a circadian pacemaker can entrain to an environmental 24-h zeitgeber signal depends on (a) characteristics of the entraining signal and (b) response characteristics and intrinsic stability of the pacemaker itself. Position of the sun, weather conditions, shades, and behavioral variations (eye closure, burrowing) all modulate the light signal reaching the pacemaker. A simple model of a circadian pacemaker allows researchers to explore the impact of these factors on pacemaker accuracy. Accuracy is operationally defined as the reciprocal value of the day-to-day standard deviation of the clock times at which a reference phase (0) is reached. For the purpose of this exploration, the authors used a model pacemaker characterized solely by its momentary phase and momentary velocity. The average velocity determines the time needed to complete one pacemaker cycle and, therefore, is inversely proportional to pacemaker period. The model pacemaker responds to light by shifting phase and/or changing its velocity. The authors assumed further that phase and velocity show small random fluctuations and that the velocity is subject to aftereffects. Aftereffects were incorporated mathematically in a term allowing period to contract exponentially to a stable steady-state value, with a time constant of 69 d in the absence of light. The simulations demonstrate that a pacemaker reaches highest accuracy when it responds to light by simultaneous phase shifts and changes of its velocity. Phase delays need to coincide with slowing down and advances with speeding up; otherwise, no synchronization to the zeitgeber occurs. At maximal accuracy, the changes in velocity are such that the average period of the pacemaker under entrained conditions equals 24 h. The results suggest that during entrainment, the pacemaker adjusts its period to 24 h, after which daily phase shifts to compensate for differences between the periods of the zeitgeber and the clock are no longer necessary. On average, phase shifts compensate for maladjustments of phase and velocity changes compensate for maladjustments of period.
The photopigment in the human eye that transduces light for circadian and neuroendocrine regulation, is unknown. The aim of this study was to establish an action spectrum for light-induced melatonin suppression that could help elucidate the ocular photoreceptor system for regulating the human pineal gland. Subjects (37 females, 35 males, mean age of 24.5 +/- 0.3 years) were healthy and had normal color vision. Full-field, monochromatic light exposures took place between 2:00 and 3:30 A.M. while subjects' pupils were dilated. Blood samples collected before and after light exposures were quantified for melatonin. Each subject was tested with at least seven different irradiances of one wavelength with a minimum of 1 week between each nighttime exposure. Nighttime melatonin suppression tests (n = 627) were completed with wavelengths from 420 to 600 nm. The data were fit to eight univariant, sigmoidal fluence-response curves (R(2) = 0.81-0.95). The action spectrum constructed from these data fit an opsin template (R(2) = 0.91), which identifies 446-477 nm as the most potent wavelength region providing circadian input for regulating melatonin secretion. The results suggest that, in humans, a single photopigment may be primarily responsible for melatonin suppression, and its peak absorbance appears to be distinct from that of rod and cone cell photopigments for vision. The data also suggest that this new photopigment is retinaldehyde based. These findings suggest that there is a novel opsin photopigment in the human eye that mediates circadian photoreception.
The interaction of homeostatic and circadian processes in the regulation of waking neurobehavioral functions and sleep was studied in six healthy young subjects. Subjects were scheduled to 15–24 repetitions of a 20-h rest/activity cycle, resulting in desynchrony between the sleep-wake cycle and the circadian rhythms of body temperature and melatonin. The circadian components of cognitive throughput, short-term memory, alertness, psychomotor vigilance, and sleep disruption were at peak levels near the temperature maximum, shortly before melatonin secretion onset. These measures exhibited their circadian nadir at or shortly after the temperature minimum, which in turn was shortly after the melatonin maximum. Neurobehavioral measures showed impairment toward the end of the 13-h 20-min scheduled wake episodes. This wake-dependent deterioration of neurobehavioral functions can be offset by the circadian drive for wakefulness, which peaks in the latter half of the habitual waking day during entrainment. The data demonstrate the exquisite sensitivity of many neurobehavioral functions to circadian phase and the accumulation of homeostatic drive for sleep.
Background: Morningness-eveningness refers to interindividual differences in preferred timing of behavior (ie, bed and wake times). Older people have earlier wake times and rate themselves as more morning-like than young adults. It has been reported that the phase of circadian rhythms is earlier in morning-types than in evening types, and that older people have earlier phases than young adults. These changes in phase have been considered to be the chronobiological basis of differences in preferred bed and wake times and age-related changes therein. Whether such differences in phase are associated with changes in the phase relationship between endogenous circadian rhythms and the sleep-wake cycle has not been investigated previously. Methods: We investigated the association between circadian phase, the phase relationship between the sleep-wake cycle and circadian rhythms, and morningness-eveningness, and their interaction with aging. In this circadian rhythm study, 68 young and 40 older subjects participated. Results: Among the young subjects, the phase of the melatonin and core temperature rhythms occurred earlier in morning than in evening types and the interval between circadian phase and usual wake time was longer in morning types. Thus, while evening types woke at a later clock hour than morning types, morning types actually woke at a later circadian phase. Comparing young and older morning types we found that older morning types had an earlier circadian phase and a shorter phase-wake time interval. The shorter phase-wake-time interval in older "morning types" is opposite to the change associated with morningness in young people, and is more similar to young evening types. Conclusions: These findings demonstrate an association between circadian phase, the relationship between the sleep-wake cycle and circadian phase, and morningness-eveningness in young adults. Furthermore, they demonstrate that age-related changes in phase angle cannot be attributed fully to an age-related shift toward morningness, These findings have important implications for understanding individual preferences in sleep-wake timing and age-related changes in the timing of sleep.
This paper is an attempt to integrate in a general model the major findings reported earlier in this series on: lability and history dependence of circadian period, tau (Pittendrigh and Daan, 1976); dependence of tau and α on light intensity as described by Aschoff's Rule (Daan and Pittendrigh, 1976); the interrelationships between tau and phase response curves (Daan and Pittendrigh, 1976); and those inconsistencies between experimental facts on entrainment and theoretical predictions based on a single oscillator with fixed parameters tau and PRC, which pointed to a more complex system (Pittendrigh and Daan, 1976). The qualitative model developed consists of 2 oscillators. The evidence that 2 separate oscillators are involved in circadian activity rhythms rests largely on the 'splitting' phenomenon, known to occur in several species of mammals and birds. The empirical regularities of 'splitting' in hamsters exposed to constant illumination (LL) are described. Splitting, i.e., the dissociation of a single activity band into 2 components which become stably coupled in ca 180° antiphase, occurs in about 50% of the animals in 100 to 200 lux, and has not been observed in lower light intensities. Splitting never occurred before 40 days after the transition to LL, unless the pretreatment had been LL of low intensity. In some animals, the unsplit condition returned spontaneously. The attainment of antiphase is usually accompanied by a decrease in tau, and refusion of the 2 components by an increase in tau. These data show that both the split and the unsplit condition are metastable states, characterized by different phase relationships (psiEM) of 2 constituent oscillators. psiEM is history dependent and determines tau of the coupled system. Observations in Peromyscus leucopus transferred from LL to DD to LD 12:12 show that the 2 components of the bimodal activity peak in (LD) can for some time run at different frequencies (in DD), suggesting that bimodality of activity peaks and splitting are based on the same 2 oscillator systems. The model developed assumes the existence of 2 oscillators or principal groups of oscillators E and M, with opposite dependence of spontaneous frequency on light intensity. The dependence of the phase relationship (psiEM) between the 2 on light intensity and the dependence of tau and psiEM account for all the history dependent characteristics of circadian pacemakers, and for the interdependence of tau, PRC, and tau lability. The model qualitatively accommodates the interdependence of tau and α summarized in Aschoff's Rule. It is noted that the major intuitive elements in the model have been found to characterize an explicit version of it in computer stimulations. The relevance of the model to seasonal change in photoperiod is discussed. A pacemaker comprising 2 oscillators mutually interacting but coupled separately to sunrise and sunset enhances its competence to accommodate to seasonal change in the daily pattern of external conditions; and it could well be involved in the pacemaker's known ability to discriminate between daylengths in the phenomena of photoperiodic induction.
gives an overview of the current understanding of how the output of the circadian pacemaker interacts with the sleep–wake-dependent oscillatory process to generate the daily time course of alertness and cognitive performance under normal entrained conditions, under free-running conditions, among night workers, and during desynchronization of the sleep–wake and circadian systems, such as is seen in blind individuals (PsycINFO Database Record (c) 2012 APA, all rights reserved)
1.
The seasonal variations in time of daily onset and end of locomotor activity are described for 3 species of mammals and 5 species of birds kept in captivity at the arctic circle and at lower latitude. These variations are most pronounced at high latitude.
2.
The duration of daily activity plotted versus the photoperiod can be described as an S-curve in all species studied so far, both in nature and in captivity. In both male and female fringillid birds activity times were longer before the summer solstice (spring) than after the summer solstice at equal photoperiods.
3.
The seasonal changes in activity time result from roughly mirror-image changes in the times of onset and end of activity relative to sunrise and sunset, cancelling out each other. Therefore the midpoint of activity stays relatively stable; remaining minor changes in the midpoint of activity do not produce a general seasonal pattern.
4.
At high latitude, a large seasonal fluctuation in the day-to-day variability (or precision) of activity timing is detected. These patterns of precision of the rhythm cannot be attributed to a single Zeitgeber property without complex assumptions. Onset and ends of activity become more precise when occurring during the civil twilight, i.e. at times of day with most rapid changes in light intensity. This may reflect direct action of light on the rhythm rather than a property of the entrainment mechanism.
5.
The data do not give compelling evidence for any formal model of the oscillations driving the activity rhythms. Predictions concerning the relation between phase and activity time derived from a single oscillator model are not matched by the data. On the other hand, the general seasonal patterns can be easily described in terms of a two-oscillator model.
6.
Seasonal variations in duration of activity are larger in birds than in mammals. Day-to-day variations in timing are larger in mammals than in birds. The implications for photoperiodic time measurement are discussed.
1 Phase response curves for 15′ bright light pulses of four species of nocturnal rodents are described. All show delay phase shifts early in the subjective night, advance shifts in the late subjective night, and relative insensitivity during the subjective day. 2 The broad scatter in measured phase-shifts is largely due to error of measurement: the response of the pacemakers to light stimuli is more accurate than we observe. 3. Indications are found that the response to a resetting stimulus at a given phase of the rhythm is correlated with the individual {Mathematical expression} (freerunning period). Fast pacemakers (short {Mathematical expression}) tend to be more delayed or less advanced by the light than slow pacemakers (long {Mathematical expression}). 4. Within individual mice (Mus musculus) the circadian pacemaker adjusts its resetting response to variations in its frequency: when τ is long (induced as after-effect of prior light treatment) light pulses at a defined phase of the oscillation (ct 15) produce smaller delay phase shifts than when τ is short. 5. Among species there are conspicuous differences in the shape of the phase response curve: where {Mathematical expression} is long, advance phase shifts are large and delay phase shifts small (Mesocricetus auratus); where {Mathematical expression} is short, advance shifts are small, and delay shifts are large (Mus musculus;Peromyscus maniculatus). 6. The functional meaning of the interrelationships of τ and PRC is briefly discussed.
1.
In thefirst part of the paper, the model of non-parametric entrainment of circadian pacemakers is tested for the case of nocturnal rodents. The model makes use of the available data on freerunning period ([`(t)]\bar \tau
is close to 24 h. Thus the verycircadian nature of these pacemakers helps to conserve[`(t)]\bar \tau
=24 h. The effect of-instability is further reduced by entrainment with 2 pulses (dawn and dusk), made possible by the PRC's having both an advance and a delay section.
8.
To analyze the contributions to-conservation with seasonally changing photoperiod, we have assumed that it is of functional significance to conserve the phase of activity with respect to dusk (nocturnal animals) or to dawn (diurnal animals). We distinguish three contributions of nocturnal pacemaker behaviour to this type of-conservation: increased amplitude of the PRC (i), asymmetry in the PRC, such that the slope of the delay-part is steeper than the slope of the advance-part (ii), and a short freerunning period in DD (iii).
9.
A further contribution must derive from parametric effects of light, which are not traceable by the model, but certainly effective in preventing in complete photoperiods the-jump which is seen in skeleton photoperiods. The existence of parametric effects is further demonstrated by the change of with light intensity in LL, described by Aschoff's Rule, which presumably reflects differences in PRC-shape between nocturnal and diurnal animals (Daan and Pittendrigh, 1976b).
10.
The paper concludes with an attempt to distinguish the features of circadian clocks that are analytically necessary for entrainment to occur (i), or have functional meaning, either in the measurement of the lapse of time (ii) or in the identification of local time (iii).
We recently reported that humans have conserved mechanisms, like those that exist in other animals, which detect changes in day length and make corresponding adjustments in the duration of nocturnal periods of secretion of melatonin and of other functions. We detected these responses in individuals who were exposed to artificial "days" of different durations. The purpose of the present study was to determine whether men who are exposed to natural and artificial light in an urban environment at 39 degrees N are still able to detect and respond to seasonal changes in duration of the natural photoperiod. We measured profiles of circadian rhythms during 24-h periods of constant darkness (< 1 lx) and found no summer-winter differences in durations of nocturnal periods of active secretion of melatonin, rising levels of cortisol, high levels of thyrotropin, and low levels of rectal temperature. The results of this and our previous study suggest that modern men's use of artificial light suppresses responses to seasonal changes in the natural photoperiod that might otherwise occur at this latitude.
Obesity has reached crisis proportions in industrialized societies. Many factors converge to yield increased body mass index (BMI). Among these is sleep duration. The circadian clock controls sleep timing through the process of entrainment. Chronotype describes individual differences in sleep timing, and it is determined by genetic background, age, sex, and environment (e.g., light exposure). Social jetlag quantifies the discrepancy that often arises between circadian and social clocks, which results in chronic sleep loss. The circadian clock also regulates energy homeostasis, and its disruption-as with social jetlag-may contribute to weight-related pathologies. Here, we report the results from a large-scale epidemiological study, showing that, beyond sleep duration, social jetlag is associated with increased BMI. Our results demonstrate that living "against the clock" may be a factor contributing to the epidemic of obesity. This is of key importance in pending discussions on the implementation of Daylight Saving Time and on work or school times, which all contribute to the amount of social jetlag accrued by an individual. Our data suggest that improving the correspondence between biological and social clocks will contribute to the management of obesity.
Key points
The human circadian pacemaker generates near‐24‐h rhythms that set the timing of many physiological, metabolic and behavioural body rhythms, and is synchronized to environmental time primarily by the 24 h light–dark cycle.
The magnitude and direction of the resetting response of the pacemaker to light depends on the time of day of exposure, and the change in responses over the day is summarized in a phase response curve (PRC).
A previous PRC showed that a 6.7 h bright white light exposure maximally shifted the circadian pacemaker by over 3 h.
We show that a PRC to a 1 h bright white light pulse maximally shifted the circadian pacemaker by ∼2 h, despite representing only ∼15% of the exposure duration.
This study demonstrates that the circadian pacemaker is sensitive to short‐duration light pulses with a non‐linear relationship between light duration and the amount of resetting.
Abstract The phase resetting response of the human circadian pacemaker to light depends on the timing of exposure and is described by a phase response curve (PRC). The current study aimed to construct a PRC for a 1 h exposure to bright white light (∼8000 lux) and to compare this PRC to a <3 lux dim background light PRC. These data were also compared to a previously completed 6.7 h bright white light PRC and a <15 lux dim background light PRC constructed under similar conditions. Participants were randomized for exposure to 1 h of either bright white light ( n = 18) or <3 lux dim background light ( n = 18) scheduled at 1 of 18 circadian phases. Participants completed constant routine (CR) procedures in dim light (<3 lux) before and after the light exposure to assess circadian phase. Phase shifts were calculated as the difference in timing of dim light melatonin onset (DLMO) during pre‐ and post‐stimulus CRs. Exposure to 1 h of bright white light induced a Type 1 PRC with a fitted peak‐to‐trough amplitude of 2.20 h. No discernible PRC was observed in the <3 lux dim background light PRC. The fitted peak‐to‐trough amplitude of the 1 h bright light PRC was ∼40% of that for the 6.7 h PRC despite representing only 15% of the light exposure duration, consistent with previous studies showing a non‐linear duration–response function for the effects of light on circadian resetting.
To examine the effect of ambulatory daytime light exposure on phase delays and on the advances produced by timed exposure to bright evening or morning light.
As a subset of a larger study, 32 older (63.0 ± 6.43 years) adults with primary insomnia were randomized to an at-home, single-blind, 12-week, parallel-group study entailing daily exposure to 45 min of scheduled evening or morning bright (∼4000 lux) light. Light exposure patterns during the baseline and the last week of treatment were monitored using actigraphs with built-in illuminance detectors. Circadian phase was determined through analysis of in-laboratory collected plasma melatonin.
Less daytime light exposure during the last week of treatment was significantly associated with larger phase delays in response to evening light (r's>0.78). Less daytime light exposure during the last week of treatment was also associated with a significant delay in wake time (r's>-0.75). There were no such relationships between light exposure history and phase advances in response to morning light.
Greater light exposure during the daytime may decrease the ability of evening light, but not morning light, exposure to engender meaningful changes of circadian phase.
We examined the effects of an advanced sleep/wake schedule and morning short wavelength (blue) light in 25 adults (mean age±SD=21.8±3 years; 13 women) with late sleep schedules and subclinical features of delayed sleep phase disorder (DSPD).
After a baseline week, participants kept individualized, fixed, advanced 7.5-h sleep schedules for 6days. Participants were randomly assigned to groups to receive "blue" (470nm, ∼225lux, n=12) or "dim" (<1lux, n=13) light for 1h after waking each day. Head-worn "Daysimeters" measured light exposure; actigraphs and sleep diaries confirmed schedule compliance. Salivary dim light melatonin onset (DLMO), self-reported sleep, and mood were examined with 2×2 ANOVA.
After 6days, both groups showed significant circadian phase advances, but morning blue light was not associated with larger phase shifts than dim-light exposure. The average DLMO advances (mean±SD) were 1.5±1.1h in the dim light group and 1.4±0.7h in the blue light group.
Adherence to a fixed advanced sleep/wake schedule resulted in significant circadian phase shifts in young adults with subclinical DSPD with or without morning blue light exposure. Light/dark exposures associated with fixed early sleep schedules are sufficient to advance circadian phase in young adults.
In humans, modulation of circadian rhythms by light is thought to be mediated primarily by melanopsin-containing retinal ganglion cells, not rods or cones. Melanopsin cells are intrinsically blue light-sensitive but also receive input from visual photoreceptors. We therefore tested in humans whether cone photoreceptors contribute to the regulation of circadian and neuroendocrine light responses. Dose-response curves for melatonin suppression and circadian phase resetting were constructed in subjects exposed to blue (460 nm) or green (555 nm) light near the onset of nocturnal melatonin secretion. At the beginning of the intervention, 555-nm light was equally effective as 460-nm light at suppressing melatonin, suggesting a significant contribution from the three-cone visual system (lambda(max) = 555 nm). During the light exposure, however, the spectral sensitivity to 555-nm light decayed exponentially relative to 460-nm light. For phase-resetting responses, the effects of exposure to low-irradiance 555-nm light were too large relative to 460-nm light to be explained solely by the activation of melanopsin. Our findings suggest that cone photoreceptors contribute substantially to nonvisual responses at the beginning of a light exposure and at low irradiances, whereas melanopsin appears to be the primary circadian photopigment in response to long-duration light exposure and at high irradiances. These results suggest that light therapy for sleep disorders and other indications might be optimized by stimulating both photoreceptor systems.
Humans are a diurnal species usually exposed to light while engaged in cognitive tasks. Light not only guides performance on these tasks through vision but also exerts non-visual effects that are mediated in part by recently discovered retinal ganglion cells maximally sensitive to blue light. We review recent neuroimaging studies which demonstrate that the wavelength, duration and intensity of light exposure modulate brain responses to (non-visual) cognitive tasks. These responses to light are initially observed in alertness-related subcortical structures (hypothalamus, brainstem, thalamus) and limbic areas (amygdala and hippocampus), followed by modulations of activity in cortical areas, which can ultimately affect behaviour. Light emerges as an important modulator of brain function and cognition.<