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High Sensitivity of Human Melatonin, Alertness, Thermoregulation, and Heart Rate to Short Wavelength Light


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Light can elicit acute physiological and alerting responses in humans, the magnitude of which depends on the timing, intensity, and duration of light exposure. Here, we report that the alerting response of light as well as its effects on thermoregulation and heart rate are also wavelength dependent. Exposure to 2 h of monochromatic light at 460 nm in the late evening induced a significantly greater melatonin suppression than occurred with 550-nm monochromatic light, concomitant with a significantly greater alerting response and increased core body temperature and heart rate ( approximately 2.8 x 10(13) photons/cm(2)/sec for each light treatment). Light diminished the distal-proximal skin temperature gradient, a measure of the degree of vasoconstriction, independent of wavelength. Nonclassical ocular photoreceptors with peak sensitivity around 460 nm have been found to regulate circadian rhythm function as measured by melatonin suppression and phase shifting. Our findings-that the sensitivity of the human alerting response to light and its thermoregulatory sequelae are blue-shifted relative to the three-cone visual photopic system-indicate an additional role for these novel photoreceptors in modifying human alertness, thermophysiology, and heart rate.
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High Sensitivity of Human Melatonin, Alertness,
Thermoregulation, and Heart Rate to Short
Wavelength Light
Christian Cajochen, Mirjam Mu¨ nch, Szymon Kobialka, Kurt Kra¨uchi, Roland Steiner, Peter Oelhafen,
Selim Orgu¨ l, and Anna Wirz-Justice
Centre for Chronobiology (C.C., M.M., S.K., K.K., A.W.-J.), Psychiatric University Clinic, CH-4025 Basel, Switzerland;
Institute of Physics (R.S., P.O.), University of Basel, CH-4056 Basel, Switzerland; and Eye Clinic, University Hospital
(S.O.), CH-4012 Basel, Switzerland
Light can elicit acute physiological and alerting responses in
humans, the magnitude of which depends on the timing, in-
tensity, and duration of light exposure. Here, we report that
the alerting response of light as well as its effects on thermo-
regulation and heart rate are also wavelength dependent. Ex-
posure to2hofmonochromatic light at 460 nm in the late
evening induced a significantly greater melatonin suppres-
sion than occurred with 550-nm monochromatic light, con-
comitant with a significantly greater alerting response and
increased core body temperature and heart rate (2.8 10
/sec for each light treatment). Light diminished
the distal-proximal skin temperature gradient, a measure of
the degree of vasoconstriction, independent of wavelength.
Nonclassical ocular photoreceptors with peak sensitivity
around 460 nm have been found to regulate circadian rhythm
function as measured by melatonin suppression and phase
shifting. Our findings—that the sensitivity of the human alert-
ing response to light and its thermoregulatory sequelae
are blue-shifted relative to the three-cone visual photopic
system—indicate an additional role for these novel photore-
ceptors in modifying human alertness, thermophysiology, and
heart rate. (J Clin Endocrinol Metab 90: 1311–1316, 2005)
HE HUMAN CIRCADIAN timing system is sensitive to
ocular light exposure. The effects of light depend on the
circadian phase at which light is administered: light given
after the core body temperature (CBT) nadir advances the
phase of circadian rhythms, whereas light given before the
CBT nadir induces delays. This can be quantified by a
so-called “human phase-response curve to light” (1, 2).
Besides the timing of exposure, the intensity of light (i.e.
irradiance) also plays a crucial role in human circadian-
phase resetting (3, 4). The irradiance dose-response func-
tion to a single episode of light in the phase-delay region
can be characterized by a logistic function with high sen-
sitivity, such that half of the maximal resetting response
achieved in response to bright light (9100 lux) is obtained
with just 1% of this light (dim room light of 100 lux; see
Ref. 4). Recent results indicate that very low intensity
monochromatic light in the short-wave range (460 nm)
also affects the human circadian timing system and is
capable of inducing a significantly greater phase shift than
monochromatic light at 555 nm (the peak of the three-cone
photopic visual system) (5). Furthermore, short wave-
length light between 436 and 456 nm induced a phase
advance similar to that for polychromatic light (i.e. white
light) containing 185-fold more photons (6). These studies
clearly demonstrate that the human circadian timing sys-
tem is highly sensitive to ocular light exposure, particu-
larly in the short wavelength range.
Besides circadian phase shifts, light also elicits acute phys-
iological effects in humans such as a rapid suppression of
melatonin at night (for review, see Ref. 7), an increase in CBT
(8–11) and heart rate (12), and an immediate dose-dependent
alerting response, measured subjectively and objectively via
the electroencephalogram (10). Brainard et al. (13, 14) have
consistently shown that short wavelength light at around 460
nm is most effective in acutely suppressing human melatonin
levels. Furthermore, Hankins and Lucas (15) have recently
shown that acute light responses in the human electroreti-
nogram (ERG) are highly dependent on wavelength, such
that light at 483 nm elicited the strongest reduction in cone
ERG b wave–implicit time.
The acute effects of light, as well as the circadian effects,
seem to be mediated by the eyes. Thus, acute elevation of
body temperature and suppression of melatonin are not
observed when the eyes are covered (11, 16) or when light
is administered to the skin in the popliteal region (17–19).
There is mounting evidence that nonrod and noncone
photoreceptors might form the basis of this nonimage-
forming photoreceptive pathway mediating both the
circadian and direct effects of light in rodents (20, 21)
(for review, see Ref. 22). Therefore, we hypothesized that
the acute effect of light on melatonin, alertness, thermo-
regulation, and heart rate is blue-shifted, such that short
wavelength light at 460 nm induces a greater melatonin-
suppressing, alerting, hyperthermic, and tachycardic effect
than light at 550 nm.
First Published Online December 7, 2004
Abbreviations: CBT, Core body temperature; CP, constant posture;
DPG, distal-proximal skin temperature gradient; ERG, electroretino-
gram; SCN, suprachiasmatic nuclei.
JCEM is published monthly by The Endocrine Society (http://www., the foremost professional society serving the en-
docrine community.
0021-972X/05/$15.00/0 The Journal of Clinical Endocrinology & Metabolism 90(3):1311–1316
Printed in U.S.A. Copyright © 2005 by The Endocrine Society
doi: 10.1210/jc.2004-0957
on April 11, 2005 jcem.endojournals.orgDownloaded from
Subjects and Methods
Study participants
Ten male volunteers (age range, 21–29 yr; mean, 25.9 3.8 sd) were
studied. All study participants were nonsmokers, free from medical,
psychiatric, and sleep disorders as assessed by history, a physical ex-
amination, and questionnaires. An ophthalmological examination was
carried out before the study began and after completion of the study by
one of our coauthors (S.O.) to exclude volunteers with visual impair-
ments as well as to be certain that our light application was not harmful.
The volunteers were instructed to abstain from caffeine and alcohol for
1 wk before the study; their compliance was verified with urinary
toxicological analysis on the day of admission to the laboratory. They
were asked to keep a regular sleep-wake schedule (bedtimes and wa-
ketimes within 30 min of self-selected target time) during the week
before their admission to the laboratory. Adherence to this regular
schedule was verified with a wrist actigraph (Cambridge Neurotech-
nologies, Cambridge, UK) and daily sleep diaries. All volunteers gave
written informed consent. The protocol, screening questionnaires, and
consent form were approved by the Ethical Committee of Basel (Basel,
Switzerland) and were in agreement with the Declaration of Helsinki.
Study protocol
The study consisted of three arms, performed in a balanced order,
separated by a 1-wk intervening period (Fig. 1). There were no changes
in sleep quality or sleep-wake cycles during the intervening week. On
the basis of the habitual bedtimes of the volunteers, a constant posture
(CP) protocol started 10 h after usual waketime in the early evening (e.g.
1800 h) and ended the next day, 2 h after usual waketime (e.g. 1000 h).
Under CP conditions, the volunteers experienced a controlled, lying-
down episode of 1.5 h under 2 lux, followed by a 2-h dark adaptation
episode under complete darkness (zero lux). After that, light exposure
was initiated for the next 2 h. During this 2-h episode, the volunteers
received either monochromatic light at 460 nm, monochromatic light at
550 nm, or no light (zero lux). After this, the volunteers remained awake
for another 1.5-h episode under 2 lux (polychromatic white light), before
they were allowed to sleep for 7.75 h. One study participant developed
a mild cold during one of the study legs and was therefore excluded from
additional analysis.
Light exposure
Monochromatic light exposure (2 h) was scheduled at a circadian
phase at which polychromatic, white light exposure induces robust
phase delays (1, 2, 23) and alerting effects (24). The monochromatic light
was generated by a 300-W arc-ozone-free Xenon lamp (Thermo Oriel,
Spectra Physics, Stratford, CT), filtered by either 460 or 550 nm (Inter-
ference filter, 10 nm half-peak bandwidth, Spectra Physics, Stratford,
CT). Monochromatic light was transmitted via two glass-fiber bundles
(L.O.T. Oriel-Suisse, Romanel-sur Morges, Switzerland) through the
wall, into the soundproofed and temperature-controlled chronobiology
suite, onto the goggles that covered the volunteers’ eyes. The custom-
built goggles (K. Haug AG, Basel, Switzerland) consisted of two spheres
(27.5-mm inner radius) coated with white reflectance paint (two com-
ponents polyurethane-acryl antifading paint; Lachenmeier & Co. AG,
Basel, Switzerland). Each sphere was illuminated via three branches of
the main fiber-optic cable to provide constant uniform illumination.
Equal photon densities (2.8 10
/sec) for the 460- and
550-nm wavelength light were administered. This irradiance level (12.1
for 460 nm and 10.05
for 550 nm) was chosen ac
cording to recently reported results on monochromatic light on the
human circadian timing system (5). Irradiances were measured with a
laser power meter (Laser Check, Coherent, Auburn, CA) before the
beginning and at the end of each light exposure. During light exposure
as well as during the no-light condition, volunteers were asked to keep
their eyes open and to fix their gaze on the middle of the spheres. A
technician checked the latter by online monitoring the polysomno-
graphic recordings and also verifying that the subjects remained awake.
The volunteers’ pupils were not dilated to avoid possible repercussions
of the dilation agent per se on thermoregulation, heart rate, and alertness.
However, we tested the effects of the light stimulus on pupil constriction
by applying monocular light exposure (light via the goggle of the right
eye) and concomitantly measuring the pupil size on the left eye via an
infrared camera. The entire control protocol was conducted at the same
time of day (evening), with the same light intensity on six subjects.
Results from the control experiment revealed a significantly smaller
pupil size after the short wavelength light at 460 nm than after light at
550 nm in comparison to the dark condition [P 0.01; Duncan’s multiple
range test performed after a one-way ANOVA for repeated measures
with the factor light condition (P 0.02; dark, 460 and 550 nm)].
Assessment of subjective sleepiness
Subjective sleepiness was assessed every 30 min on the Karolinska
Sleepiness Scale (25), with a visual analog scale throughout scheduled
wakefulness. Because the participants wore goggles during the light
exposure and during the no-light condition, the Karolinska Sleepiness
Scale and the visual analog scale were read out loud by a technician and
transmitted via the interphone to the volunteers’ room.
CBT and eight surface skin temperatures from different body regions
were recorded continuously throughout the study, using a rectal probe
and skin thermocouples, with data stored in 20-sec epochs. Distal and
proximal skin temperatures as well as the distal-proximal skin temper-
ature gradient (DPG) were calculated according to the procedures de-
scribed in Ref. 26.
Heart rate
Standard electrocardiogram leads were placed on the lateral thorax
and on the sternum. The signal was recorded on the Vitaport-3 digital
system at 256 Hz. An off-line algorithm (System Hofstetter, SHS,
Allschwil, Switzerland) detected heart rate by the length of R-R intervals.
Salivary melatonin
Saliva was collected at 30-min intervals during scheduled wakeful-
ness. A direct double-antibody RIA was used for the melatonin assay,
validated by gas-chromatography-mass spectroscopy (Bu¨ hlmann Lab-
FIG. 1. Overview of the protocol design.
After 1.5 h under 2 lux, subjects were
dark adapted for 2 h, followed by an-
other2hindarkness or light exposure
at 460 nm or 550 nm (for details about
the light exposures, see Subjects and
Methods). Subsequently, subjects spent
1.5 h under 2 lux before they were al-
lowed to sleep for 8 h. The entire pro-
tocol was carried out under constant re-
cumbent posture conditions in bed.
Saliva samples were collected, and
sleepiness ratings were taken, both in
half-hourly intervals.
1312 J Clin Endocrinol Metab, March 2005, 90(3):1311–1316 Cajochen et al. Effects of Monochromatic Light on Humans
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oratories, Scho¨nenbuch, Switzerland) (27). The minimum detectable
dose of melatonin (analytical sensitivity) was determined to be 0.2 pg/
ml. The functional least-detectable dose using the less than 20% coef-
ficient of interassay variation criterion was less than 0.65 pg/ml, and
individual serum and saliva melatonin profiles showed excellent par-
allelism (r 0.977– 0.999; slopes 0.21–0.63) (27).
Statistical analysis of the time course was carried out for each variable
using two-way ANOVAs for repeated measures on factor light condition
and time interval with Huynh-Feldt’s statistics. P values were based on
corrected degrees of freedom, but the original degrees of freedom are
reported. The statistical package SAS (version 6.12, SAS Institute Inc.,
Cary, NC) was used. For post hoc comparisons, one-sided Duncan’s
multiple range tests were used. To correct for multiple comparisons, the
resulting P values were alpha-corrected according to the procedures
described in Ref. 28.
Melatonin suppression and subjective sleepiness
Monochromatic light exposure caused a wavelength-
dependent suppression of salivary melatonin (Fig. 1, top
panel), as indicated by a significant two-way interaction of the
factors light condition and time interval (F
3.6; P
0.001). Post hoc comparisons yielded a significant melatonin
suppression after light at 460 nm compared with no light and
to light at 550 nm 30 min after the start of light exposure,
which continued for the remainder of the light-exposure
episode (for post hoc comparisons, see Fig. 2). Salivary mel-
atonin levels during monochromatic light at 550 nm were
only slightly but significantly suppressed during the first
hour of light exposure (Fig. 2). Subjective sleepiness ratings
changed in parallel [interaction light condition time in-
terval (F
1.7; P 0.05)]. Post hoc comparisons yielded
a significant decrease in subjective sleepiness during the
460-nm light exposure compared with 550-nm light exposure
and no light, starting 30 min after lights on (Fig. 2, second
panel). There was no significant difference in sleepiness be-
tween the 550-nm light exposure and the no-light condition.
Thermoregulation and heart rate
Light exposure changed the time course of CBT [interac-
tion light condition time interval (F
2.9; P 0.02)
(Fig. 2, third panel)]. The evening decline of CBT was signif-
icantly attenuated by light at 460 nm starting about 1 h after
lights on (Fig. 2), remaining significantly higher throughout
the remainder of the 1.5-h interval before sleep. No differ-
ences were found between the condition with light at 550 nm
and the no-light condition. A similar pattern was found for
heart rate, as indexed by beats per min [interaction light
condition time interval (F
3.1; P 0.01) (Fig. 2,
bottom panel)]. Although the effect was short lasting, post hoc
comparisons revealed a significantly higher heart rate during
the 460- nm light condition starting 1.5 h after lights on
compared with 550 nm and the no-light condition and lasting
for the first 20 min of the after-light exposure episode.
Although repercussions of light at 460 and 550 nm were
clearly visible in the time course of both proximal and distal
skin temperatures, no significant interaction terms were
found (Fig. 3, top two panels). However, the derived measure
of the DPG that is used as an estimate of the degree of
vasodilation (29) yielded a significant interaction term (F
1.8; P 0.03) (Fig. 3, bottom panel). Post hoc comparisons
revealed a significant decrease in the DPG during both 460-
and 550-nm light exposures compared with the no-light
These results demonstrate that the alerting response to
light is wavelength dependent, such that short wavelength
light (460 nm) is more effective than longer wavelength light
(550 nm) in reducing sleepiness in the evening. Furthermore,
our controlled study provides evidence that the effects of
light on thermoregulation and heart rate are similarly wave-
length dependent.
Our data are in good agreement with recent findings that
the human circadian pacemaker is highly sensitive to short
wavelength light (13, 30), as indexed by action spectra for
human melatonin suppression and assessment of human
circadian phase resetting (5, 6). On the basis of these previous
studies, we expected a significantly more pronounced atten-
uation of the nocturnal melatonin increase after light at the
shorter wavelength (460 nm), a hypothesis that was clearly
verified. We have obtained very similar results as Brainard
et al. (13) who reported approximately 60% suppression of
melatonin after2hoflight at 460 nm and at 12.1
Therefore, melatonin levels in 460 nm did not increase during
the light exposure, whereas in the 550-nm condition, they
additionally increased very similarly as shown by Lockley et
al. (5). To our knowledge, this is the first report showing that
human alertness levels as well as thermophysiology are
highly sensitive to this short wavelength light. With the
exception of the proximal and distal skin temperatures, all
other variables (i.e. salivary melatonin, subjective sleepiness,
CBT, and heart rate) responded more strongly to 460- than
550-nm light. However, light at 550 nm was not inactive
because it induced a subtle, short-lasting but significant mel-
atonin suppression. What is interesting is that both wave-
lengths decreased the DPG to a similar extent. Why the
effects of light on the skin temperatures were not wavelength
dependent remains to be elucidated. Although our study was
conducted under very controlled laboratory conditions (i.e.
CP, room temperature, and food intake), skin temperatures
exhibit large inter- and intraindividual variance (31). There-
fore, it may be that this measure did not provide enough
power to differentiate between the two wavelengths. How-
ever, there are two possibilities: 1) the DPG may indeed be
a very sensitive measure for subtle illuminance changes;
and/or 2) that it immediately reflects a minute-to-minute
level of cognitive arousal independent of the sensory mo-
dality of the signal. The DPG increase during the dark-ad-
aptation episode, which was unusual at this circadian phase
as previously measured under 8 lux of ambient-light levels
in a constant routine protocol (26, 31), can be interpreted in
both ways—a diminution to zero lux and a diminution of
sensory input, leading to relaxation. Interestingly, this in-
creased DPG was also paralleled by an unusually early in-
crease in subjective sleepiness and an unusually early
evening melatonin onset. Furthermore, evidence for the re-
sponsiveness of DPG comes from the decrease in this mea-
Cajochen et al. Effects of Monochromatic Light on Humans J Clin Endocrinol Metab, March 2005, 90(3):1311–1316 1313
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sure seen after the 4-h dark episode in the no-light condition,
when the volunteers were under 2 lux (Fig. 2, bottom panel).
The DPG reflected very sensitively whether the lights were
on or off. Besides the DPG, the increase in heart rate during
the 460-nm light exposure may well be another indication
that the autonomous nervous system acutely responds to
light with an increase in sympathetic tone—a response that
seems particularly susceptible to short wavelength light.
Many studies have shown that exposure to white poly-
chromatic light during the evening or nighttime increases
alertness (8, 10, 24, 32–35) and CBT (8 –10, 19, 34, 35). There
is also evidence that light may acutely affect heart rate (12).
Previously, we have found a dose-response relationship be-
tween the magnitude of the alerting response to light and its
irradiance, such that half of the maximum alerting response
to bright light at 9100 lux was obtained with room light of
approximately 100 lux (10). However, the duration of light
exposure in this study was rather long (6.5 h), and the dose
relationship was only present in the latter part of the light
exposure (10). In contrast, the present study revealed that
light at 460 nm of very low intensity (5 photopic lux or 116.6
scotopic lux) was already effective after about 40 min of
exposure, which corroborates high specificity for light in the
short wavelength range, and shows that the nonimage-form-
ing visual system does not simply count or average photons,
but rather depends on exposure to particular wavelengths of
energy. In fact, during the 460-nm light condition, volunteers
in our experiment probably received fewer photons, because
their pupil size was smaller than in the 550-nm light condi-
tion (based on our data from the control experiment; see
Subjects and Methods). Therefore, the melatonin suppression
and the alerting response was underestimated from what
they would have been had the subjects’ pupils been artifi-
cially dilated. Despite fewer photons, 460-nm light was more
efficient on the above-described variables than 550 nm,
which corroborates its effectiveness also in the absence of
pupil dilators.
Our results demonstrate that besides regulating human
circadian rhythms, the nonclassical photoreceptors are also
involved in the regulation of the acute effects of light, which
has until now only been shown for ERG responses (15).
Although it is possible that the central circadian pacemaker
located in the suprachiasmatic nuclei (SCN) is involved in
both phase shifting and acute responses to light, it is not clear
that these share a common mechanism. It has been proposed
that acute changes in CBT may be primary events mediating
circadian phase-shift responses (36). There is, however, con-
trary evidence; previous administration of melatonin can
completely reverse the acute CBT elevation induced by
nighttime bright light without greatly altering light-induced
phase shifts (23, 37, 38). Whether the delayed decline in CBT
represents only an acute effect of light or whether it is the
initiation of a circadian phase delay would have required a
longer study. We may interpret the sustained evening max-
imum in CBT only after 460-nm light exposure as evidence
FIG. 2. Effects of a 2-h light exposure at 460 nm (F), 550 nm (Œ), and
no light (f) in the evening under CP conditions (i.e. supine in bed) on
salivary melatonin levels, subjective sleepiness as rated on the Karo-
linska Sleepiness Scale, CBT, and heart rate [mean values (n 9) and
SEM]. For clarity, the SEM values for the 550-nm light condition were
not plotted. Significant post hoc comparisons (P 0.05; Duncan’s
multiple range test corrected for multiple comparisons) are indicated
by the following symbols: *, 460-nm light vs. no light; E, 550-nm light
vs. no light; and ƒ, 460-nm light vs. 550-nm light. The prelight ex-
posure episode represents a 2-h dark adaptation episode under zero
lux, whereas the light level in the 1.5-h post-light exposure was 2 lux.
1314 J Clin Endocrinol Metab, March 2005, 90(3):1311–1316 Cajochen et al. Effects of Monochromatic Light on Humans
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for the latter explanation of a selective circadian phase delay
The mechanisms by which light induces acute physiolog-
ical responses and shifts circadian phase seem to diverge at
some level. Besides the SCN, candidate retinal projections for
the acute effects of light are the pretectal area (39), the in-
tergeniculate leaflet (40), and the ventromedial preoptic area
(41). It is clear that for light to rapidly suppress melatonin
secretion, retinal projections to the SCN are necessary. There-
fore, it has been suggested that the mechanism by which light
exposure may reduce sleepiness is by its suppression of
melatonin synthesis (8, 10, 24). However, there is recent
evidence that these effects appear to be mediated by mech-
anisms that are separate from melatonin suppression (42). It
is more likely to be the ventromedial preoptic area that in-
nervates all of the major nuclei of the ascending monoam-
inergic and, in particular, the histaminergic system and plays
a key role in wakefulness and electroencephalogram arousal
(43, 44).
All of the above-mentioned brain regions receive projec-
tions from intrinsically photosensitive retinal ganglion cells
for which the photopigment melanopsin has recently been
identified (45). Melanopsin is present in the human retina
(46), and melanopsin-containing retinal ganglion cells are
directly photosensitive at a
of 484 nm in the rat (47).
Melanopsin expression defines a subset of retinal ganglion
cells that play a broad role in the regulation of nonvisual
photoreception, providing projections that contribute to cir-
cadian entrainment, negative masking, the regulation of
sleep-wake states, and the pupillary reflex (for citations, see
Ref. 45). Our results add to these functions, suggesting that
changes in human alertness and aspects of autonomic control
(thermoregulation and heart rate) are influenced, if not reg-
ulated, by the nonvisual system via the photopigment mela-
nopsin. A definite answer to this would be to investigate
people lacking the classical receptors, or having a melanop-
sin deficiency, to the see repercussions this may have on
light-induced changes in alertness, thermoregulation, and
heart rate. At least in blind mice, melanopsin is required for
nonimage-forming photic responses. However, there is still a
debate (see Ref. 48) as to whether the photopigment melanopsin
is the only candidate for nonvisual ocular photoreception.
It will be interesting to test whether short wavelength light
is more efficient in the workplace environment, where high
alertness levels are required, and in the treatment of seasonal
affective disorder; although in all putative applications, the
blue-light damage potential needs to be evaluated (49). An
important physiological question is whether the decline in
alertness and thermoregulation with age is a consequence of
age-related changes in lens transmittance at the short-wave
range (50).
We thank Dr. Corinna Schnitzler for medical screenings; Claudia
Renz, Giovanni Balestrieri, and Marie-France Dattler for their help in
data acquisition; and the volunteers for participating.
Received May 27, 2004. Accepted November 23, 2004.
Address all correspondence and requests for reprints to: Christian
Cajochen, Ph.D., Centre for Chronobiology, Psychiatric University
Clinic, Wilhelm Kleinstr. 27, CH-4025 Basel, Switzerland. E-mail:
This research was supported by the Velux Foundation (Glarus, Swit-
zerland) and in part by The Swiss National Foundation Grants START
3130-054991.98 and 3100-055385.98 (to C.C.).
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1316 J Clin Endocrinol Metab, March 2005, 90(3):1311–1316 Cajochen et al. Effects of Monochromatic Light on Humans
on April 11, 2005 jcem.endojournals.orgDownloaded from
... To assess if presented results were acquired with sufficient validity, a power calculation was conducted. Cajochen et al., (2005) reported a significant effect of BL exposure on distal proximal gradient (DPG) with an effect size of 5.16. With alpha set to 0.05 and power to 0.8, a total of 3 participants per light condition should be included to find a statistically significant difference in DPG. ...
... With alpha set to 0.05 and power to 0.8, a total of 3 participants per light condition should be included to find a statistically significant difference in DPG. (Cajochen et al., (2005) tested effects of light on DPG in the evening hours, and therefore included bright light induced melatonin suppression, which in turn can induce temperature changes. Therefore, an additional power calculation was conducted using (Aizawa and Tokura, 1998). ...
... P. Johnson et al., 1992;Wyatt et al., 1999). Fluctuations in CBT levels coincide with rhythms of melatonin production (Cajochen et al., 1998(Cajochen et al., , 2005Kräuchi et al., 1997b); peak melatonin values are associated with the nadir in CBT, while the subsequent decline in melatonin is followed by an increase in CBT (Cagnacci et al., 1997). The CBT decreasing effect of melatonin is partly caused by increased heat loss due to peripheral vasodilatation by melatonin (Lok et al., 2019). ...
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Human thermoregulation is strictly regulated by the preoptic area of the hypothalamus, which is directly influenced by the suprachiasmatic nucleus (SCN). The main input pathway of the SCN is light. Here, thermoregulatory effects of light were assessed in humans in a forced desynchrony (FD) design. The FD experiment was performed in dim light (DL, 6 lux) and bright white light (BL, 1300 lux) in 8 men in a semi-randomized within-subject design. A 4 × 18 h FD protocol (5 h sleep, 13 h wake) was applied, with continuous core body temperature (CBT) and skin temperature measurements at the forehead, clavicles, navel, palms, foot soles and toes. Skin temperature parameters indicated sleep-wake modulations as well as internal clock variations. All distal skin temperature parameters increased during sleep, when CBT decreased. Light significantly affected temperature levels during the wake phase, with decreased temperature measured at the forehead and toes and increased navel and clavicular skin temperatures. These effects persisted when the lights were turned off for sleep. Circadian amplitude of CBT and all skin temperature parameters decreased significantly during BL exposure. Circadian proximal skin temperatures cycled in phase with CBT, while distal skin temperatures cycled in anti-phase, confirming the idea that distal skin regions reflect heat dissipation and proximal regions approximate CBT. In general, we find that increased light intensity exposure may have decreased heat loss in humans, especially at times when the circadian system promotes sleep.
... These cells project to hypothalamic regions including the SCN (the master pacemaker in the brain) via the retinohypothalamic tract (Hattar et al., 2002). NIF effects of light have proven essential for entrainment of the circadian system (Cajochen et al., 2005(Cajochen et al., , 2011Geerdink et al., 2016), which can affect sleep quality (Grønli et al., 2016), defined as chronic effects of light exposure. For example, light exposure in the (late) evening decreases sleep quality (Cajochen et al., 1992;Dijk et al., 1991) since it stimulates the phase delay portion of the human phase response curve (Khalsa et al., 2003;Papantoniou et al., 2014). ...
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Under real-life conditions, increased light exposure during wakefulness seems associated with improved sleep quality, quantified as reduced time awake during bed time, increased time spent in non-rapid eye movement (NREM) sleep, or increased power of the electroencephalogram delta band (0.5-4 Hz). The causality of these important relationships and their dependency on circadian phase and/or time awake has not been studied in depth. To disentangle possible circadian and homeo-static interactions, we employed a forced desynchrony protocol under dim light (6 lux) and under bright light (1300 lux) during wakefulness. Our protocol consisted of a fast cycling sleep-wake schedule (13 h wakefulness-5 h sleep; 4 cycles), followed by 3 h recovery sleep in a within-subject cross-over design. Individuals (8 men) were equipped with 10 polysomnography electrodes. Subjective sleep quality was measured immediately after wakening with a questionnaire. Results indicated that circa-dian variation in delta power was only detected under dim light. Circadian variation in time in rapid eye movement (REM) sleep and wakefulness were uninfluenced by light. Prior light exposure increased accumulation of delta power and time in NREM sleep, while it decreased wakefulness, especially during the circadian wake phase (biological day). Subjective sleep quality scores showed that participants rated their sleep quality better after bright light exposure while sleeping when the circadian system promoted wakefulness. These results suggest that high environmental light intensity either increases sleep pressure buildup during wakefulness or prevents the occurrence of micro-sleep, leading to improved quality of subsequent sleep.
... Light exposure may counteract initial decrements in performance due to sleep inertia after awakening. The significant improvement of alertness by light in the middle of the subjective night (circadian phase 150) might be considered to be in line with findings of light-induced improvements in the early night under entrained conditions (Cajochen et al., 2005). ...
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Light-induced improvements in alertness are more prominent during nighttime than during the day, suggesting that alerting effects of light may depend on internal clock time or wake duration. Relative contributions of both factors can be quantified using a forced desynchrony (FD) designs. FD designs have only been conducted under dim light conditions (<10 lux) since light above this amount can induce non-uniform phase progression of the circadian pacemaker (also called relative coordination). This complicates the mathematical separation of circadian clock phase from homeostatic sleep pressure effects. Here we investigate alerting effects of light in a novel 4 × 18 h FD protocol (5 h sleep, 13 h wake) under dim (6 lux) and bright light (1300 lux) conditions. Hourly saliva samples (melatonin and cortisol assessment) and 2-hourly test sessions were used to assess effects of bright light on subjective and objective alertness (electroencephalography and performance). Results reveal (1) stable free-running cortisol rhythms with uniform phase progression under both light conditions, suggesting that FD designs can be conducted under bright light conditions (1300 lux), (2) subjective alerting effects of light depend on elapsed time awake but not circadian clock phase, while (3) light consistently improves objective alertness independent of time awake or circadian clock phase. Reconstructing the daily time course by combining circadian clock phase and wake duration effects indicates that performance is improved during daytime, while subjective alertness remains unchanged. This suggests that high-intensity indoor lighting during the regular day might be beneficial for mental performance, even though this may not be perceived as such.
... This result suggests that the human circadian system mainly depends on the effect of short-wavelength light (Warman et al., 2003). In line with this, blue light exposure in the evening has been shown to suppress the evening onset of melatonin and lead to a phase delay of the circadian rhythm (Cajochen et al., 2005Chellappa et al., 2011;Alkozei et al., 2016). Moreover, morning exposure to blue light is also known to suppress melatonin and leads to a phase advance of the circadian rhythm (Wright et al., 2004;Lack et al., 2007;Sletten et al., 2009). ...
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Sleep deprivation is known to have adverse effects on various cognitive abilities. In particular, a lack of sleep has been reported to disrupt memory consolidation and cognitive control functions. Here, focusing on long-term memory and cognitive control processes, we review the consistency and reliability of the results of previous studies of sleep deprivation effects on behavioral performance with variations in the types of stimuli and tasks. Moreover, we examine neural response changes related to these behavioral changes induced by sleep deprivation based on human fMRI studies to determine the brain regions in which neural responses increase or decrease as a consequence of sleep deprivation. Additionally, we discuss about the possibility that light as an environmentally influential factor affects our sleep cycles and related cognitive processes.
... Unlike the circuit that operates through SCNs for circadian rhythm regulation, some researchers believe that the circuit for acute non-visual effects manifests in other areas of the brain and is mediated by certain subpopulations of ipRGCs [52,53]. The possibility that some types of ipRGCs project to other regions of the brain and influence acute body temperature suggests that more pathways and signs of acute thermal responses are caused by light [54]. ...
The acute thermal effects of artificial light during the daytime have the potential to promote energy savings and enhance indoor comfort. Although the long-held hue-heat hypothesis suggests that the visual features of lighting may alter human thermal responses, there currently exists no systematic review or cross-study analysis that synthesizes and characterizes light's impact along both visual and non-visual pathways during the daytime. This review highlights evidence and physiological and psychological measures examining the presence of light's thermal effects during the daytime, as obtained from the literature. This review searched articles from PubMed, Scopus, Web of Science, and other sources and screened for articles with thorough lighting information and microclimatic conditions describing experiments conducted during the daytime and using static artificial light. Eighteen articles describing 18 studies were selected based on the inclusion criteria; these studies featured different experimental and intervention designs for the indoor environment and measurements of psychological, physiological, and behavioral responses. This research specifically reviews the experimental designs and settings in terms of lighting and microclimatic characteristics and also identifies the effective and appropriate physiological and psychological measures of light's thermal effects. Much of the literature described in this review suggests that lighting exposure during the daytime is associated with thermal-related psychological and physiological responses; the analysis results across different studies showed statistically significant associations with certain psychological (e.g., thermal sensation, preferred temperature) and physiological measures (e.g., proximal skin temperature, heart rate variability). However, some cross-study results were limited by the unavailability of key measurements and reports on lighting, microclimatic conditions, and/or thermal responses and possibly high levels of heterogeneity. In general, the findings of this review will facilitate continuing advances in this area, providing more comprehensive scientific rationales and strategies for adopting smart lighting technologies in sustainable and smart buildings.
... However, in 2013, the scientific research of Heath, M. and others pointed out that when the brightness of a flat screen is below 80 lux and the use time is below one hour, the sleep time of the flat screen user is not significantly different from that of someone using a screen with 1 lux filtered blue light [21]. This is different from other scientific studies where there is significant melatonin suppression in the continuous viewing of electronic devices for more than 2 h and exposure to human light [22][23][24]. ...
Full-text available
With the rapid development of science and technology, social competition becomes more and more fierce, and work pressure becomes more and more intense. The problem of sleep has become a disease of modern civilization, and in order to alleviate sleep problems and improve sleep quality, related research methods are constantly updated, and there are more and more ways to address these problems. In this paper, we propose a dynamic assisted sleep light that adjusts the color temperature, illuminance, and frequency of change by controlling the backlight of the smart mobile device, so that the user experiences an improved quality of sleep. In this study, the methods of subjective questionnaire survey and non-subjective physiological parameter measurement were adopted for analysis. The results showed that the continuous use of the light source could continuously enhance the user’s delta energy during sleep and promote the improvement of sleep quality.
... Nevroendokrini odzivi regulirajo, kako možgani regulirajo sintezo hormonov [Berson, 2003], predvsem sintezo melatonina, ki posredno vpliva tudi na cirkadiane odzive, ki usklajujejo našo notranjo 24-urno uro s solarnim ciklom [Czeisler, 2007]. Med nevrovedenjske odzive pa uvrščamo odnos med živčnim sistemom in človeškim vedenjem, med takšne procese uvrščamo pozornost [Rahman, 2017], razpoloženje [Milosavljevic, 2019], kognitivno učinkovitost [Jamrozik, 2019] kot tudi srčni utrip [Cajochen, 2005], telesno temperaturo [Lok, 2019] itd. ...
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Light, especially daylight, has been recognized by advances in science over the past decades as the main coordinator of the daily biological rhythm of people with a 24-hour solar rhythm. The photoreceptor responsible for the non-visual perception is more sensitive to the blue component of light than the visual perception system. Therefore, it is necessary to evaluate the indoor environment in terms of the spectral composition of light to assess the effects of light on the non-visual system. The purpose of this study was to verify whether the existing tools for multispectral evaluation of the indoor environment are accurate enough to evaluate the non-visual luminous environment. The reliability of the studied tools was tested under clear sky conditions on a north oriented scale model of an office the luminous environment of which was determined experimentally. Experimental results were compared to the simulations evaluated in terms of accuracy and speed. The accuracy of the simulations was evaluated based on the root mean square error (RMSE) of the relative spectral distribution of light (RSPD) and the relative error of the relative melanopic efficiency (RME). The analysis shows that low RMSE of ALFA and Lark tools were achieved when considering RSPD measured outside on the horizontal plane. When evaluating accuracy from the perspective of a hypothetical office user, ALFA adequately simulated the lighting environment with a maximum RMSE = 0.08 and a maximum error in RME = 2.8 %. Lark proves to be less accurate with a maximum RMSE = 0.18 and a maximum error in RME = 16.2 %. Additionally, the ALFA tool proves to be much more time-efficient with more than 20 times shorter simulation runs compared to Lark.Svetlobo, predvsem dnevno svetlobo smo z napredki v znanosti preteklih desetletij prepoznali kot usklajevalec vsakodnev-nega biološkega ritma-cirkadianega ritma ljudi s 24-urnim solarnim ritmom. Vrsta fotoreceptorja, zadolžena za nevizualno zaznavo, je bolj občutljiva za modri del svetlobe kakor sistem vidne zaznave in zato za oceno učinkov svetlobe na nevizualni sistem zahteva sposobnost vrednotenja okolja tudi z vidika spektralne sestave svetlobe. Namen te študije je bil na podlagi opravljenega eksperimenta ocene notranjega okolja pomanjšane pisarne, postavljene na strehi Fakultete za gradbeništvo in geodezijo Univerze v Ljubljani, preveriti, ali so obstoječa orodja večspektralnega vrednotenja notranjega okolja dovolj točna za vrednotenje nevizualnega svetlobnega okolja. Simulacije ovrednotimo z vidika točnosti in hitrosti pri jasnem nebu in severni orientaciji analiziranega prostora. Točnost simulacij ovrednotimo na podlagi korena povprečnega kvadrata napake (RMSE) relativne spektralne porazdelitve svetlobe (RSPD) in relativne napake v relativni melanopski učinkovitosti (RMU). Analiza pokaže nizke RMSE orodij ALFA in Lark, ko obravnavamo RSPD dnevne svetlobe, merjene na horizontalni ravnini zunaj. Kadar točnost vrednotimo z vidika navideznega uporabnika prostora, ALFA vestno simulira svetlobno okolje z največjo RMSE = 0,08 in največjo napako v RMU = 2,8 %. Lark se izkaže za manj natančnega z največjo RMSE = 0,18 in največjo napako v RMU = 16,2 %. Dodatno se izkaže, da je orodje ALFA časovno mnogo učinkovitejše z več kot 20-krat krajšimi simulacijskimi časi v primerjavi z Larkom. Ključne besede: dnevna svetloba, nevizualni učinki svetlobe, spektralne simulacije, pisarna asist. dr. Jaka Potočnik, mag. inž. arh. izr. prof. dr. Mitja Košir, univ. dipl. inž. arh.
... The effect of bright light on the psychological variables was time independence, since nighttime and daytime bright light reduced sleepiness. (22) Light influences the daily rhythm and humans well-being in a physiological, psychological and biological way beside visual photoreceptors, the human eye also contains non visual photoreceptors. ...
This study shed the light on the effect of light intensity on some of the dependent variables, such as blood oxygen saturation (SPO2%), heart pulse rate (P.R), arterial blood pressure (systolic (SBP), diastolic (DBP)), and tympanic temperature (T) of nurses, in their shift work. The sample consists of 207 nurses of both genders (104 female, 103 male), with mean age 29 years, and the mean duration of employment 6 years, were randomly chosen as a sample to fulfill the aim meant. This sample was taken from four hospitals in Nablus city. The values of light intensity in all hospital ranged from 220 Lux to 1000 Lux, at the day shift, and from 500 Lux to 1700 Lux, at the night shift. Number of measurements concerning the blood oxygen saturation, heart pulse rate, arterial blood pressure (systolic and diastolic), and tympanic temperature at different light intensities were taken for the selected sample before and an after exposure to light. Strong positive correlation (Pearson Correlation Coefficient) with light intensity was found for all measured variables. The statistical result for the dependent variables (SPO2%, P.R, SBP, DBP, T) showed that Pearson correlation coefficient (R) between light intensity and the dependent variables are approximately equal to one, and the Probabilities (P) are < 0.05. It was also found that blood oxygen saturation has Pearson's Coefficient R = 0.980 and probability P = 0.020, whereas heart pulse rate has R = 0.966 and probability P = 0.034, while systolic blood pressure has R = 0.985 and P = 0.015. In addition, diastolic blood pressure has R = 0.989 and P = 0.011, and the values for temperature are R = 0.990 and P = 0.010. This study shows that the health effects of light intensity depend on the light intensity itself, more specifically, nurses exposed to light intensity 1700 Lux, have a significant shift of the measured mean values ( blood oxygen saturation, heart pulse rate, arterial blood pressure (systolic and diastolic ), and tympanic temperature), more than nurses exposed to light intensity less than 500 Lux.
School non-attendance due to difficulties waking up is increasing in Japan, and affected students are commonly diagnosed with orthostatic dysregulation (OD); however, OD-associated sleep problems are overlooked. To date, no sleep-medicine-based treatment for wake-up difficulties in non-school-attending students has been established. This study aimed to assess the efficacy of a novel combination therapy for these students. We assessed the combined effect of sleep hygiene guidance, low-dose aripiprazole administration (3 mg/day), and blue-light exposure on wake-up difficulty in 21 non-school-attending teenage patients. The patients were evaluated using sleep studies and questionnaires before and after treatment. The average subjective total sleep time calculated from sleep diaries before treatment in the patients was 10.3 h. The therapy improved wake-up difficulty by 85.7% and further improved school non-attendance by 66.7%. The subjective sleep time significantly decreased by 9.5 h after treatment (p = 0.0004). The self-rating Depression Scale and mental component summary of the 36-item Short-Form Health Survey significantly improved after treatment (p = 0.002 and p = 0.01, respectively). Wake-up difficulties were caused by the addition of a delayed sleep phase to the patients’ long sleep times. The novel combination therapy was effective in improving wake-up difficulty and mental quality of life in non-school-attending teenage students.
Objectives To determine the efficacy and safety of blue-light therapy in seasonal and non-seasonal major depressive disorder (MDD), by comparison to active and inactive control conditions. Methods We searched Web of Science, EMBASE, Medline, PsycInfo, and through January 17, 2022, for randomized controlled trials (RCTs) using search terms for blue/blue-enhanced, light therapy, and depression/seasonal affective disorder. Two independent reviewers extracted data. The primary outcome was the difference in endpoint scores on the Structured Interview Guide for the Hamilton Depression Rating Scale - Seasonal Affective Disorder (SIGH-SAD) or the Structured Interview Guide for the Hamilton Depression Rating Scale with Atypical Depression Supplement (SIGH-ADS) between blue light and comparison conditions. Secondary outcomes were response (≥ 50% improvement from baseline to endpoint on a depression scale) and remission rates (endpoint score in the remission range). Results Of 582 articles retrieved, we included nine RCTs ( n = 347 participants) assessing blue-light therapy. Seven studies had participants with seasonal MDD and two studies included participants with non-seasonal MDD. Four studies compared blue light to an inactive light condition (efficacy studies), and five studies compared it to an active condition (comparison studies). For the primary outcome, a meta-analysis with random-effects models found no evidence for the efficacy of blue-light conditions compared to inactive conditions (mean difference [MD] = 2.43; 95% confidence interval [CI], −1.28 to 6.14, P = 0.20); however, blue-light also showed no differences compared to active conditions (MD = −0.11; 95% CI, −2.38 to 2.16, P = 0.93). There were no significant differences in response and remission rates between blue-light conditions and inactive or active light conditions. Blue-light therapy was overall well-tolerated. Conclusions The efficacy of blue-light therapy in the treatment of seasonal and non-seasonal MDD remains unproven. Future trials should be of longer duration, include larger sample sizes, and attempt to better standardize the parameters of light therapy.
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In seven subjects sleep was recorded after a single 3-hour (2100-0000 hours) exposure to either bright light (BL, approx. 2,500 lux) or dim light (DL, approx. 6 lux) in a crossover design. The latency to sleep onset was increased after BL. Whereas rectal temperature before onset and during the first 4 hours of sleep was higher after BL than after DL, the time course of electroencephalographic (EEG) slow-wave activity (SWA, EEG power density in the range of 0.75-4.5 Hz) in nonrapid eye movement sleep (NREMS) differed only slightly between the conditions. After BL, SWA tended to be lower than after DL in the first NREMS-REMS cycle and was higher in the fourth cycle at the time when the rectal temperature did not differ. The differences in SWA may have been due to a minor sleep-disturbing aftereffect of BL, which was followed by a rebound. The data are not in support of a close relationship between SWA and core body temperature.
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Using 'classical' experimental protocols, a human phase-response curve (PRC) to a single 3-h bright light pulse has been established. When the light pulse was centred slightly before the time of body temperature minimum, the circadian system delayed, whilst a pulse slightly after the minimum advanced it. Maximum phase shifts were about 2 h. When light pulses over 3 successive cycles were used, larger shifts (4-7 h) were produced. It is concluded that the human PRC does not differ in principle from that found in other species, except with respect to the light intensity required.
The present study validated the nine-point Karolinska Sleepiness Scale (KSS) and the new Accumulated Time with Sleepiness (ATS) scale against performance of laboratory tasks. The ATS scale was designed as a method for integrating subjective sleepiness over longer time periods. The subjects were asked if certain symptoms of sleepiness had occurred and, if so, for how long. Six subjects participated twice. Each time they were kept awake during the night (except for a short nap occurring during one of the nights in a counterbalanced order) and were tested at 2200, 0200, 0400 and 0600 hours. The tests included a 10-minute rest period, a 28-minute visual vigilance task and an 11 -minute single reaction time task. KSS and visual analogue scale (VAS) ratings were given before each test, and ATS ratings were given after. Performance deteriorated clearly, and all three rating scales reflected increased sleepiness with time of night. Scores on the KSS and VAS showed high correlations with performance tasks (mean intraindividual correlations were between 0.49 and 0.71). Performance correlated even higher with the ATS ratings (r = 0.73–0.79). Intercorrelations between rating scales were also high (r = 0.65–0.86). It was concluded that there were strong relations between ratings of sleepiness and performance, that the ATS rating scale was at least as good as the other scales and that the ratings were affected by type of task.
Bright (3000 lux) vs dim (100 lux) illuminance levels were compared for associated effects on oral temperature and sustained human performance. A counter balanced repeated measures design was used to assess the 9 male subjects during each of the two illuminance conditions. After practice sessions (0700-1800 hrs), cognitive performance tests were administered by a computer workstation every 2 hrs throughout the test period (1800-1000 hrs). Oral temperature taken immediately after each performance test was elevated during the bright over the dim condition at 2130, 0130, 0330 hrs. Performance on tests for cognitive abilities was improved for the bright over the dim light condition particularly at 2400, 0200 and 0400 hrs. This effect did not endure after the bright light exposure ended. These data suggest that bright illumination may improve performance otherwise susceptible to fatigue, particularly during early morning shifts.
This study examined the effects of bright light exposure, as compared to dim light, on daytime subjective sleepiness, incidences of slow eye movements (SEMs), and psychomotor vigilance task (PVT) performance following 2 nights of sleep restriction. The study had a mixed factorial design with 2 independent variables: light condition (bright light, 1,000 lux; dim light, < 5 lux) and time of day. The dependent variables were subjective sleepiness, PVT performance, incidences of SEMs, and salivary melatonin levels. Sleep research laboratory at Monash University. Sixteen healthy adults (10 women and 6 men) aged 18 to 35 years (mean age 25 years, 3 months). Following 2 nights of sleep restriction (5 hours each night), participants were exposed to modified constant routine conditions. Eight participants were exposed to bright light from noon until 5:00 pm. Outside the bright light exposure period (9:00 am to noon, 5:00 pm to 9:00 pm) light levels were maintained at less than 5 lux. A second group of 8 participants served as controls for the bright light exposure and were exposed to dim light throughout the entire protocol. Bright light exposure reduced subjective sleepiness, decreased SEMs, and improved PVT performance compared to dim light. Bright lights had no effect on salivary melatonin. A significant positive correlation between PVT reaction times and subjective sleepiness was observed for both groups. Changes in SEMs did not correlate significantly with either subjective sleepiness or PVT performance. Daytime bright light exposure can reduce the impact of sleep loss on sleepiness levels and performance, as compared to dim light. These effects appear to be mediated by mechanisms that are separate from melatonin suppression. The results may assist in the development of treatments for daytime sleepiness.
This constant routine study (n=9 men) compared the phase delay of the circadian system induced by a single pulse of evening light (5000 lx at 2100–2400 h) in the presence or absence of exogenous melatonin (5 mg p.o. at 2040 h). On the treatment day, light and melatonin protracted and accelerated, respectively, the evening decline in core body temperature (CBT). Subjective sleepiness ratings showed parallel shifts, the earlier the decline in CBT, the sleepier. On the post-treatment day, light induced a phase delay in the mid-range crossing time of CBT decline independent of whether melatonin was co-administered or not. Subjective sleepiness was delayed in parallel. The phase delay of the circadian system by evening light appears to be independent of an immediate hyperthermic effect and is not mediated by melatonin.
The immediate psychophysiological and behavioral effects of photic stimulation on humans [bright light (BL) of 5K lux or dim light (DL) of 50 lux] were assessed in male subjects (N = 43) under four different conditions. For one condition the same subjects (N = 16) received alternating 90-min blocks of BL and DL during the nighttime h (2300-0800 h) under sustained wakefulness conditions. A second condition was similar to the first except that subjects (N = 8) received photic stimulation during the daytime hours. For the third and fourth conditions different subjects received either continuous BL (N = 10) or continuous DL (N = 9) during the nighttime hours. For the nighttime alternating condition body temperature decreased under DL but either increased or maintained under BL. For the continuous light condition, body temperature dropped sharply across the night under DL but dropped only slightly under BL. Sleepiness was considerably greater under DL than under BL, and the difference became larger as the night progressed. Similarly, alertness, measured by EEG beta activity, was greater under BL, and nighttime performance on behavioral tasks was also generally better. There were no differential effects between BL and DL on any measure during the daytime. These data indicate that light exerts a powerful, immediate effect on physiology and behavior in addition to its powerful influence on circadian organization.
Seven human subjects were exposed to bright light (BL, approx. 2500 lux) and dim light (DL, approx. 6 lux) during 3 h prior to nocturnal sleep, in a cross-over design. At the end of the BL exposure period core body temperature was significantly higher than at the end of the DL exposure period. The difference in core body temperature persisted during the first 4 h of sleep. The latency to sleep onset was increased after BL exposure. Rapid-eye movement sleep (REMS) and slow-wave sleep (SWS; stage 3 + 4 of non-REMS) were not significantly changed. Eight subjects were exposed to BL from 20.30 to 23.30 h while their eyes were covered or uncovered. During BL exposure with uncovered eyes, core body temperature decreased significantly less than during exposure with covered eyes. We conclude that bright light immediately affects core body temperature and that this effect is mediated via the eyes.