Effects of Artificial Dawn and Morning Blue Light on Daytime Cognitive Performance,Well-being,Cortisol and Melatonin Levels

Light exposure elicits numerous effects on human physiology and behavior, such as better cognitive performance and mood. Here we investigated the role of morning light exposure as a countermeasure for impaired cognitive performance and mood under sleep restriction (SR). Seventeen participants took part of a 48h laboratory protocol, during which three different light settings (separated by 2?wks) were administered each morning after two 6-h sleep restriction nights: a blue monochromatic LED (light-emitting diode) light condition (BL; 100?lux at 470?nm for 20?min) starting 2?h after scheduled wake-up time, a dawn-simulating light (DsL) starting 30?min before and ending 20?min after scheduled wake-up time (polychromatic light gradually increasing from 0 to 250?lux), and a dim light (DL) condition for 2?h beginning upon scheduled wake time (<8?lux). Cognitive tasks were performed every 2?h during scheduled wakefulness, and questionnaires were administered hourly to assess subjective sleepiness, mood, and well-being. Salivary melatonin and cortisol were collected throughout scheduled wakefulness in regular intervals, and the effects on melatonin were measured after only one light pulse. Following the first SR, analysis of the time course of cognitive performance during scheduled wakefulness indicated a decrease following DL, whereas it remained stable following BL and significantly improved after DsL. Cognitive performance levels during the second day after SR were not significantly affected by the different light conditions. However, after both SR nights, mood and well-being were significantly enhanced after exposure to morning DsL compared with DL and BL. Melatonin onset occurred earlier after morning BL exposure, than after morning DsL and DL, whereas salivary cortisol levels were higher at wake-up time after DsL compared with BL and DL. Our data indicate that exposure to an artificial morning dawn simulation light improves subjective well-being, mood, and cognitive performance, as compared with DL and BL, with minimal impact on circadian phase. Thus, DsL may provide an effective strategy for enhancing cognitive performance, well-being, and mood under mild sleep restriction.     

Suppression of nocturnal plasma melatonin and 6-sulphatoxymelatonin by bright and dim light in man

Previous studies have shown that bright light (2500 lux) suppresses nocturnal secretion of melatonin, while dim light (500 lux) has little or no effect. We have studied the effect of varying intensities of light on 5 normal male volunteers (age 18-28). The experiment was divided into 3 parts which took place at weekly intervals. Subjects remained under artificial light (fluorescent strip 150-250 lux) between 2000 h-2300 h, they then retired to bed in darkness. On each occasion, between 0030 h and 0100 h, the subjects were required to get up and were treated with light of different intensities; (a) less than 1 lux, (b) 300 lux and (c) 2500 lux respectively. Subjects returned to bed in darkness until 0700 h. Blood was sampled hourly from 2000 h-1000 h with additional samples at 2330 h, 0015 h, 0030 h, 0045 h, 0115 h and 0130 h. Plasma melatonin and 6-sulphatoxymelatonin (aMT6s), the major melatonin metabolite, were measured by radioimmunoassay. Dim (300 lux) and bright (2500 lux) light, both significantly suppressed melatonin levels compared to less than 1 lux (P less than 0.05 and P less than 0.01 respectively) at the following time points 0100 h, 0115 h and 0130 h. One subject did not show suppression with 300 lux. There was also a significant suppression of aMT6s levels, compared to less than 1 lux, after both 300 lux and 2500 lux at 0115 h (P less than 0.05, P less than 0.01), 0130 h (P less than 0.01, P less than 0.01) and 0200 h (P less than 0.01, P less than 0.001) respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Non-Visual Effects of Light on Melatonin,Alertness and Cognitive Performance: Can Blue-Enriched Light Keep Us Alert?

Background

Light exposure can cascade numerous effects on the human circadian process via the non-imaging forming system, whose spectral relevance is highest in the short-wavelength range. Here we investigated if commercially available compact fluorescent lamps with different colour temperatures can impact on alertness and cognitive performance.

Methods

Sixteen healthy young men were studied in a balanced cross-over design with light exposure of 3 different light settings (compact fluorescent lamps with light of 40 lux at 6500K and at 2500K and incandescent lamps of 40 lux at 3000K) during 2 h in the evening.

Results

Exposure to light at 6500K induced greater melatonin suppression, together with enhanced subjective alertness, well-being and visual comfort. With respect to cognitive performance, light at 6500K led to significantly faster reaction times in tasks associated with sustained attention (Psychomotor Vigilance and GO/NOGO Task), but not in tasks associated with executive function (Paced Visual Serial Addition Task). This cognitive improvement was strongly related with attenuated salivary melatonin levels, particularly for the light condition at 6500K.

Conclusions

Our findings suggest that the sensitivity of the human alerting and cognitive response to polychromatic light at levels as low as 40 lux, is blue-shifted relative to the three-cone visual photopic system. Thus, the selection of commercially available compact fluorescent lights with different colour temperatures significantly impacts on circadian physiology and cognitive performance at home and in the workplace.     

Thermoregulatory responses in humans during exercise after exposure to two different light intensities

The effects after exposure to two different light intensities (dim, 50 lx and bright, 5000 lx) on thermoregulatory responses during exercise in a climatic chamber (27 degrees C, 60% relative humidity) were studied in nine untrained female subjects, aged 19-22 years. The subjects were in either the dim or bright light intensities from 0600 hours to 1200 hours. They were then instructed to exercise on a cycle ergometer at an intensity of 60% maximal oxygen uptake from 1200 hours to 1300 hours in a light intensity of 500 Ix. The main results can be summarized as follows. Firstly, exercise-induced increases of core temperature were significantly smaller, after exposure to the bright than after the dim light intensities, although both tests were performed in the same light intensity. Secondly, body mass loss after exercise was significantly greater after exposure to the bright light intensity. Thirdly, an increase in salivary lactic acid during exercise was significantly lower after the bright intensity. Fourthly although the salivary melatonin level was not different between the two light intensities both before and after the exercise, it increased significantly during exercise only after the bright intensity. These results are discussed in terms of the establishment of a lower set-point in the core temperature after exposure to a bright light intensity.     

BLUE LIGHT Exposure Reduces Objective Measures of Sleepiness during Prolonged Nighttime Performance Testing

This study examined the effects of nocturnal exposure to dim, narrowband blue light (460 nm, ～1 lux, 2 µW/cm²), compared to dim broad spectrum (white) ambient light (～0.2 lux, 0.5 µW/cm²), on subjective and objective indices of sleepiness during prolonged nighttime performance testing. Participants were also exposed to a red light (640 nm, ～1 lux, 0.7µW/cm²) placebo condition. Outcome measures were driving simulator and psychomotor vigilance task (PVT) performance, subjective sleepiness, salivary melatonin, and electroencephalographic (EEG) activity. The study had a repeated-measures design, with three counterbalanced light conditions and a four-week washout period between each condition. Participants (n?=?8) maintained a regular sleep-wake schedule for 14 days prior to the ～14 h laboratory study, which consisted of habituation to light conditions followed by neurobehavioral performance testing from 21:00 to 08:30 h under modified constant-routine conditions. A neurobehavioral test battery (2.5 h) was presented four times between 21:00 and 08:30 h, with a 30 min break between each. From 23:30 to 05:30 h, participants were exposed to blue or red light, or remained in ambient conditions. Compared to ambient light exposure, blue light exposure suppressed EEG slow wave delta (1.0–4.5 Hz) and theta (4.5–8 Hz) activity and reduced the incidence of slow eye movements. PVT reaction times were significantly faster in the blue light condition, but driving simulator measures, subjective sleepiness, and salivary melatonin levels were not significantly affected by blue light. Red light exposure, as compared to ambient light exposure, reduced the incidence of slow eye movements. The results demonstrate that low-intensity, blue light exposure can promote alertness, as measured by some of the objective indices used in this study, during prolonged nighttime performance testing. Low intensity, blue light exposure has the potential to be applied to situations where it is desirable to increase alertness but not practical or appropriate to use bright light, such as certain occupational settings.     

Effectiveness of a red-visor cap for preventing light-induced melatonin suppression during simulated night work

Bright light at night improves the alertness of night workers. Melatonin suppression induced by light at night is, however, reported to be a possible risk factor for breast cancer. Short-wavelength light has a strong impact on melatonin suppression. A red-visor cap can cut the short-wavelength light from the upper visual field selectively with no adverse effects on visibility. The purpose of this study was to investigate the effects of a red-visor cap on light-induced melatonin suppression, performance, and sleepiness at night. Eleven healthy young male adults (mean age: 21.2±0.9 yr) volunteered to participate in this study. On the first day, the subjects spent time in dim light (<15 lx) from 20:00 to 03:00 to measure baseline data of nocturnal salivary melatonin concentration. On the second day, the subjects were exposed to light for four hours from 23:00 to 03:00 with a nonvisor cap (500 lx), red-visor cap (approx. 160 lx) and blue-visor cap (approx. 160 lx). Subjective sleepiness and performance of a psychomotor vigilance task (PVT) were also measured on the second day. Compared to salivary melatonin concentration under dim light, the decrease in melatonin concentration was significant in a nonvisor cap condition but was not significant in a red-visor cap condition. The percentages of melatonin suppression in the nonvisor cap and red-visor cap conditions at 4 hours after exposure to light were 52.6±22.4% and 7.7±3.3%, respectively. The red-visor cap had no adverse effect on performance of the PVT, brightness and visual comfort, though it tended to increase subjective sleepiness. These results suggest that a red-visor cap is effective in preventing melatonin suppression with no adverse effects on vigilance performance, brightness and visibility.     

Quantal melatonin suppression by exposure to low intensity light in man

Plasma melatonin concentrations were examined following three relatively low intensities of artificial light. Six normal, healthy control subjects were all exposed to (a) 200 lux, (b) 400 lux and (c) 600 lux for a three hour duration from midnight to 0300 h. Blood was also collected on a control night where light intensity was less than 10 lux throughout. Significant suppression of melatonin was observed following light of 400 lux and 600 lux intensity when compared to the control night (p less than 0.05; Mann-Whitney U-test). 200 lux light did not produce a statistically significant melatonin suppression when compared with control samples. Each light intensity produced its own individual maximal melatonin suppression by one hour of exposure. Increased duration of exposure to the light had no further influence on melatonin plasma concentrations. These data confirm a dose response relationship between light and melatonin suppression, and indicate that there is no reciprocal relationship between the effects of light intensity and the duration of exposure on maximal melatonin suppression in man.     

Short-wavelength attenuated polychromatic white light during work at night: limited melatonin suppression without substantial decline of alertness

Exposure to light at night increases alertness, but light at night (especially short-wavelength light) also disrupts nocturnal physiology. Such disruption is thought to underlie medical problems for which shiftworkers have increased risk. In 33 male subjects we investigated whether short-wavelength attenuated polychromatic white light (<530?nm filtered out) at night preserves dim light melatonin levels and whether it induces similar skin temperature, alertness, and performance levels as under full-spectrum light. All 33 subjects participated in random order during three nights (at least 1 wk apart) either under dim light (3 lux), short-wavelength attenuated polychromatic white light (193 lux), or full-spectrum light (256 lux). Hourly saliva samples for melatonin analysis were collected along with continuous measurements of skin temperature. Subjective sleepiness and activation were assessed via repeated questionnaires and performance was assessed by the accuracy and speed of an addition task. Our results show that short-wavelength attenuated polychromatic white light only marginally (6%) suppressed salivary melatonin. Average distal-to-proximal skin temperature gradient (DPG) and its pattern over time remained similar under short-wavelength attenuated polychromatic white light compared with dim light. Subjects performed equally well on an addition task under short-wavelength attenuated polychromatic white light compared with full-spectrum light. Although subjective ratings of activation were lower under short-wavelength attenuated polychromatic white light compared with full-spectrum light, subjective sleepiness was not increased. Short-wavelength attenuated polychromatic white light at night has some advantages over bright light. It hardly suppresses melatonin concentrations, whereas performance is similar to the bright light condition. Yet, alertness is slightly reduced as compared with bright light, and DPG shows similarity to the dim light condition, which is a physiological sign of reduced alertness. Short-wavelength attenuated polychromatic white light might therefore not be advisable in work settings that require high levels of alertness. (Author correspondence: maan.van.de.werken@gmail.com)     

The Effect on Body Temperature and Melatonin of A 39-H Constant Routine with Two Different Light Levels at Nighttime

Eight healthy subjects were studied during 39-h spans (from 07:00 on one day until 22:00 the second) in which they remained awake. During one experiment, subjects were exposed to 100 lux of light between 18:00 and 8:00, and during a second experiment, they were exposed to 1000 lux during the same time span. Throughout the daytime period, they were exposed to normal daylight (1500 lux or more). The nighttime 1000-lux light treatment suppressed the melatonin metabolite aMT6s, while the 100 lux treatment did not. On the treatment day, the 1000 lux, in comparison to the 100 lux, light treatment resulted in both an elevated temperature minimum and a delay in its clock-time occurrence overnight. No real circadian phase shift in the temperature, urinary melatonin, or Cortisol rhythms was detected after light treatment. This study confirmed that nocturnal exposure to lower light intensities is capable of modifying circadian variables more than previously estimated. The immediate effects of all-night light treatment are essentially not different from those of evening light. This may be important if bright light is used to improve alertness of night workers. Whether subsequent daytime alertness and sleep recovery are affected by the protocol used in our study remains to be determined.     

Interactions between light and melatonin on the circadian clock of mice.

Melatonin and light synchronize the biological clock and are used to treat sleep/wake disturbances in humans. However, the two treatments affect circadian rhythms differently when they are combined than when they are administered individually. To elucidate the nature of the interaction between melatonin and light, the present study assessed the effect of melatonin on circadian timing and immediate-early gene expression in the suprachiasmatic nucleus (SCN) when administered in the presence of light. Male C3H/HeN mice, housed in constant dark in cages equipped with running wheels, were treated with either melatonin (90 microg, s.c.) or vehicle (3% ethanol-saline) 5 min prior to exposure to light (15 min, 300 lux) at various times in the circadian cycle. Combined treatment resulted in lower magnitude phase delays of circadian activity rhythms than those obtained with light alone during the early subjective night and advances in phase when melatonin and light were administered during the subjective day (p < .001). The reduction in phase delays with combined treatment at Circadian Time (CT) 14 was significant when light exposure measured 300 lux but not at lower light levels (p < .05). When light preceded melatonin administration, the inhibition of phase delays attained significance only when the light exposure reached 1000 lux (p < .05). Neither basal nor light-induced expression of c-fos mRNA in the SCN was modified by melatonin administration at CT 14 or CT 22. Together, these results suggest that combined administration of melatonin and light affect circadian timing in a manner not predicted by summing the two treatments given individually. Furthermore, the interaction is not likely to be due to inhibition of photic input to the clock by melatonin but might arise from a photically induced enhancement of melatonin's actions on circadian timing.     

The Use of a Light Visor During Night Work by Nurses

Five nurses have been investigated for the two nights of a rapidly rotating shift schedule on four occasions: once with normal ward lighting (40 lux on average) and three times while wearing a light visor (Bio-Brite Inc., MD, USA). The visor was worn for four periods of 40 minutes each, at about 2 hour intervals during each night shift, the intensity giving 400-600, 1500 and 3200 lux for the three studies. The nurses recorded subjective evaluations of mood, physical fitness, sleepiness and fatigue, and carried out some performance measures (Simple Auditory Reaction Time, Flicker Fusion Frequency, Search and Memory test) at the start, middle and end of each night shift. Plasma cortisol was measured at the end of the shift, and 6-sulphatoxymelatonin was measured in urine collected at the middle and end of each shift. Oral temperature was also recorded for 48 h covering the two shifts. No significant effects of light treatment (even at 3200 lux) upon within-shift decline in mood and performance were seen. The acceptability of the wearer of the visor was moderate since the upper visual field was impaired and, at the highest light intensity, there was difficulty in seeing clearly objects in the dimly-lit environment. Furthermore, no significant falls in melatonin excretion and cortisol excretion were noted, but there was some evidence that the circadian rhythm of oral temperature was stabilised by the light visors. This is thoroughly desirable in rapidly rotating shift systems.     

The Use of a Light Visor During Night Work by Nurses

Five nurses have been investigated for the two nights of a rapidly rotating shift schedule on four occasions: once with normal ward lighting (40 lux on average) and three times while wearing a light visor (Bio-Brite Inc., MD, USA). The visor was worn for four periods of 40 minutes each, at about 2 hour intervals during each night shift, the intensity giving 400-600, 1500 and 3200 lux for the three studies. The nurses recorded subjective evaluations of mood, physical fitness, sleepiness and fatigue, and carried out some performance measures (Simple Auditory Reaction Time, Flicker Fusion Frequency, Search and Memory test) at the start, middle and end of each night shift. Plasma cortisol was measured at the end of the shift, and 6-sulphatoxymelatonin was measured in urine collected at the middle and end of each shift. Oral temperature was also recorded for 48 h covering the two shifts. No significant effects of light treatment (even at 3200 lux) upon within-shift decline in mood and performance were seen. The acceptability of the wearer of the visor was moderate since the upper visual field was impaired and, at the highest light intensity, there was difficulty in seeing clearly objects in the dimly-lit environment. Furthermore, no significant falls in melatonin excretion and cortisol excretion were noted, but there was some evidence that the circadian rhythm of oral temperature was stabilised by the light visors. This is thoroughly desirable in rapidly rotating shift systems.     

Nasal versus temporal illumination of the human retina: effects on core body temperature, melatonin, and circadian phase

The mammalian retina contains both visual and circadian photoreceptors. In humans, nocturnal stimulation of the latter receptors leads to melatonin suppression, which might cause reduced nighttime sleepiness. Melatonin suppression is maximal when the nasal part of the retina is illuminated. Whether circadian phase shifting in humans is due to the same photoreceptors is not known. The authors explore whether phase shifts and melatonin suppression depend on the same retinal area. Twelve healthy subjects participated in a within-subjects design and received all of 3 light conditions--1) 10 lux of dim light on the whole retina, 2) 100 lux of ocular light on the nasal part of the retina, and 3) 100 lux of ocular light on the temporal part of the retina--on separate nights in random order. In all 3 conditions, pupils were dilated before and during light exposure. The protocol consisted of an adaptation night followed by a 23-h period of sustained wakefulness, during which a 4-h light pulse was presented at a time when maximal phase delays were expected. Nasal illumination resulted in an immediate suppression of melatonin but had no effect on subjective sleepiness or core body temperature (CBT). Nasal illumination delayed the subsequent melatonin rhythm by 78 min, which is significantly (p= 0.016) more than the delay drift in the dim-light condition (38 min), but had no detectable phase-shifting effect on the CBT rhythm. Temporal illumination suppressed melatonin less than the nasal illumination and had no effect on subjective sleepiness and CBT. Temporal illumination delayed neither the melatonin rhythm nor the CBT rhythm. The data show that the suppression of melatonin does not necessarily result in a reduction of subjective sleepiness and an elevation ofCBT. In addition, 100 lux of bright white light is strong enough to affect the photoreceptors responsible for the suppression of melatonin but not strong enough to have a significant effect on sleepiness and CBT. This may be due to the larger variability of the latter variables.     

Jet lag: minimizing it's effects with critically timed bright light and melatonin administration

The symptoms of jet lag cause distress to an increasing number of travelers. Potentially they may impair sleep, mood and cognitive performance. Critically timed exposure to bright light and melatonin administration can help to reduce symptoms. Bright light is one of the most powerful synchronizers of human rhythms and melatonin serves as a "dark pulse" helping to induce nighttime behaviors. Thus, enhancing day and night signals to the brain, appropriate to the environmental light/dark cycle of the new time zone, can serve to reestablish adaptive timing relationships between the body's internal biological rhythms and the external environment, and thereby reduce the symptoms of jet lag. Specific recommendations using bright light and melatonin for eastward and westward travel before and after departure are provided for time zone changes of up to 6, 7-9 and 10 or more hours.     

Variations in the light-induced suppression of nocturnal melatonin with special reference to variations in the pupillary light reflex in humans

The purpose of the present study was to elucidate the existence of individual differences of pupil response to light stimulation, and to confirm the reproducibility of this phenomenon. Furthermore, the relationship between the individual differences in nocturnal melatonin suppression induced by lighting and the individual differences of pupillary light response (PLR) was examined. The pupil diameter and salivary melatonin content of 20 male students were measured at the same period of time (00:00-02:30 hr) on different days, accordingly. Illumination (530 nm) produced by a monochromatic light-emitting diode (LED) was employed as the light stimulation: pupil diameter was measured with 4 different levels of illuminance of 1, 3, 30 and 600 lux and melatonin levels were measured at 30 and 600 lux (respective controls were taken at 0 lux). Oral temperature, blood pressure and subjective index of sleepiness were taken in experiments where melatonin levels were measured. Changes of the pupil diameter in response to light were expressed as PLR and light-induced melatonin suppression was expressed as a control-adjusted melatonin suppression score (control-adjusted MSS), which was compared to the melatonin level measured at 0 lux. In the PLR, the coefficients of variation obtained at 30 lux or less were large (51.5, 45.0, 28.4 and 6.2% at 1, 3, 30 and 600 lux, respectively). Correlations of illuminance of any combination at 30 lux or less were statistically significant at less than 1% level (1 vs. 3 lux: r=0.68; 1 vs. 30 lux: r=0.64; 3 vs. 30 lux: r=0.73), which showed the reproducibility of individual differences. The control-adjusted MSS at 600 lux (-1.14+/-1.16) was significantly (p<0.05) lower than that registered at 30 lux (-0.22+/-2.12). PLR values measured at 30 and 600 lux were then correlated with control-adjusted MSS; neither indicated a significant linear relationship. However, the control-adjusted MSS showed around 0 under any of the illuminance conditions in subjects with high PLR. In control-adjusted MSS of low values (i.e., melatonin secretions were easily suppressed), subjects indicated typically low PLR. In subjects with low control-adjusted MSS (n=3), characteristic changes in the autonomic nervous system, such as body temperature and blood pressure, were noted in subjects exposed to low illuminance of 30 lux. The fact that the relationship between PLR and control-adjusted MSS portray a similar pattern even under different luminance conditions suggests that MSS may not be affected in those with high PLR at low illuminance, regardless of the illuminance condition.     

The Test Re-Test Reliability of the Melatonin Suppression by White Light

The melatonin supersensitivity to light has been suggested as a biological marker of bipolar disorder. However previous studies have been inconsistent with regard to light induced suppression of melatonin and raising questions regarding its reproducibility and reliability. The present study examined the test re-test reliability of the melatonin suppression by light in healthy subjects. Study was divided into two parts. The first examined the melatonin suppression by 200 lux of light while the second examined effect 500 lux of light. Subjects were tested twice, separated by one week for each part of the study. On each night subjects reported to the study at 1800 h. The first sample was collected at 2100 h (in the light). Subjects were then placed in a dark room, with a background light intensity of 10–20 lux. Further blood samples were collected at regular intervals. After each collection, blood samples were centrifuged and plasma separated and stored frozen at –20ºC. Plasma melatonin concentrations were determined by a specific radioimmunoassay. Results showed poor test re-test reliability for nights 1 and 2 for both light intensities suggesting that the melatonin suppression by light is not reproducible and has poor reliability. The poor test re-test reliability may provide an explanation for the inconsistencies in previous studies.     

The Test Re-Test Reliability of the Melatonin Suppression by White Light

The melatonin supersensitivity to light has been suggested as a biological marker of bipolar disorder. However previous studies have been inconsistent with regard to light induced suppression of melatonin and raising questions regarding its reproducibility and reliability. The present study examined the test re-test reliability of the melatonin suppression by light in healthy subjects. Study was divided into two parts. The first examined the melatonin suppression by 200 lux of light while the second examined effect 500 lux of light. Subjects were tested twice, separated by one week for each part of the study. On each night subjects reported to the study at 1800 h. The first sample was collected at 2100 h (in the light). Subjects were then placed in a dark room, with a background light intensity of 10-20 lux. Further blood samples were collected at regular intervals. After each collection, blood samples were centrifuged and plasma separated and stored frozen at -20ºC. Plasma melatonin concentrations were determined by a specific radioimmunoassay. Results showed poor test re-test reliability for nights 1 and 2 for both light intensities suggesting that the melatonin suppression by light is not reproducible and has poor reliability. The poor test re-test reliability may provide an explanation for the inconsistencies in previous studies.     

Melatonin suppression during a simulated night shift in medium intensity light is increased by 10-minute breaks in dim light and decreased by 10-minute breaks in bright light

ABSTRACT

Exposure to light at night results in disruption of endogenous circadian rhythmicity and/or suppression of pineal melatonin, which can consequently lead to acute or chronic adverse health problems. In the present study, we investigated whether exposure to very dim light or very bright light for a short duration influences melatonin suppression, subjective sleepiness, and performance during exposure to constant moderately bright light. Twenty-four healthy male university students were divided into two experimental groups: Half of them (mean age: 20.0 ± 0.9 years) participated in an experiment for short-duration (10 min) light conditions of medium intensity light (430 lx, medium breaks) vs. very dim light (< 1 lx, dim breaks) and the other half (mean age: 21.3 ± 2.5 years) participated in an experiment for short-duration light conditions of medium intensity light (430 lx, medium breaks) vs. very bright light (4700 lx, bright breaks). Each simulated night shift consisting of 5 sets (each including 50-minute night work and 10-minute break) was performed from 01:00 to 06:00 h. The subjects were exposed to medium intensity light (550 lx) during the night work. Each 10-minute break was conducted every hour from 02:00 to 06:00 h. Salivary melatonin concentrations were measured, subjective sleepiness was assessed, the psychomotor vigilance task was performed at hourly intervals from 21:00 h until the end of the experiment. Compared to melatonin suppression between 04:00 and 06:00 h in the condition of medium breaks, the condition of dim breaks significantly promoted melatonin suppression and the condition of bright breaks significantly diminished melatonin suppression. However, there was no remarkable effect of either dim breaks or bright breaks on subjective sleepiness and performance of the psychomotor vigilance task. Our findings suggest that periodic exposure to light for short durations during exposure to a constant light environment affects the sensitivity of pineal melatonin to constant light depending on the difference between light intensities in the two light conditions (i.e., short light exposure vs. constant light exposure). Also, our findings indicate that exposure to light of various intensities at night could be a factor influencing the light-induced melatonin suppression in real night work settings.     

Human nonvisual responses to simultaneous presentation of blue and red monochromatic light

Blue light sensitivity of melatonin suppression and subjective mood and alertness responses in humans is recognized as being melanopsin based. Observations that long-wavelength (red) light can potentiate responses to subsequent short-wavelength (blue) light have been attributed to the bistable nature of melanopsin whereby it forms stable associations with both 11-cis and all-trans isoforms of retinaldehyde and uses light to transition between these states. The current study examined the effect of concurrent administration of blue and red monochromatic light, as would occur in real-world white light, on acute melatonin suppression and subjective mood and alertness responses in humans. Young healthy men (18-35 years; n = 21) were studied in highly controlled laboratory sessions that included an individually timed 30-min light stimulus of blue (λ(max) 479 nm) or red (λ(max) 627 nm) monochromatic light at varying intensities (10(13)-10(14) photons/cm(2)/sec) presented, either alone or in combination, in a within-subject randomized design. Plasma melatonin levels and subjective mood and alertness were assessed at regular intervals relative to the light stimulus. Subjective alertness levels were elevated after light onset irrespective of light wavelength or irradiance. For melatonin suppression, a significant irradiance response was observed with blue light. Co-administration of red light, at any of the irradiances tested, did not significantly alter the response to blue light alone. Under the current experimental conditions, the primary determinant of the melatonin suppression response was the irradiance of blue 479 nm light, and this was unaffected by simultaneous red light administration.     

Relationship between individual difference in melatonin suppression by light and habitual bedtime

The purpose of this study was to determine the relationship between individual difference in melatonin suppression by exposure to light and habitual bedtime. Seventeen healthy male students (mean age: 22.6+/-2.4 yr) volunteered to participate in the study. The subjects were exposed to light (1000 lx) for 2 hours from 2 hours before the time of peak salivary melatonin concentration. Two hours after exposure to the light, melatonin suppression had occurred in fifteen subjects. No significant correlation was found between the rate of melatonin suppression and habitual bedtime in the fifteen subjects in whom melatonin suppression occurred. However, the habitual bedtime of the two subjects in whom melatonin suppression did not occur was earlier than that of the other subjects. These results suggest that there are some people with very low sensitivity to light and that this may affect habitual bedtime.