Effects of different light intensities in the morning on dim light melatonin onset

The present study evaluated the effects of exposure to light intensity in the morning on dim light melatonin onset (DLMO). The tested light intensities were 750 lux, 150 lux, 3000 lux, 6000 lux and 12,000 lux (horizontal illuminance at cornea), using commercial 5000 K fluorescent lamps. Eleven healthy males aged 21-31 participated in 2-day experiments for each light condition. On the first experimental day (day 1), subjects were exposed to dim light (<30 lux) for 3 h in the morning (09:00-12:00). On the same day, saliva samples were taken in dim light (<30 lux) every 30 min from 21:00 to 01:00 to determine the DLMO phase. The subjects were allowed to sleep from 01:00 to 08:00. On the second experimental day (day 2), the subjects were exposed to experimental light conditions for 3 h in the morning. The experimental schedule after light exposure was the same as on day 1. On comparing day 2 with day 1, significant phase advances of DLMO were obtained at 3000 lux, 6000 lux and 12,000 lux. These findings indicate that exposure to a necessary intensity from an ordinary light source, such as a fluorescent lamp, in the morning within one day affects melatonin secretion.     

Effects of dim or bright-light exposure during the daytime on human gastrointestinal activity

On the basis of our previous findings that bright-light exposure during the daytime has profound influence on physiological parameters such as melatonin secretion and tympanic temperature in humans, we proposed the hypothesis that bright vs. dim light-exposure during the daytime has a different influence on the activity of the digestive system via the endocrine and/or autonomic nervous system. To examine this hypothesis, we conducted a series of counterbalanced experiments in which subjects stayed the daytime (7:00 to 15:00h) under either a dim (80 lux) or bright (5,000 lux) light condition. We measured gastrointestinal activity using a breath hydrogen (indicative of carbohydrate malabsorption) and an electrogastrography (EGG, indicative of gastric myoelectric activity) test. The results showed the postprandial breath hydrogen excretion during the following nighttime period after daytime exposure to the dim-light condition was significantly higher than under the bright-light condition (p < 0.05). In addition, the spectrum total power of the EGG recorded after taking the evening meal was significantly lower for the dim than bright-light condition (p < 0.05). These results support our hypothesis and indicate that dim-light exposure during the daytime suppresses the digestion of the evening meal, resulting in malabsorption of dietary carbohydrates in it.     

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.     

Time-of-day-dependent effects of bright light exposure on human psychophysiology: comparison of daytime and nighttime exposure

Bright light can influence human psychophysiology instantaneously by inducing endocrine (suppression of melatonin, increasing cortisol levels), other physiological changes (enhancement of core body temperature), and psychological changes (reduction of sleepiness, increase of alertness). Its broad range of action is reflected in the wide field of applications, ranging from optimizing a work environment to treating depressed patients. For optimally applying bright light and understanding its mechanism, it is crucial to know whether its effects depend on the time of day. In this paper, we report the effects of bright light given at two different times of day on psychological and physiological parameters. Twenty-four subjects participated in two experiments (n = 12 each). All subjects were nonsmoking, healthy young males (18-30 yr). In both experiments, subjects were exposed to either bright light (5,000 lux) or dim light <10 lux (control condition) either between 12:00 P.M. and 4:00 P.M. (experiment A) or between midnight and 4:00 A.M. (experiment B). Hourly measurements included salivary cortisol concentrations, electrocardiogram, sleepiness (Karolinska Sleepiness Scale), fatigue, and energy ratings (Visual Analog Scale). Core body temperature was measured continuously throughout the experiments. Bright light had a time-dependent effect on heart rate and core body temperature; i.e., bright light exposure at night, but not in daytime, increased heart rate and enhanced core body temperature. It had no significant effect at all on cortisol. The effect of bright light on the psychological variables was time independent, since nighttime and daytime bright light reduced sleepiness and fatigue significantly and similarly.     

Effect of Evening Exposure to Dim or Bright Light on the Digestion of Carbohydrate in the Supper Meal

In a previous study we found that daytime exposure to bright as compared to dim light exerted a beneficial effect on the digestion of the evening meal. This finding prompted us to examine whether the digestion of the evening meal is also affected by evening light intensity. Subjects lived in light of 200 lux during the daytime (08:00–17:00 h) and took their evening meal at 17:00 h under 20 lux (evening dim‐light condition: 17:00–02:00 h) or 2000 lux (evening bright‐light condition: 17:00–02:00 h) until retiring at 02:00 h. Assessment of carbohydrate digestion of the evening meal was accomplished by a breath hydrogen test that is indicative of the malabsorption of dietary carbohydrate. Hydrogen excretion in the breath in the evening under the dim‐light condition was significantly less than under the bright‐light condition (p < 0.05). This finding is the opposite to that obtained in previous experiments in which subjects were exposed to the different intensities of light during the daytime, and indicates that the exposure to dim light in the evening exerts a better effect on carbohydrate digestion in the evening meal than does the exposure to bright light.     

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.     

Salivary Secretion under the Influence of Bright/Dim Light Exposure in the Morning and Evening in Humans

Recent studies show that bright and dim light intensities during the daytime have important regulatory functions. Our present study was performed to evaluate the effect of exposure to different light intensities during the morning and evening on salivary secretion and its sodium concentration. The study involved 6 healthy, female volunteers who were exposed to dim light (100 lx) from 7:00 to 17:00 and to bright light (3000 lx) from 17:00 to 23:00 one day, and to bright light (3000 lx) from 7:00 to 17:00 and dim light (100 lx) from 17:00 to 23:00 on the next day. We collected salivary samples every 10 minutes during 2 hours in the morning and in the evening by means of a Lashley cup. Saliva secretion was stimulated by sweet candy. The amount of saliva secreted was significantly greater in the morning under bright light exposure, while it was significantly greater in the evening under dim light exposure. We discuss these findings in terms of changes in activity of the parasympathetic nervous system (PNS) and sympathetic (SNS) nervous system produced by exposure to different light intensities at different times of the day.     

The Influence of the Bright Light Exposure during Daytime on Melatonin Excreting Rate in Urine

The present study was conducted to know the effects of different light intensities exposed during daytime for several hours on melatonin excreting rate in urine and tympanic temperature. Eleven healthy female subjects were exposed to bright light of 6000 lx (Bright) or dim light of 100 lx (Dim) during daytime from 09:00 h to 13:30 h, and then the light condition was kept at 100 lx until the end of test at 14:30 h. The urinary samples were collected from 10:00 h to 14:30 h every 1.5 hours, and melatonin excreting rate in urine was measured by enzyme immunoassay. Melatonin excreting rate in urine was significantly higher in Bright than in Dim at 11:30 h and 14:30 h, and not significant but at high level at 13:00 h (p &lt;0.07). Moreover, average tympanic temperatures were significantly lower in Bright than in Dim from 11:43 h to 14:30 h. These results showed that the bright light exposure during daytime could reduce tympanic temperature, which might result from the increase of melatonin level.     

A compromise phase position for permanent night shift workers: circadian phase after two night shifts with scheduled sleep and light/dark exposure

Night shift work is associated with a myriad of health and safety risks. Phase-shifting the circadian clock such that it is more aligned with night work and day sleep is one way to attenuate these risks. However, workers will not be satisfied with complete adaptation to night work if it leaves them misaligned during days off. Therefore, the goal of this set of studies is to produce a compromise phase position in which individuals working night shifts delay their circadian clocks to a position that is more compatible with nighttime work and daytime sleep yet is not incompatible with late nighttime sleep on days off. This is the first in the set of studies describing the magnitude of circadian phase delays that occurs on progressively later days within a series of night shifts interspersed with days off. The series will be ended on various days in order to take a "snapshot" of circadian phase. In this set of studies, subjects sleep from 23:00 to 7:00 h for three weeks. Following this baseline period, there is a series of night shifts (23:00 to 07:00 h) and days off. Experimental subjects receive five 15 min intermittent bright light pulses (approximately 3500 lux; approximately 1100 microW/cm2) once per hour during the night shifts, wear sunglasses that attenuate all visible wavelengths--especially short wavelengths ("blue-blockers")--while traveling home after the shifts, and sleep in the dark (08:30-15:30 h) after each night shift. Control subjects remain in typical dim room light (<50 lux) throughout the night shift, wear sunglasses that do not attenuate as much light, and sleep whenever they want after the night shifts. Circadian phase is determined from the circadian rhythm of melatonin collected during a dim light phase assessment at the beginning and end of each study. The sleepiest time of day, approximated by the body temperature minimum (Tmin), is estimated by adding 7 h to the dim light melatonin onset. In this first study, circadian phase was measured after two night shifts and day sleep periods. The Tmin of the experimental subjects (n=11) was 04:24+/-0.8 h (mean+/-SD) at baseline and 7:36+/-1.4 h after the night shifts. Thus, after two night shifts, the Tmin had not yet delayed into the daytime sleep period, which began at 08:30 h. The Tmin of the control subjects (n=12) was 04:00+/-1.2 h at baseline and drifted to 4:36+/-1.4 h after the night shifts. Thus, two night shifts with a practical pattern of intermittent bright light, the wearing of sunglasses on the way home from night shifts, and a regular sleep period early in the daytime, phase delayed the circadian clock toward the desired compromise phase position for permanent night shift workers. Additional night shifts with bright light pulses and daytime sleep in the dark are expected to displace the sleepiest time of day into the daytime sleep period, improving both nighttime alertness and daytime sleep but not precluding adequate sleep on days off.     

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)     

Effects of tryptophan-rich breakfast and light exposure during the daytime on melatonin secretion at night

Background

The purpose of the present study is to investigate effects of tryptophan intake and light exposure on melatonin secretion and sleep by modifying tryptophan ingestion at breakfast and light exposure during the daytime, and measuring sleep quality (by using actigraphy and the OSA sleep inventory) and melatonin secretion at night.

Methods

Thirty three male University students (mean ± SD age: 22 ± 3.1 years) completed the experiments lasting 5 days and 4 nights. The subjects were randomly divided into four groups: Poor*Dim (n = 10), meaning a tryptophan-poor breakfast (55 mg/meal) in the morning and dim light environment (<50 lx) during the daytime; Rich*Dim (n = 7), tryptophan-rich breakfast (476 mg/meal) and dim light environment; Poor*Bright (n = 9), tryptophan-poor breakfast and bright light environment (>5,000 lx); and Rich*Bright (n = 7), tryptophan-rich breakfast and bright light.

Results

Saliva melatonin concentrations on the fourth day were significantly lower than on the first day in the Poor*Dim group, whereas they were higher on the fourth day in the Rich*Bright group. Creatinine-adjusted melatonin in urine showed the same direction as saliva melatonin concentrations. These results indicate that the combination of a tryptophan-rich breakfast and bright light exposure during the daytime could promote melatonin secretion at night; further, the observations that the Rich*Bright group had higher melatonin concentrations than the Rich*Dim group, despite no significant differences being observed between the Poor*Dim and Rich*Dim groups nor the Poor*Bright and Rich*Bright groups, suggest that bright light exposure in the daytime is an important contributor to raised melatonin levels in the evening.

Conclusions

This study is the first to report the quantitative effects of changed tryptophan intake at breakfast combined with daytime light exposure on melatonin secretion and sleep quality. Evening saliva melatonin secretion changed significantly and indicated that a tryptophan-rich breakfast and bright light exposure during the daytime promoted melatonin secretion at this time.     

Effect of evening exposure to dim or bright light on the digestion of carbohydrate in the supper meal

In a previous study we found that daytime exposure to bright as compared to dim light exerted a beneficial effect on the digestion of the evening meal. This finding prompted us to examine whether the digestion of the evening meal is also affected by evening light intensity. Subjects lived in light of 200 lux during the daytime (08:00-17:00 h) and took their evening meal at 17:00 h under 20 lux (evening dim-light condition: 17:00-02:00 h) or 2000 lux (evening bright-light condition: 17:00-02:00 h) until retiring at 02:00 h. Assessment of carbohydrate digestion of the evening meal was accomplished by a breath hydrogen test that is indicative of the malabsorption of dietary carbohydrate. Hydrogen excretion in the breath in the evening under the dim-light condition was significantly less than under the bright-light condition (p < 0.05). This finding is the opposite to that obtained in previous experiments in which subjects were exposed to the different intensities of light during the daytime, and indicates that the exposure to dim light in the evening exerts a better effect on carbohydrate digestion in the evening meal than does the exposure to bright light.     

Influence of bright light exposure for several hours during the daytime on cutaneous vasodilatation and local sweating induced by an exercise heat load

The aim of the present study was to investigate the effect of exposure to differing light intensities for several hours during the daytime on the cutaneous vasodilatation and local forearm sweat rate induced by exercise. Seven healthy female subjects were exposed to bright light of 6000 lux (bright) or dim light of 100 lux (dim) during the daytime between 0900 hours to 1330 hours, followed by exposure to 150 lux until the test was over at 1600 hours. They spent their time in neutral conditions (29°C, 40% relative humidity) from 0900 hours to 1500 hours, and then exercised on a cycle ergometer for 30 min at 50% maximal physical work capacity. Average tympanic temparatures were significantly lower in bright than in dim from 1133 hours to 1430 hours. The onset of cutaneous vasodilatation and local forearm sweating occurred at significantly lower tympanic temperature (T _ty) during exercise after bright than after dim. After exercise, the cessation of forearm sweating and the rapid change of skin blood flow occurred at significantly lower T _ty after bright than after dim. It was concluded that exposure to bright light over several hours during the daytime could reduce T _ty and shift the threshold T _ty for cutaneous vasodilatation and forearm sweating to a lower level. Accepted: 30 March 1998     

BIOLOGICAL RHYTHM RESEARCH,Circadian rhythms,monoamines and behaviorTitlepage and Contents

A spectral analysis of heart rate was carried out on 11 young female adults in order to evaluate the effects of bright light exposure on autonomic nervous activity. Bright light (5,000 lx) was provided by fluorescent lamps during the daytime (07:00–15:00) on day 1. Dim light (200 lx) was given on day 2. High frequency components (HF: 0.15–0.4Hz) were used as a marker of parasympathetic activity and the ratio of low frequency (LF: 0.04–0.15 HZ) to high frequency (LF/HF) as an indicator of sympathetic activity. The average value during the sleep period (23:30–06:30) was compared following diurnal exposure to bright or dim light. HF component was significantly greater from 23:30 to 02:00 after diurnal exposure of bright light, being accompanied by lower heart rate during these periods. There existed negative correlation between heart rate and HF component from 23:30 to 02:00 under diurnal exposure to bright and dim lights. The results indicate that bright light exposure during the daytime (07:00–15:00) could enhance parasympathetic activity around midnight.     

Increase in Parasympathetic Nerve Activity During the Nighttime Following Bright Light Exposure During the Daytime

A spectral analysis of heart rate was carried out on 11 young female adults in order to evaluate the effects of bright light exposure on autonomic nervous activity. Bright light (5,000 lx) was provided by fluorescent lamps during the daytime (07:00-15:00) on day 1. Dim light (200 lx) was given on day 2. High frequency components (HF: 0.15-0.4Hz) were used as a marker of parasympathetic activity and the ratio of low frequency (LF: 0.04-0.15 HZ) to high frequency (LF/HF) as an indicator of sympathetic activity. The average value during the sleep period (23:30-06:30) was compared following diurnal exposure to bright or dim light. HF component was significantly greater from 23:30 to 02:00 after diurnal exposure of bright light, being accompanied by lower heart rate during these periods. There existed negative correlation between heart rate and HF component from 23:30 to 02:00 under diurnal exposure to bright and dim lights. The results indicate that bright light exposure during the daytime (07:00-15:00) could enhance parasympathetic activity around midnight.     

Effect of bright light on EEG activities and subjective sleepiness to mental task during nocturnal sleep deprivation

The purpose of this study was to investigate the effect of the exposure to bright light on EEG activity and subjective sleepiness at rest and at the mental task during nocturnal sleep deprivation. Eight male subjects lay awake in semi-supine in a reclining seat from 21:00 to 04:30 under the bright (BL; >2500 lux) or the dim (DL; <150 lux) light conditions. During the sleep deprivation, the mental task (Stroop color-word conflict test: CWT) was performed each 15 min in one hour. EEG, subjective sleepiness, rectal and mean skin temperatures and urinary melatonin concentrations were measured. The subjective sleepiness increased with time of sleep deprivation during both rest and CWT under the DL condition. The exposure to bright light delayed for 2 hours the increase in subjective sleepiness at rest and suppressed the increase in that during CWT. The bright light exposure also delayed the increase in the theta and alpha wave activities in EEG at rest. In contrast, the effect of the bright light exposure on the theta and alpha wave activities disappeared by CWT. Additionally, under the BL condition, the entire theta activity during CWT throughout nocturnal sleep deprivation increased significantly from that in a rest condition. Our results suggest that the exposure to bright light throughout nocturnal sleep deprivation influences the subjective sleepiness during the mental task and the EEG activity, as well as the subjective sleepiness at rest. However, the effect of the bright light exposure on the EEG activity at the mental task diminishes throughout nocturnal sleep deprivation.     

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.     

Physiological significance of 3-h bright and dim light exposure prior to taking a bath for core and forehead skin temperatures and heart rate during 1-h bathing of 38.5°C

1. The study investigated the effect of exposure to 3-h bright light (2500 lx) or dim light (200 lx) just prior to taking a hot bath upon thermophysiological responses during the 1-h bath (at 38.5°C water temperature). 2. Core and forehead skin temperature increases during the bath were significantly lower after bright than after dim light exposure. 3. Heart rate during the bath was significantly lower after exposure to bright light than dim light. 4. These results are discussed in terms of a reduced set-point of core temperature due to a probable higher secretion of melatonin under the bright light condition.     

A Compromise Phase Position for Permanent Night Shift Workers: Circadian Phase after Two Night Shifts with Scheduled Sleep and Light/Dark Exposure

Night shift work is associated with a myriad of health and safety risks. Phase‐shifting the circadian clock such that it is more aligned with night work and day sleep is one way to attenuate these risks. However, workers will not be satisfied with complete adaptation to night work if it leaves them misaligned during days off. Therefore, the goal of this set of studies is to produce a compromise phase position in which individuals working night shifts delay their circadian clocks to a position that is more compatible with nighttime work and daytime sleep yet is not incompatible with late nighttime sleep on days off. This is the first in the set of studies describing the magnitude of circadian phase delays that occurs on progressively later days within a series of night shifts interspersed with days off. The series will be ended on various days in order to take a “snapshot” of circadian phase. In this set of studies, subjects sleep from 23:00 to 7:00 h for three weeks. Following this baseline period, there is a series of night shifts (23:00 to 07:00 h) and days off. Experimental subjects receive five 15 min intermittent bright light pulses (～3500 lux; ～1100 µW/cm²) once per hour during the night shifts, wear sunglasses that attenuate all visible wavelengths—especially short wavelengths (“blue‐blockers”)—while traveling home after the shifts, and sleep in the dark (08:30–15:30 h) after each night shift. Control subjects remain in typical dim room light (<50 lux) throughout the night shift, wear sunglasses that do not attenuate as much light, and sleep whenever they want after the night shifts. Circadian phase is determined from the circadian rhythm of melatonin collected during a dim light phase assessment at the beginning and end of each study. The sleepiest time of day, approximated by the body temperature minimum (T_min), is estimated by adding 7 h to the dim light melatonin onset. In this first study, circadian phase was measured after two night shifts and day sleep periods. The T_min of the experimental subjects (n=11) was 04:24±0.8 h (mean±SD) at baseline and 7:36±1.4 h after the night shifts. Thus, after two night shifts, the T_min had not yet delayed into the daytime sleep period, which began at 08:30 h. The T_min of the control subjects (n=12) was 04:00±1.2 h at baseline and drifted to 4:36±1.4 h after the night shifts. Thus, two night shifts with a practical pattern of intermittent bright light, the wearing of sunglasses on the way home from night shifts, and a regular sleep period early in the daytime, phase delayed the circadian clock toward the desired compromise phase position for permanent night shift workers. Additional night shifts with bright light pulses and daytime sleep in the dark are expected to displace the sleepiest time of day into the daytime sleep period, improving both nighttime alertness and daytime sleep but not precluding adequate sleep on days off.     

Bright-light exposure during daytime sleeping affects nocturnal melatonin secretion after simulated night work

The guidelines for night and shift workers recommend that after night work, they should sleep in a dark environment during the daytime. However, staying in a dark environment during the daytime reduces nocturnal melatonin secretion and delays its onset. Daytime bright-light exposure after night work is important for melatonin synthesis the subsequent night and for maintaining the circadian rhythms. However, it is not clear whether daytime sleeping after night work should be in a dim- or a bright-light environment for maintaining melatonin secretion. The aim of this study, therefore, was to evaluate the effect of bright-light exposure during daytime sleeping on nocturnal melatonin secretion after simulated night work. Twelve healthy male subjects, aged 24.8 ± 4.6 (mean ± SD), participated in 3-day sessions under two experimental conditions, bright light or dim light, in a random order. On the first day, the subjects entered the experimental room at 16:00 and saliva samples were collected every hour between 18:00 and 00:00 under dim-light conditions. Between 00:00 and 08:00, they participated in tasks that simulated night work. At 10:00 the next morning, they slept for 6 hours under either a bright-light condition (>3000 lx) or a dim-light condition (<50 lx). In the evening, saliva samples were collected as on the first day. The saliva samples were analyzed for melatonin concentration. Activity and sleep times were recorded by a wrist device worn throughout the experiment. In the statistical analysis, the time courses of melatonin concentration were compared between the two conditions by three-way repeated measurements ANOVA (light condition, day and time of day). The change in dim light melatonin onset (ΔDLMO) between the first and second days, and daytime and nocturnal sleep parameters after the simulated night work were compared between the light conditions using paired t-tests. The ANOVA results indicated a significant interaction (light condition and3 day) (p = .006). Post hoc tests indicated that in the dim-light condition, the melatonin concentration was significantly lower on the second day than on the first day (p = .046); however, in the bright-light condition, there was no significant difference in the melatonin concentration between the days (p = .560). There was a significant difference in ΔDLMO between the conditions (p = .015): DLMO after sleeping was advanced by 11.1 ± 17.4 min under bright-light conditions but delayed for 7.2 ± 13.6 min after sleeping under dim-light conditions. No significant differences were found in any sleep parameter. Our study demonstrated that daytime sleeping under bright-light conditions after night work could not reduce late evening melatonin secretion until midnight or delay the phase of melatonin secretion without decreasing the quality of the daytime sleeping. Thus, these results suggested that, to enhance melatonin secretion and to maintain their conventional sleep–wake cycle, after night work, shift workers should sleep during the daytime under bright-light conditions rather than dim-light conditions.