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Lack of short-wavelength light during the school day delays dim light melatonin onset (DLMO) in middle school students


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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.
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Neuroendocrinol Lett 2010; 31(1):92–96
Neuroendocrinology Letters Volume 31 No. 1 2010
Lack of short-wavelength light during the
school day delays dim light melatonin
onset (DLMO) in middle school students
Mariana G. F and Mark S. R
Lighting Research Center, Rensselaer Polytechnic Institute, Troy, New York, USA
Correspondence to: Mariana Figueiro
Lighting Research Center
21 Union St., Troy, NY 12180, USA.
: +1 (518) 687-7100; : +1 (518) 687-7120; -:
Submitted: 2009-10-21 Accepted: 2009-12-15 Published online: 2010-02-16
Key words: daylight; melatonin; circadian system; adolescents; sleep
Neuroendocrinol Lett 2010; 31(1):92–96 PMID: 20150866 NEL310110A04 © 2010 Neuroendocrinology Letters ww
OBJECT IVE: 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 dura-
tion 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.
METHOD S: 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.
RESULTS : DLMO was significantly delayed (30 minutes) after the five-day inter-
vention, demonstrating that short-wavelength light exposure during the day can
be important for advancing circadian rhythms in students.
CONCLUS IONS: 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 stud-
ies, are directly relevant to lighting practice in schools.
In terrestrial mammals, circadian rhythms are
regulated by the interaction of the internal bio-
logical clock located in the suprachiasmatic nuclei
(SCN) of the hypothalamus with the earth’s natu-
ral 24-hour light-dark pattern (Refinetti 2006).
The SCN are self-sustaining oscillators with an
intrinsic period that is typically slightly longer or
shorter than 24 hours. The timing of the SCN is
set by the local light-dark pattern, usually ensur-
ing that the organism’s behavioral and physiologi-
cal rhythms are synchronized with its photic niche
(nocturnal, diurnal, or crepuscular).
Light incident on human retinas will entrain or
phase shift SCN timing, depending upon the time,
duration, spectrum and intensity of the stimulus
(Stevens & Rea 2001). These fundamental light
Neuroendocrinology Letters Vol. 31 No. 1 2010 • Article available online:
Lack of short-wavelength light during the school day delays dim light melatonin onset
characteristics affect the circadian system differently
than they affect the visual system. Although we now
know the human circadian system is more sensitive to
light than was originally thought (Lewy et al. 1980), it is
much less sensitive to light than the visual system (Rea
et al. 2002). It is also well established that the human
circadian system is maximally sensitive to short-wave-
length (450 nm to 480 nm) light (Brainard et al. 2001,
Thapan et al. 2001, and Rea et al. 2005). Most electric
light sources illuminating our indoor environments
are designed to support the visual system by providing
relatively low levels of light dominated by wavelengths
near 555 nm, the peak of the photopic luminous effi-
ciency function (CIE 1978). Moreover, for conve-
nience, electric light sources are available night or day
and for variable durations. More and more then, people
throughout the world are living under a roof illumi-
nated by electric light sources, exposing them to dim
days and extended dim light at night, separating them
from the robust, natural light-dark cycle.
Studies have shown that adolescents report going to
bed later as they get older (Crowley et al. 2007). These
age-related changes in bedtimes have been associated
with reduced parental influence on bedtimes, increased
homework and extra-curricular activities, and other
activities such as playing on computers and watching
television. Evidence to date supports the hypothesis
that adolescents have a late circadian phase, contribut-
ing to these late bed times. With a highly structured
school schedule requiring early rising, these adoles-
cents typically experience reduced sleep durations.
Indeed, on unrestricted weekends, adolescents rise 1.5
to 3 hours later than they do on weekdays (Crowley et
al. 2007).
Light during the day is important for entrainment;
that is, for aligning circadian phase to the rest-activ-
ity cycle required by attending school. For reasons
described above, however, electric lighting, including
that common in schools, may not provide adequate
light for circadian entrainment. Without a robust light
stimulus during the day then, adolescents would logi-
cally be expected to exhibit late circadian phase and
therefore go to bed late and experience restricted sleep.
Daily morning short-wavelength light exposures
(after minimum core body temperature) are expected
to slightly advance the clock every day and thereby
maintain entrainment to the solar day (Jewett et al.
1997). The impact of reduced daily short-wavelength
light exposure on the circadian system of young adults,
as might be experienced by students without adequate
daylight (or electric light) exposure, has never been for-
mally investigated. A simple before-and-after, within-
subjects field experiment was conducted in a school
with documented good daylight design to determine
whether removal of short-wavelength light on five con-
secutive school days would delay circadian phase rela-
tive to a baseline measurement obtained prior to the
The study was conducted at Smith Middle School,
Chapel Hill, North Carolina in May 2009. Smith Middle
School is unusual with respect to current architectural
practice in terms of the amount of daylight provided to
interior spaces (LRC 2004). The building uses south-
facing roof monitor skylights in most spaces to deliver
daylight to the interior spaces. Diffuse toplight ing pre-
vents occlusions due to blinds or wall displays typi cal
of sidelighting. To minimize glare from direct sunlight
entering the spaces, light entering the roof monitor is
baffled with cloth partitions; only diffuse light enters
the conditioned room. The electric lighting system is
controlled with motion sensors and photosensors that
modulate the fluores cent lamp output with dimming
ballasts. This strategy allows electric lights to be off
most of the day for electric energy savings.
The daylighting conditions were evaluated as part
of an extensive case study in 2004 (LRC 2004). On a
sunny afternoon in March 2004, researchers measured
light levels on several surfaces in a classroom with a
calibrated illuminance meter having a photopic spectral
response (CIE 1978). At the time of the site measure-
ments, all illu mination was provided by daylight. Hori-
zontal light level measurements were made by placing
the illuminance meter on desks; these ranged from
1330–2150 lux. Vertical illu minances on the chalkboard
ranged from 996–1 265 lux. The vertical light measure-
ments were made by placing the illuminance meter on
the chalkboard at eye level. Typical levels found in spaces
illuminated only by electric light sources are approxi-
mately 80% lower. That is, these illuminance levels
were approximately 5× higher than commonly found in
buildings only illuminated by electric lights. Based on
calculations using the model of human circadian pho-
totransduction developed by Rea and colleagues (2005),
these vertical illuminance levels would result in at least
60% melatonin suppression (at night), suggesting that
the light stimulus students receive in Smith’s classrooms
is strong enough to activate the circadian system. Based
upon results by Zeitzer et al. (2000) who showed that
the half maximum saturation for phase shifting was
80–160 lux from cool white fluorescent light sources for
a 6.5 hr exposure, the light levels measured in Smith
Middle School would also be highly effective for phase
shifting and, therefore, entrainment. Battery-powered
monitoring devices also recorded illumin ances on the
teacher’s desk over a long period of time. The desk was
located near the perimeter of the room rather than
directly under the roof monitor. These illuminance
levels aver aged 550 lux on sunny days and 320 lux on
partly cloudy days. Based upon the previous measure-
ments, it was expected that students at Smith Middle
School would be exposed to some of the highest illu-
minances typically found in an in door classroom envi-
ronment, making this an appropri ate site for the study.
Copyright © 2010 Neuroendocrinology Letters ISSN 0172–780X
Mariana G. Figueiro and Mark S. Rea
The within-subjects study began at the school on a
Friday morning. Eleven subjects (nine males and two
females, ages 13–14 years) were given an explanation
about the study and were asked to wear orange glasses
that attenuated all light of wavelengths shorter than
about 525 nm from reaching the eyes (Figure 1) during
the study period, Monday to Friday on the following
week, from the time they awoke until they returned
home after school (approximately 15:00); thus, subjects
were required to wear the orange glasses during school
and on the commute to school in the morning when
they are likely to be exposed to daylight. Participants
were then asked to refrain from consuming caffeinated
products for the remainder of the day because saliva
samples would be gathered from them in the evening.
Finally, subjects were instructed to return to the school
at 19:00 for saliva sample collection. The participants
stayed in the dimly illuminated school library during
sample collection. All electric lighting was kept off and
all blinds were pulled down to avoid daylighting in the
space. The room was lit with a dim red light (less than
5 lux at the cornea), dur ing which time the participants
were allowed to watch movies, play games, read or
study. Serial saliva samples (Salivette, Sarstedt, Newton,
NC, USA) were collected every 30 mi nutes (from 19:30
to 23:00) to determine DLMO. The subjects chewed on
a plain cotton cylinder until saturated. These samples
were then, in turn, centrifuged and refrigerated by
the researcher. To pre vent contamination of the saliva
samples, the subjects were not allowed to eat or drink
between sample times. The re frigerated samples were
later sent to Pharmascan, Os ceola, WI, for melatonin
assay. On the next Friday evening, participants returned
to school at 19:00 to repeat the saliva sample data col-
lection in the dimly illuminated library. The study was
approved by Rensselaer Polytechnic Institute’s Institu-
tional Review Board and meets the international ethical
standards of this journal (Portaluppi et al. 2008).
DLMO, in decimal hours, was calculated for each subject
using a threshold of 4.0 pg/ml. DLMO was determined
by using linear interpolation between the melatonin
values that fell above and below threshold. DLMO for
the eleven subjects on the Friday prior to wearing the
orange glasses averaged 21.15±0.61 and DLMO for the
same subjects averaged 21.66±0.81 after five consecu-
tive days of wearing the orange glasses. One subject had
not achieved the threshold value (4 pg/ml) at 23:00 on
the second Friday, suggesting that his DLMO occurred
later than 23:00, but as a conservative estimate, we
used 23:00 as his DLMO time. Using a two-tail paired
students’ t-test, this difference was significant with a
probability of 0.006 of a Type 1 error. Figure 2 shows
the cumulative frequencies of DLMO for the partici-
pants before and after the orange glasses intervention.
In one respect the results of this field study are trivial
because they simply confirm what has been shown
before (Warman et al. 2003) namely that removing
short-wavelength light exposure in the morning delays
circadian phase. In another respect, however, the
results of this study are quite important because they
Figure 1. The spectral irradiance distribution (SIR) of daylight varies
continuously throughout the day at every location on the earth.
Shown here is the relative SIR of one, standard phase of daylight
(left ordinate), defined by CIE as illuminant D65 (Wyszecki &
Stiles 1982) to represent natural daylight at 6500 K. Also shown
is the spectral transmittance of the orange glasses (amber/
orange, UV Process Supply), in percent (right ordinate) used in
the study. Irrespective of the actual and highly variable SIR in
daylight present in Smith Middle School during the experiment,
the orange glasses would have attenuated all short-wavelength
light from both the natural and the electric sources seen by the
Figure 2. Cumulative frequencies of DLMO for the students before
and after the orange glasses intervention.
Neuroendocrinology Letters Vol. 31 No. 1 2010 • Article available online:
Lack of short-wavelength light during the school day delays dim light melatonin onset
validate controlled laboratory findings with actual field
measurements. Specifically, these data are consistent
with the inference that removing short-wavelength light
during school days delays circadian phase in 8th-grade
students. After five consecutive school days of wearing
orange glasses, DLMO was delayed by about 30 min-
utes. Although it is known that individual SCN clocks
can have different periods, the phase delay of about 6
minutes per day observed here is consistent with the
typical free-running period in humans of 24.18±0.04 hrs
(Czeisler et al. 1999). Therefore, both the direction and
the magnitude of the predicted effects from laboratory
studies were obtained in this field study.
It has been estimated that bedtime occurs approxi-
mately two to three hours after DLMO (Burgess et al.
2003; Burgess & Fogg 2008). Since the present results
showed that removing short-wavelength light during
the school day will delay DLMO, sleep times are likely
to be delayed as well. Wake-up times are fixed for most
students, so those who do not receive short-wavelength
light during the day will probably have reduced sleep on
school nights. One study showed that students who had
poorer performance in school were those who obtained
about 25 minutes less sleep per night and went to bed
on average 40 minutes later on school nights than those
who were good performers (Wolfson and Carskadon
1998). By extension then, those who do not get enough
short-wavelength light during the school day would
exhibit reduced scholastic performance.
These findings, bridging controlled laboratory
results to a real school environment, should have
important, and practical, implications for school design
because it seems necessary to expose students to short-
wavelength light during the early part of the day to
maintain entrainment. Conscious delivery of short-
wavelength light in schools may be a simple, effective,
non-pharmacological treatment for students to help
them increase sleep duration and, perhaps, scholastic
performance. Daylight in a school like that provided in
Smith Middle School appears to be an ideal source to
accomplish this goal because it can deliver the proper
quantity and spectrum as well as the proper timing
and duration of light exposure. Electric lighting could
also serve this purpose, but current electric lighting is
manufactured, designed and specified to meet visual
requirements. Electric lighting could have an advantage
over daylight for the purpose of circadian entrainment,
because electric lighting can be precisely controlled, not
only during the day, but during the night when expo-
sure to light emulating daylight would be counterpro-
ductive for entrainment. Indeed, electric lighting can
provide a complete 24-hour light exposure pattern to
help ensure entrainment, but these deliberations repre-
sent an entirely new framework for architectural light-
ing design and practice (Figueiro 2008).
The authors would like to acknowledge the U.S.
Green Building Council (USGBC) for sponsoring this
research. The Trans-NIH Genes, Environment and
Health Initiative Grant U01 DA023822 also provided
support this project. The authors would also like to
thank Dr. Mary Carskadon and Dr. Stephanie Crowley
of Brown University and Bradley Hospital for helping
with the DLMO calculations. A. Bierman, D. Guyon, J.
Brons, B. Plitnick, R. Leslie, and J. Taylor of the Lighting
Research Center, Rensselaer Polytechnic Institute, are
thanked for their assistance with this project and manu-
script. We would also like to thank the staff, teachers,
and principal of Smith Middle School for making this
project possible. And, finally, we would like to thank the
students and parents who participated in this research
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... Light can serve as a zeitgeber for the human clock (Merrow and Roenneberg 2001), and can help regulate melatonin suppression and phase shifts (Boyce et al. 2003;Duffy and Czeisler 2009). The lack of short wavelength light in the morning was shown to delay dim light melatonin onset (Figueiro and Rea 2010). Daylight in buildings influences occupant's visual and thermal comfort as well as their psychophysiological state. ...
Sunlight is a multisensory phenomenon that can enhance occupant's comfort, health, and connection to the outside environment through its dynamic luminous and thermal attributes. One gap in the existing literature on sunlight exposure is in addressing the visual interest of sunlight patterns and its potential effects on visual comfort. This study employed an experimental procedure where 33 office workers were subjected to three different window and sunlight patterns: fractal pattern, striped pattern, and clear at an office building over three days (one condition per day). Subjective ratings and physical environmental measurements were collected and analyzed to understand differences among the three conditions. Results showed no significant differences in visual comfort or visual interest of sunlight patterns among the three conditions. Desk layout influenced visual interest and view quality ratings. The fractal and striped patterns negatively influenced view quality compared to the clear condition. These results suggest that the shape of window and sunlight patterns might have limited to no impact on visual comfort and interest in offices when workers are preoccupied performing typical office work.
Рreference for later bedtimes and rise times characterize evening chronotypes. Evening chronotypes suffer from early work start times thereby contradicting their circadian rhythms, as a result, a late wake-up time on free days reflect an attempt to compensate for a sleep debt accumulated on work days. This leads to a misalignment in sleep timing between weekdays and weekends, known as social jetlag (SJL), which is associated with increased health risk. Here we analyze the risks that evening preferences related with SJL bear and their potential impact on health, and also talk about possible correction measures, primarily of a behavioral nature, using literature data from PubMed and Embase database. Evening chronotype can compromise the maintenance of a healthy lifestyle. Evening chronotypes are more prone to bedtime screen use, which can suppress melatonin rise and extend wakefulness activities far into the night, thus dragging sleep and meal timing to later periods. Preference towards later time-of-day is linked with higher intake of total calories and fats, as well as unhealthy dietary habits (breakfast skipping, snacking, longer eating duration). Evening chronotype also has been associated with high caffeinated drinks intake, alcohol consumption and smoking, low physical activities. It has been found that unhealthy behavior might function as the promoting factors to circadian misalignment and greater SJL. Interventions to prevent and control unhealthy behaviors among evening types should be included in preventive measures of SJL.
The issues of light non-visual influence on the human body during the work of office workers and students of educational institutions are considered. The necessity of creating a high-quality lighting environment of educational premises and offices by means of the corresponding dynamic lighting scenario taking into account non-visual effects of visible light is substantiated. The necessity of light levels revision of on working surfaces taking into account circadian effects is shown. The rapid development and increasing growth of LED lighting, which allows to obtain dynamic light scenes, as well as progress in knowledge of physiological mechanisms that regulate circadian rhythms, and their relationship with light stimuli allows to implement really high quality artificial lighting. The light environment not only affects the visual characteristics, but also has a significant impact on people through the so-called non-visual effects or those that do not form images (BOZ-effects) - mood, vivacity, circadian rhythms and more. It is important to combine visual and non-visual requirements with innovative lighting systems. Circadian lighting should always be evaluated at eye level, in the typical location of human existence in a specific internal environment. The improved lighting scenario involves synchronizing the light with the activity and circadian rhythms of consumers over a 24-hour cycle. Lighting in the auditoriums of educational institutions should meet the necessary visual requirements and create comfort for pupils and students. Audience lighting can be more efficient than lighting dynamics or dynamic lighting levels and color temperature. Automatic lighting control, which depends on the dynamics of daylight and should usually be built into the smart installation of dynamic lighting, should be a must.
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Internationally, it is said that the “era of sleeping difficulty” has arrived, and for Japanese children, this is no exception. On the other hand, it is well-established that daytime light reception promotes phase advances in melatonin secretion. Thus, it is undeniable that the sleep situation and melatonin secretion patterns of schoolchildren may depend on whether the classroom seat where they spend a relatively long time during a school day is on the window side of the classroom. This study examined the relationship between classroom seat location and children's sleep situation and melatonin secretion patterns. Our subjects were 88 elementary school children (47 boys and 41 girls) from the 5th to 6th grades enrolled in public elementary schools in Tokyo; we analyzed the data of 73 (37 boys and 36 girls) with whom there was no data loss. The study was carried out on weekdays from September to October 2018. From the analysis, a 1.7-times difference in the average illuminance median was observed between seats that were on the window side and those on the corridor side (window side group: 362.2 lx, control group: 207.7 lx) In addition, the odds ratio of children with high melatonin (night to morning) was 10-times higher in the window side group than in the control group (OR=10.179, 95% CI=1.492-69.455). Based on our findings, we conclude that the sleep situation of children should be an important determinant in classroom seating.
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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.
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Alzheimer's disease (AD) patients exhibit random patterns of rest and activity rather than the consolidated sleep/wake cycle found in normal, older people. Light treatment has been shown to improve rest and activity rhythms and sleep efficiency of AD patients, presumably through consolidation of their circadian rhythms. The circadian system is maximally sensitive to short-wavelength radiation. Two independent studies summarised here show that 30 lux at the cornea of blue light (λ max = 470 nm) from light emitting diodes (LEDs) for 2 h in the early evening improved sleep efficiency of older adults, including those with AD compared to exposure to the same dose of red light. Because compliance to blue light treatment may be difficult for adults with AD, we conceived of a lighting scheme that might be more practical and as effective. White light dosages of different spectra and amounts for night and for day, based on a computational model for human circadian phototransduction, might be more readily accepted by seniors and by their caregivers. Implications for an improved visual environment and for better sleep efficiency of older adults are discussed.
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Regulation of circadian period in humans was thought to differ from that of other species, with the period of the activity rhythm reported to range from 13 to 65 hours (median 25.2 hours) and the period of the body temperature rhythm reported to average 25 hours in adulthood, and to shorten with age. However, those observations were based on studies of humans exposed to light levels sufficient to confound circadian period estimation. Precise estimation of the periods of the endogenous circadian rhythms of melatonin, core body temperature, and cortisol in healthy young and older individuals living in carefully controlled lighting conditions has now revealed that the intrinsic period of the human circadian pacemaker averages 24.18 hours in both age groups, with a tight distribution consistent with other species. These findings have important implications for understanding the pathophysiology of disrupted sleep in older people.
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A century of research and practice have optimized the use of electric lighting in buildings to support human vision. However, recent lines of research show that light is also important to human circadian regulation, as reflected in such diverse phenomena as depression, sleep quality, alertness, and, perhaps, even health. Although light is essential to both vision and circadian regulation, research shows that the biophysical processes that govern circadian regulation are very different from those that govern vision. This growing body of research will probably influence the architectural lighting community and manufacturers to reoptimize the use of electric lighting in buildings to support both human vision and circadian functions. The present paper is concerned with establishing a framework for lighting practice and applied research that will assist lighting practitioners and manufacturers in interpreting this emerging research.
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The absolute and spectral sensitivities to light by the human circadian system, measured through melatonin suppression or phase shifting response, are beginning to emerge after a quarter century of active research. The present paper outlines a hypothesized model of human circadian phototransduction that is consistent with the known neuroanatomy and physiology of the human visual and circadian systems. Spectral opponency is fundamental to the model, providing a parsimonious explanation of some recently published data. The proposed model offers a framework for hypothesis testing and subsequent discussion of the practical aspects of architectural lighting with respect to light and health.
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The main objectives of this article are to update the ethical standards for the conduct of human and animal biological rhythm research and recommend essential elements for quality chronobiological research information, which should be especially useful for new investigators of the rhythms of life. A secondary objective is to provide for those with an interest in the results of chronobiology investigations, but who might be unfamiliar with the field, an introduction to the basic methods and standards of biological rhythm research and time series data analysis. The journal and its editors endorse compliance of all investigators to the principles of the Declaration of Helsinki of the World Medical Association, which relate to the conduct of ethical research on human beings, and the Guide for the Care and Use of Laboratory Animals of the Institute for Laboratory Animal Research of the National Research Council, which relate to the conduct of ethical research on laboratory and other animals. The editors and the readers of the journal expect the authors of submitted manuscripts to have adhered to the ethical standards dictated by local, national, and international laws and regulations in the conduct of investigations and to be unbiased and accurate in reporting never-before-published research findings. Authors of scientific papers are required to disclose all potential conflicts of interest, particularly when the research is funded in part or in full by the medical and pharmaceutical industry, when the authors are stock-holders of the company that manufactures or markets the products under study, or when the authors are a recent or current paid consultant to the involved company. It is the responsibility of the authors of submitted manuscripts to clearly present sufficient detail about the synchronizer schedule of the studied subjects (i.e., the sleep-wake schedule, ambient light-dark cycle, intensity and spectrum of ambient light exposure, seasons when the research was conducted, shift schedule in studies involving shift work, and menstrual cycle stage in studies involving young women). Rhythm analysis of time series data should be performed with the perspective that rhythms of different periods might be superimposed upon the observed temporal pattern of interest. A variety of different and complementary statistical procedures can be used for rhythm detection. Fitting a mathematical model to the time series data provides a better and more objective analysis of time series data than simple data inspection and narrative description, and if rhythmicity is documented by objective methods, its characterization is required by relevant parameters such as the rhythm's period (tau), MESOR (time series average), amplitude (range of temporal variation), acrophase (time of peak value), and bathyphase (time of trough value). However, the assumptions underlying the time series modeling must be satisfied and applicable in each case, especially the assumption of sinusoidality in the case of cosinor analysis, before it can be accepted as appropriate. An important aspect of the peer review of manuscripts submitted to Chronobiology International entails judgment of the conformity of research protocols and methods to the standards described in this article.
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Background The aim of this study was to examine individual differences in a large sample of complete melatonin profiles not suppressed by light and search for possible associations between the amount and timing of melatonin secretion and a multitude of lifestyle variables. The melatonin profiles were derived from saliva samples collected every 30 minutes in dim light from 85 healthy women and 85 healthy men aged 18–45 years. There was a large individual variability in the amount of melatonin secreted with peak values ranging from 2 to 84 pg/ml. The onset of melatonin secretion ranged from 18:13 to 00:26 hours. The use of hormonal birth control, reduced levels of employment, a smaller number of days on a fixed sleep schedule, increased day length and lower weight were associated with an increased amplitude of melatonin secretion. The use of hormonal birth control, contact lenses, a younger age, and lower ratings of mania and paranoia were associated with a longer duration of melatonin secretion. An earlier occurrence of the onset of melatonin secretion was associated with an earlier wake time, more morningness and the absence of a bed partner. Lifestyle and behavioral variables were only able to explain about 15% of the individual variability in the amount of melatonin secretion, which is likely because of a substantial genetic influence on the levels of melatonin secretion.
This paperback reprint of a classic book deals with all phases of light, color, and color vision, providing comprehensive data, formulas, concepts, and procedures needed in basic and applied research in color vision, colorimetry, and photometry.
Life in industrialized societies is primarily life inside buildings. Illumination from electric lighting in the built environment is quite different from solar radiation in intensity, spectral content, and timing during the 24-hour daily period. Humans evolved over millions of years with the day–night pattern of solar radiation as the primary circadian cue. This pattern maintained a 24-hour rhythm of melatonin release, as well as a host of other physiological rhythms including the sleep–wake cycle. Electric lighting in the built environment is generally more than sufficient for visual performance, but may be inappropriate for the maintenance of normal neuroendocrine rhythms in humans; e.g., insufficient during the day and too much at night. Lighting standards and engineering stress visual performance, whereas circadian function is not currently emphasized. The molecular biological research on the circadian clock and on mechanisms of phototransduction makes it clear that light for vision and light for circadian function are not identical systems. In particular, if electric lighting as currently employed contributes to `circadian disruption' it may be an important cause of `endocrine disruption' and thereby contribute to a high risk of breast cancer in industrialized societies.