Influence of two different light intensities from 16:00 to 20:30 hours on evening dressing behavior in the cold

The present experiment tested our hypothesis that the subjects will wear more clothing in the evening cold under the influence of bright light exposure in the late afternoon and evening. Nine young female adults participated in this study. Light intensity was controlled from 9:00 h to 16:00 h at 100 lx, and from 16:00 h to 20:30 h either at 3000 lx in the bright light (Brighte) or at 10 lx in the dim light ("Dim") conditions. Light intensity was maintained at 10 lx from 20:30 h to 23:00 h. They were instructed to wear garments to maintain themselves to feel comfortable during the fall of ambient temperature from 30 degrees C to 15 degrees C (21:00 h - 22:00 h) and its constant temperature at 15 degrees C (22:00 h - 23:00 h). Most subjects dressed in heavier clothing in the "Bright" than in the "Dim" conditions. The evening fall of core temperature was significantly smaller and the urinary melatonin secretion was significantly lower in the "Bright" condition, suggesting that the set-point of core temperature has been set at a higher level during the evening and at night, being influenced by the less amount of melatonin secretion. Thus, it is concluded that the late afternoon and evening bright light exposure could accelerate the dressing behavior in the evening cold.     

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.     

Exposure to bright light for several hours during the daytime lowers tympanic temperature

 The present study investigates the effect on thympanic temperature of exposure to different light intensities for several hours during the daytime. Nine healthy young adult volunteers (two male, seven female) were exposed to bright light of 4000 lx or dim light of 100 lx during the daytime from 0930 to 1800 hours; the light condition was then kept at 100 lx for a further hour. Tympanic temperature was measured continuously at a neutral condition (28° C, 60% relative humidity) from 1000 to 1800 hours. Urinary samples were collected from 1100 to 1900 hours every 2 h, and melatonin excretion rate was measured by enzyme immunoassay. Of nine subjects, six showed clearly lower tympanic temperatures in the bright compared with the dim condition from 1400 to 1800 hours. Average tympanic temperatures were significantly lower in the bright than in the dim condition from 1645 to 1800 hours. Melatonin excretion rate tended to be higher in the bright than in the dim condition. It was concluded that exposure to bright light of 4000 lx during the daytime for several hours could reduce tympanic temperature, compared with that measured in dim light of 100 lx. Received: 22 January 1997 / Revised: 5 April 1997 / Accepted: 26 June 1997     

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.     

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.     

Bright Light Exposure During the Daytime Affects Circadian Rhythms of Urinary Melatonin and Salivary Immunoglobulin A

The effects of bright light exposure during the daytime on circadian urinary melatonin and salivary immunoglobulin A (IgA) rhythms were investigated in an environmental chamber controlled at a global temperature of 27°C ± 0.2°C and a relative humidity of 60% ± 5%. Seven diurnally active healthy females were studied twice, in bright and dim light conditions. Bright light of 5000 lux was provided by placing fluorescent lamps about 1 meter in front of the subjects during the daytime exposure (06:30-19:30) from 06:30 on day 1 to 10:30 on day 3. Dim light was controlled at 200 lux, and the subjects were allowed to sleep from 22:30 to 06:30 under both light exposure conditions. Urine and saliva were collected at 4h intervals for assessing melatonin and IgA. Melatonin excretion in the urine was significantly greater during the nighttime (i.e., at 06:30 on day 1 and at 02:30 on day 2) after the bright light condition than during the dim light condition. Furthermore, the concentration and the amount of salivary IgA tended to be higher in the bright light than in the dim light condition, especially during the nighttime. Also, salivary IgA concentration and the total amount secreted in the saliva were significantly positively correlated with urinary melatonin. These results are consistent with the hypothesis that bright light exposure during the daytime enhances the nocturnal melatonin increase and activates the mucosal immune response.     

Influence of the Amount of Light Received during the Day and Night Times on the Circadian Rhythm of Melatonin Secretion in Women Living Diurnally

The study investigated the relationship between the circadian variation of salivary melatonin and the amount of light received during the day and night. Forty one females served as subjects. An illuminance meter worn on the wrist of the non-dominant arm measured the amount of light which subjects leading a diurnal lifestyle received during two consecutive days. Light received from the time of rising to 18:00h was defined as ‘daytime light’, and that from 18:00h to the time of retiring as ‘nighttime light’. The average amount of light over the two days was 48 × 10 4 lx during the daytime and 11 × 10 4 lx during the nighttime. Saliva was collected every 4h in order to measure melatonin secretion. Peaks of melatonin secretion were observed at 14:00h and 18:00h in the subjects who had received lesser amounts of light during the daytime and nighttime. Melatonin secretion was high around 22:00h and peaked around 02:00h in the subjects who had received greater amounts of light during the daytime and lesser amounts of light during the nighttime. Nocturnal melatonin secretion was suppressed in the subjects who received greater amounts of light during the nighttime. Thus, the amount of light received during the daytime and the nighttime during the course of a diurnal lifestyle could have a profound influence on the circadian pattern of melatonin secretion.     

Influence of the Amount of Light Received during the Day and Night Times on the Circadian Rhythm of Melatonin Secretion in Women Living Diurnally

The study investigated the relationship between the circadian variation of salivary melatonin and the amount of light received during the day and night. Forty one females served as subjects. An illuminance meter worn on the wrist of the non-dominant arm measured the amount of light which subjects leading a diurnal lifestyle received during two consecutive days. Light received from the time of rising to 18:00h was defined as 'daytime light', and that from 18:00h to the time of retiring as 'nighttime light'. The average amount of light over the two days was 48 × 10 4 lx during the daytime and 11 × 10 4 lx during the nighttime. Saliva was collected every 4h in order to measure melatonin secretion. Peaks of melatonin secretion were observed at 14:00h and 18:00h in the subjects who had received lesser amounts of light during the daytime and nighttime. Melatonin secretion was high around 22:00h and peaked around 02:00h in the subjects who had received greater amounts of light during the daytime and lesser amounts of light during the nighttime. Nocturnal melatonin secretion was suppressed in the subjects who received greater amounts of light during the nighttime. Thus, the amount of light received during the daytime and the nighttime during the course of a diurnal lifestyle could have a profound influence on the circadian pattern of melatonin secretion.     

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.     

Effects of day-time exposure to different light intensities on light-induced melatonin suppression at night

Background

Bright nocturnal light has been known to suppress melatonin secretion. However, bright light exposure during the day-time might reduce light-induced melatonin suppression (LIMS) at night. The effective proportion of day-time light to night-time light is unclear; however, only a few studies on accurately controlling both day- and night-time conditions have been conducted. This study aims to evaluate the effect of different day-time light intensities on LIMS.

Methods

Twelve male subjects between the ages of 19 and 23 years (mean ± S.D., 20.8 ± 1.1) gave informed consent to participate in this study. They were exposed to various light conditions (<10, 100, 300, 900 and 2700 lx) between the hours of 09:00 and 12:00 (day-time light conditions). They were then exposed to bright light (300 lx) again between 01:00 and 02:30 (night-time light exposure). They provided saliva samples before (00:55) and after night-time light exposure (02:30).

Results

A one-tailed paired t test yielded significant decrements of melatonin concentration after night-time light exposure under day-time dim, 100- and 300-lx light conditions. No significant differences exist in melatonin concentration between pre- and post-night-time light exposure under day-time 900- and 2700-lx light conditions.

Conclusions

Present findings suggest the amount of light exposure needed to prevent LIMS caused by ordinary nocturnal light in individuals who have a general life rhythm (sleep/wake schedule). These findings may be useful in implementing artificial light environments for humans in, for example, hospitals and underground shopping malls.     

Circadian rhythm of acute phase proteins under the influence of bright/dim light during the daytime

We investigated the influence of two different light intensities, dim (100 lx) and bright (5000 lx), during the daytime on the circadian rhythms of selected acute phase proteins of C-reactive protein (CRP), alpha1-acid glycoprotein (AGP), alpha1-antichymotrypsin (ACT), transfferin (TF), alpha2-macroglobulin (alpha2-m), haptoglobin (HP), and ceruloplasmin (CP). Serum samples were collected from 7 healthy volunteers at 4 h intervals during two separate single 24 h spans during which they were exposed to the respective light intensity conditions. A circadian rhythm was detected only in ACT concentration in the bright light condition. The concentration of ACT, a positive acute phase protein (APP), increased (significantly significant differences in the ACT concentration were detected at 14:00 and 22:00 h) and AGP showed a tendency to be higher under the daytime bright compared to dim light conditions. There were no significant differences between the time point means under daytime dim and bright light conditions for alpha2-M, AGP, Tf, Cp, or Hp. The findings suggest that some, but not all, APP may be influenced by the environmental light intensity.     

Circadian Rhythm of Acute Phase Proteins under the Influence of Bright/Dim Light during the Daytime

We investigated the influence of two different light intensities, dim (100 lx) and bright (5,000 lx), during the daytime on the circadian rhythms of selected acute phase proteins of C‐reactive protein (CRP), α1‐acid glycoprotein (AGP), α1‐antichymotrypsin (ACT), transfferin (TF), α2‐macroglobulin (α2‐m), haptoglobin (HP), and ceruloplasmin (CP). Serum samples were collected from 7 healthy volunteers at 4 h intervals during two separate single 24 h spans during which they were exposed to the respective light intensity conditions. A circadian rhythm was detected only in ACT concentration in the bright light condition. The concentration of ACT, a positive acute phase protein (APP), increased (significantly significant differences in the ACT concentration were detected at 14:00 and 22:00 h) and AGP showed a tendency to be higher under the daytime bright compared to dim light conditions. There were no significant differences between the time point means under daytime dim and bright light conditions for α2‐M, AGP, Tf, Cp, or Hp. The findings suggest that some, but not all, APP may be influenced by the environmental light intensity.     

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.     

Photostimulation of Japanese quail by dim light depends upon photophase contrast, not light intensity

Three experiments were conducted to determine whether dim light is interpreted by Japanese quail as subjective day or night, and whether this interpretation depends upon absolute light intensity. Birds were exposed to 24-h days consisting of either bright light (2500-3000 lx) with dim light (0.5-5 lx) or dim light with darkness. Locomotor activity was higher in the brighter photophase, whether it was bright light or dim light, indicating that the birds interpreted the brighter phase as daytime. Dim light produced daytime activity levels when paired with darkness, but it produced nighttime activity when paired with bright light, indicating that activity rhythms are determined by relative not absolute light intensity. Similarly, photostimulation, as measured by growth of the cloacal protrusion area (CPA), depended upon photic context, not absolute light intensity. CPA growth occurred when birds were exposed to 16 h of dim light with 8 h of darkness (16dm:8dk) but not when exposed to 10 h of bright light with 14 h of dim light (10bt:14dm). Constant dim light was stimulatory regardless of previous dim light context. Photostimulation appears to depend upon subjective interpretations of day and night rather than solely upon light intensity.     

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.     

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.     

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     

Influence of bright light during daytime on sleep parameters in hospitalized elderly patients

Nurses frequently care for sleepless elderly patients on bed rest in a hospital environment. Our previous study with young adults showed that bright light exposure during the daytime affected the induction of nocturnal deep sleep. The purpose of this study is aimed at finding whether similar research could be observed with hospitalized elderly patients. Seven patients (mean age 67; range 57-77 yrs, males 3: females 4) served as participants and their informed written consent was obtained. A fluorescent lamp fixed in the bed frame near the head of the patient was turned on at 10:00 h and off at 15:00 h each day for 1 week (BL). Moreover, each patient was required to stay near this light during this period. The patients lived in a room facing north, where the ambient light intensities ranged from 50 to 300 lx during the daytime. Their activities were continuously measured using an Actiwatch (model-AWL, Mini-Mitter, USA). Salivary samples were collected at midnight for the measurement of melatonin. The findings were compared between 2 days before BL exposure (baseline) and the last 2 days during BL exposure, respectively. The bright light exposure during the daytime prolonged "Time in Bed" (p < 0.05), increased "Immobile Minutes" (p < 0.05), and delayed "Get up Time" (p < 0.01). The average melatonin secretion at midnight in four patients increased from 7.5 +/- 2.6 pg/ml to 13.3 +/- 9.2 pg/ml. These findings suggest that diurnal bright light exposure for hospitalized elderly patients lying in bed under dark condition during the daytime may favor clinically the induction of nocturnal deep sleep. Attention should be given to the illumination conditions for elderly patients in hospitals to improve their impaired sleep.     

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.     

Repeated exposures to daytime bright light increase nocturnal melatonin rise and maintain circadian phase in young subjects under fixed sleep schedule

Effects of two different light intensities during daytime were examined on human circadian rhythms in plasma melatonin, core body temperature, and wrist activity under a fixed sleep schedule. Sleep qualities as indicated by polysomnography and subjective sleepiness were also measured. In the first week, under dim light conditions ( approximately 10 lx), the onset and peak of nocturnal melatonin rise were significantly delayed, whereas the end of melatonin rise was not changed. The peak level of melatonin rise was not affected. As a result, the width of nocturnal melatonin rise was significantly shortened. In the second week, under bright light conditions ( approximately 5,000 lx), the phases of nocturnal melatonin rise were not changed further, but the peak level was significantly increased. Core body temperature at the initial sleep phase was progressively elevated during the course of dim light exposure and reached the maximum level at the first night of bright light conditions. Subjective sleepiness gradually declined in the course of dim light exposure and reached the minimum level at the first day of bright light. These findings indicate that repeated exposures to daytime bright light are effective in controlling the circadian phase and increasing the peak level of nocturnal melatonin rise in plasma and suggest a close correlation between phase-delay shifts of the onset of nocturnal melatonin rise or body temperature rhythm and daytime sleepiness.