Less exposure to daily ambient light in winter increases sensitivity of melatonin to light suppression

This study was carried out to examine the seasonal difference in the magnitude of the suppression of melatonin secretion induced by exposure to light in the late evening. The study was carried out in Akita (39 degrees North, 140 degrees East), in the northern part of Japan, where the duration of sunshine in winter is the shortest. Ten healthy male university students (mean age: 21.9+/-1.2 yrs) volunteered to participate twice in the study in winter (from January to February) and summer (from June to July) 2004. According to Japanese meteorological data, the duration of sunshine in Akita in the winter (50.5 h/month) is approximately one-third of that in summer (159.7 h/month). Beginning one week prior to the start of the experiment, the level of daily ambient light to which each subject was exposed was recorded every minute using a small light sensor that was attached to the subject's wrist. In the first experiment, saliva samples were collected every hour over a period of 24 h in a dark experimental room (<15 lux) to determine peak salivary melatonin concentration. The second experiment was conducted after the first experiment to determine the percentage of melatonin suppression induced by exposure to light. The starting time of exposure to light was set 2 h before the time of peak salivary melatonin concentration detected in the first experiment. The subjects were exposed to light (1000 lux) for 2 h using white fluorescent lamps (4200 K). The percentage of suppression of melatonin by light was calculated on the basis of the melatonin concentration determined before the start of exposure to light. The percentage of suppression of melatonin 2 h after the start of exposure to light was significantly greater in winter (66.6+/-18.4%) than summer (37.2+/-33.2%), p<0.01). The integrated level of daily ambient light from rising time to bedtime in summer was approximately twice that in winter. The results suggest that the increase in suppression of melatonin by light in winter is caused by less exposure to daily ambient light.     

Less Exposure to Daily Ambient Light in Winter Increases Sensitivity of Melatonin to Light Suppression

This study was carried out to examine the seasonal difference in the magnitude of the suppression of melatonin secretion induced by exposure to light in the late evening. The study was carried out in Akita (39° North, 140° East), in the northern part of Japan, where the duration of sunshine in winter is the shortest. Ten healthy male university students (mean age: 21.9±1.2 yrs) volunteered to participate twice in the study in winter (from January to February) and summer (from June to July) 2004. According to Japanese meteorological data, the duration of sunshine in Akita in the winter (50.5 h/month) is approximately one‐third of that in summer (159.7 h/month). Beginning one week prior to the start of the experiment, the level of daily ambient light to which each subject was exposed was recorded every minute using a small light sensor that was attached to the subject's wrist. In the first experiment, saliva samples were collected every hour over a period of 24 h in a dark experimental room (<15 lux) to determine peak salivary melatonin concentration. The second experiment was conducted after the first experiment to determine the percentage of melatonin suppression induced by exposure to light. The starting time of exposure to light was set 2 h before the time of peak salivary melatonin concentration detected in the first experiment. The subjects were exposed to light (1000 lux) for 2 h using white fluorescent lamps (4200 K). The percentage of suppression of melatonin by light was calculated on the basis of the melatonin concentration determined before the start of exposure to light. The percentage of suppression of melatonin 2 h after the start of exposure to light was significantly greater in winter (66.6±18.4%) than summer (37.2±33.2%), p<0.01). The integrated level of daily ambient light from rising time to bedtime in summer was approximately twice that in winter. The results suggest that the increase in suppression of melatonin by light in winter is caused by less exposure to daily ambient light.     

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.     

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.     

Hypersensitivity of melatonin suppression in response to light in patients with delayed sleep phase syndrome

Patients with delayed sleep phase syndrome (DSPS) experience a chronic mismatch between the usual daily schedule required by the individual's environment and their circadian sleep-wake pattern, resulting in major academic, work, and social problems. Although functional abnormalities of the circadian pacemaker system have been reported in patients with DSPS, the etiology of DSPS has not been fully elucidated. One hypothesis proposed to explain why patients with DSPS fail to synchronize their 24h sleep-wake cycle to their environment is that they might have reduced sensitivity to environmental time cues, most notably light-dark cycles. Therefore, we compared the sensitivity of melatonin suppression in response to light in patients with DSPS and normal control subjects. Fifteen patients with DSPS and age- and sex-matched healthy controls were studied. As the melatonin secretion rhythm in patients with DSPS was expected to be delayed compared to the controls, the time of peak melatonin secretion was determined in each subject in the first session. In the second session, each subject was exposed to light with an intensity of 1000 lux for 2h beginning 2h prior to his or her peak melatonin secretion. Melatonin was measured by radioimmunoassay in saliva sampled every 30 minutes during the period of light exposure. Suppression of the melatonin concentration in saliva was dependent on duration of light exposure. In addition, the suppressive effect of light on the melatonin concentration was significantly greater in patients with DSPS than in control subjects. The results suggest hypersensitivity to nighttime light exposure in patients with this syndrome. Our findings therefore suggest that evening light restriction is important for preventing patients with DSPS from developing a sleep phase delay. (Chronobiology International, 18(2), 263-271, 2001)     

HYPERSENSITIVITY OF MELATONIN SUPPRESSION IN RESPONSE TO LIGHT IN PATIENTS WITH DELAYED SLEEP PHASE SYNDROME*

Patients with delayed sleep phase syndrome (DSPS) experiencea chronic mismatch between the usual daily schedule required by the individual'senvironment and their circadian sleep-wake pattern, resulting in major academic,work, and social problems. Although functional abnormalities of the circadianpacemaker system have been reported in patients with DSPS, the etiology ofDSPS has not been fully elucidated. One hypothesis proposed to explain whypatients with DSPS fail to synchronize their 24h sleep-wake cycle to theirenvironment is that they might have reduced sensitivity to environmental timecues, most notably light-dark cycles. Therefore, we compared the sensitivityof melatonin suppression in response to light in patients with DSPS and normalcontrol subjects. Fifteen patients with DSPS and age- and sex-matched healthycontrols were studied. As the melatonin secretion rhythm in patients withDSPS was expected to be delayed compared to the controls, the time of peakmelatonin secretion was determined in each subject in the first session. Inthe second session, each subject was exposed to light with an intensity of1000 lux for 2h beginning 2h prior to his or her peak melatonin secretion.Melatonin was measured by radioimmunoassay in saliva sampled every 30 minutesduring the period of light exposure. Suppression of the melatonin concentrationin saliva was dependent on duration of light exposure. In addition, the suppressiveeffect of light on the melatonin concentration was significantly greater inpatients with DSPS than in control subjects. The results suggest hypersensitivityto nighttime light exposure in patients with this syndrome. Our findings thereforesuggest that evening light restriction is important for preventing patientswith DSPS from developing a sleep phase delay. (ChronobiologyInternational, 18(2), 263–271, 2001)     

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.     

Eye colour and aggression in juvenile guppies, Poecilia reticulata peters (Pisces: Poeciliidae)

A series of four related experiments on guppy, Poecilia reticulata, juveniles indicates that eye colour is related to motivational state. In juveniles of this species two distinct eye colour states are easily discernible, with a transient intermediate state infrequently seen. These states are dark, light, and intermediate. In aggressive interactions involving both both dark- and light-eyed fish, dark-eyed fish won 98.6% of the encounters (N=222 encounters). Eye colours are significantly correlated with bout lengths of certain behaviours (aggressive encounters, swimming, substrate biting); total time spent in certain behaviours (swimming, aggressive encounters); and type and intensity of aggressive encounters. Eye colour appears to be related to motivational state and may serve a signal function.     

Seasonal variations of melatonin secretion in young females under natural and artificial light conditions in Fukuoka, Japan

The purpose of this study was to examine the seasonal variations of melatonin secretion of subjects and of their surrounding light conditions. Eight Japanese female students (20.1+/-2.6 yrs, Mean+/-SD) living in Fukuoka, Japan, participated in the present study. Saliva samples were collected every 3 hours over the course of a day, and the light intensity during daily life was measured every 1 min for 5 days in the four seasons. Almost all subjects had different melatonin secretory profiles in autumn, with only two subjects showing similar rhythms in all four seasons. The peak values of melatonin secretion calculated by a spline interpolation were higher in autumn than those in other seasons (p<0.001, Fisher's PLSD) and its peak time in this season was significantly delayed compared with those in spring and summer (p<0.05, Fisher's PLSD). The amount of time during daytime exposure to light of >1,000 lux was at least thirty minutes in all the seasons, and there were no significant differences among them. The relationship between peak level of melatonin secretion and amount of time of daytime light exposure to >1,000 lux was significant only in the autumn. During this season, there was a significant positive correlation (r=0.83, p<0.05, n=6), except for two subjects, whose melatonin secretion remained low.     

The circadian response of intrinsically photosensitive retinal ganglion cells

Intrinsically photosensitive retinal ganglion cells (ipRGC) signal environmental light level to the central circadian clock and contribute to the pupil light reflex. It is unknown if ipRGC activity is subject to extrinsic (central) or intrinsic (retinal) network-mediated circadian modulation during light entrainment and phase shifting. Eleven younger persons (18-30 years) with no ophthalmological, medical or sleep disorders participated. The activity of the inner (ipRGC) and outer retina (cone photoreceptors) was assessed hourly using the pupil light reflex during a 24 h period of constant environmental illumination (10 lux). Exogenous circadian cues of activity, sleep, posture, caffeine, ambient temperature, caloric intake and ambient illumination were controlled. Dim-light melatonin onset (DLMO) was determined from salivary melatonin assay at hourly intervals, and participant melatonin onset values were set to 14 h to adjust clock time to circadian time. Here we demonstrate in humans that the ipRGC controlled post-illumination pupil response has a circadian rhythm independent of external light cues. This circadian variation precedes melatonin onset and the minimum ipRGC driven pupil response occurs post melatonin onset. Outer retinal photoreceptor contributions to the inner retinal ipRGC driven post-illumination pupil response also show circadian variation whereas direct outer retinal cone inputs to the pupil light reflex do not, indicating that intrinsically photosensitive (melanopsin) retinal ganglion cells mediate this circadian variation.     

The pigmentation of human iris influences the uptake and storing of zinc

Age-related macular degeneration (AMD) is more prevalent among the elderly Caucasians than in Africans. A significant association between light iris colour, fundus pigmentation and incidence of AMD is reported, suggesting a possible correlation with melanin pigment. Zinc is known to bind to melanin in pigmented tissues and to enhance antioxidant capacity by function as a cofactor or gene expression factor of antioxidant enzymes in the eye. In this in vitro study, we investigated the uptake and storage of zinc in human irides. Irides of blue and brown human eyes were used. The number of melanocytes was measured. Tissues without any treatment served as controls. The irides were incubated with 100 microM zinc chloride in culture medium for 24 h. Specimens of the tissues were stored for the uptake examination. The remained pieces were further incubated for 3 and 7 d to investigate the storage of zinc. The concentration of zinc was measured by inductively coupled plasma mass spectrometry (ICP-MS). Melanocytes count was significantly higher in the brown tissues (P < 0.0001). Zinc concentration of blue coloured irides after 24 h zinc treatment was close to the controls. We did not observe any significant storing. In contrast, the concentration of zinc in brown irides was significantly increased after 24 h (P < or = 0.01) and remained at a high level for 7 d. The uptake of zinc is likely dependent on the amount of pigmentation in human iris. Therefore, we assume that in patients suffering from AMD the degree of pigmentation of the irides and eventually fundi should be under consideration when the patients are treated with zinc supplementation.     

Light/dark patterns of interleukin-6 in relation to the pineal hormone melatonin in patients with acute myocardial infarction

AIMS: Atherosclerosis is a chronic disease that, from its origin to its ultimate complications, involves inflammatory cells, inflammatory proteins, and inflammatory responses from vascular cells. It has been demonstrated that cytokine activities are under neuroendocrine control, in part exerted by the pineal gland through the circadian secretion of its main product melatonin. Melatonin is mainly released during the night, but the precise relationship between melatonin and the light/dark rhythm of interleukin-6 in patients with acute myocardial infarction is still unclear. METHODS AND RESULTS: The study included 60 patients diagnosed with acute myocardial infarction and 60 healthy volunteers whose venous blood samples were collected at 09:00 h (light period) and 02:00 h (dark period). Our results demonstrate that interleukin-6 concentrations presented a light/dark pattern with mean serum concentrations being higher in the acute myocardial infarction group than in the control group (101.26 +/- 13.43 and 52.67 +/- 7.73 pg/ml at 02:00 h, 41.93 +/- 5.90 and 22.98 +/- 4.49 pg/ml at 09:00 h, respectively, p < 0.05). Differences in the day/night changes in melatonin levels in control subjects (48.19 +/- 7.82 at 02:00 h, 14.51+/- 2.36 at 09:00 h, pg/ml) and acute myocardial infarction patients (25.97 +/- 3.90 at 02:00 h, 12.29 +/- 4.01 at 09:00 h, pg/ml) (p < 0.05) were a result of a reduced nocturnal elevation of melatonin in the acute myocardial infarction group. CONCLUSIONS: The current findings suggest that the circadian secretion of melatonin may be responsible at least in part for light/dark variations of endogenous interleukin-6 production in patients with acute myocardial infarction. In this study, the melatonin seems to have an anti-inflammatory effect.     

Two light‐activated neuroendocrine circuits arising in the eye trigger physiological and morphological pigmentation

Two biological processes regulate light‐induced skin colour change. A fast ‘physiological pigmentation change’ (i.e. circadian variations or camouflage) involves alterations in the distribution of pigment containing granules in the cytoplasm of chromatophores, while a slower ‘morphological pigmentation change’ (i.e. seasonal variations) entails changes in the number of pigment cells or pigment type. Although linked processes, the neuroendocrine coordination triggering each response remains largely obscure. By evaluating both events in Xenopus laevis embryos, we show that morphological pigmentation initiates by inhibiting the activity of the classical retinal ganglion cells. Morphological pigmentation is always accompanied by physiological pigmentation, and a melatonin receptor antagonist prevents both responses. Physiological pigmentation also initiates in the eye, but with repression of melanopsin‐expressing retinal ganglion cell activity that leads to secretion of alpha‐melanocyte‐stimulating hormone (α‐MSH). Our findings suggest a model in which eye photoperception links physiological and morphological pigmentation by altering α‐MSH and melatonin production, respectively.     

Does pupil constriction under blue and green monochromatic light exposure change with age?

Many nonvisual functions are regulated by light through a photoreceptive system involving melanopsin-expressing retinal ganglion cells that are maximally sensitive to blue light. Several studies have suggested that the ability of light to modulate circadian entrainment and to induce acute effects on melatonin secretion, subjective alertness, and gene expression decreases during aging, particularly for blue light. This could contribute to the documented changes in sleep and circadian regulatory processes with aging. However, age-related modification in the impact of light on steady-state pupil constriction, which regulates the amount of light reaching the retina, is not demonstrated. We measured pupil size in 16 young (22.8±4 years) and 14 older (61±4.4 years) healthy subjects during 45-second exposures to blue (480 nm) and green (550 nm) monochromatic lights at low (7×10(12) photons/cm2/s), medium (3×10(13) photons/cm2/s), and high (10(14) photons/cm2/s) irradiance levels. Results showed that young subjects had consistently larger pupils than older subjects for dark adaptation and during all light exposures. Steady-state pupil constriction was greater under blue than green light exposure in both age groups and increased with increasing irradiance. Surprisingly, when expressed in relation to baseline pupil size, no significant age-related differences were observed in pupil constriction. The observed reduction in pupil size in older individuals, both in darkness and during light exposure, may reduce retinal illumination and consequently affect nonvisual responses to light. The absence of a significant difference between age groups for relative steady-state pupil constriction suggests that other factors such as tonic, sympathetic control of pupil dilation, rather than light sensitivity per se, account for the observed age difference in pupil size regulation. Compared to other nonvisual functions, the light sensitivity of steady-state pupil constriction appears to remain relatively intact and is not profoundly altered by age.     

Entrainment of Circadian Programs

Circadian rhythms were recently proposed as a measure of physiological state and prognosis in disorders of consciousness (DOC). So far, melatonin regulation was never assessed in vegetative state (VS). Aim of our research was to investigate the nocturnal melatonin levels and light-induced melatonin suppression in a cohort of VS patients. We assessed six consecutive patients (four men, age 33.3?±?9.3 years) with post-traumatic VS and nine age-matched healthy volunteers (five men, age 34.3?±?8.9 years) on two consecutive nights: one baseline and one light exposure night. During baseline, night subjects were in bed in a dim (<5?lux) room from 10?pm to 8?am. Blood samples were collected hourly 00:30–3:30?am (00:30?=?MLT1; 1:30?=?MLT2; 2:30?=?MLT3; and 3:30?=?MLT4). Identical setting was used for melatonin suppression test night, except for the exposure to monochromatic (470?nm) light from 1:30 to 3:30?am. Plasma melatonin levels were evaluated by radioimmunoassay. Magnitude of melatonin suppression was assessed by melatonin suppression score (caMSS) and suppression rate. We searched for group differences in melatonin levels, differences between repeated samples melatonin concentrations during baseline night and light exposure night, and light-induced suppression of melatonin secretion. During baseline night, controls showed an increase of melatonin (MLT4 vs MLT1, p?=?0.037), while no significant changes were observed in VS melatonin levels (p?=?0.172). Baseline night MLT4 was significantly lower in VS vs controls (p?=?0.036). During light-exposure night, controls displayed a significant suppression of melatonin (MLT3 and MLT4 vs MLT2, p?=?0.016 and 0.002, respectively), while VS patients displayed no significant changes. The magnitude of light-induced suppression of melatonin levels was statistically different between groups considering control adjusted caMSS (p?=?0.000), suppression rate (p?=?0.002) and absolute percentage difference (p?=?0.012). These results demonstrate for the first time that VS patients present an alteration in night melatonin secretion and reduced light-induced melatonin suppression. These findings confirm previous studies demonstrating a disruption of the circadian system in DOC and suggest a possible benefit from melatonin supplementation in VS.     

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.     

Melatonin in rat milk and the likelihood of its role in postnatal maternal entrainment of rhythms

The rhythm of melatonin in rat milk and the capacity of pups to synthesize and metabolize melatonin were studied. Melatonin was undetectable in milk in the light (< 21 pM), but increased rapidly 2-4 h after dark to peak at 357 +/- 66 pM at mid-dark. Oral or subcutaneous administration of melatonin to 5- and 10-day-old pups resulted in peak plasma melatonin levels 30 min after administration and rapid metabolism. Increases in pineal and plasma melatonin levels at night were detected at 5 and 6 days of age, respectively. Isoproterenol administration (2 microg/g body wt) at mid-light to day 10 pups increased plasma melatonin from 312 +/- 40 pM to 1,298 +/- 160 pM, whereas propranolol (2 microg/g body wt) suppressed nocturnal melatonin secretion from 1,270 +/- 128 pM to 395 +/- 66 pM. The rise of pineal and plasma melatonin in day 10 pups occurred 1 and 2 h after dark onset, respectively, preceding the onset in dams by 3 and 4 h, respectively. Propranolol administration to 2- and 5-day lactating dams inhibited plasma and milk melatonin at night but had no effect on their suckling pups. Transfer of melatonin via the milk is unlikely to provide an entraining signal for rat pups.     

Inferior retinal light exposure is more effective than superior retinal exposure in suppressing melatonin in humans

Illumination of different areas of the human retina elicits differences in acute light-induced suppression of melatonin. The aim of this study was to compare changes in plasma melatonin levels when light exposures of equal illuminance and equal photon dose were administered to superior, inferior, and full retinal fields. Nine healthy subjects participated in the study. Plexiglass eye shields were modified to permit selective exposure of the superior and inferior halves of the retinas of each subject. The Humphrey Visual Field Analyzer was used both to confirm intact full visual fields and to quantify exposure of upper and lower visual fields. On study nights, eyes were dilated, and subjects were exposed to patternless white light for 90 min between 0200 and 0330 under five conditions: (1) full retinal exposure at 200 lux, (2) full retinal exposure at 100 lux, (3) inferior retinal exposure at 200 lux, (4) superior retinal exposure at 200 lux, and (5) a dark-exposed control. Plasma melatonin levels were determined by radioimmunoassay. ANOVA demonstrated a significant effect of exposure condition (F = 5.91, p < 0.005). Post hoc Fisher PLSD tests showed significant (p < 0.05) melatonin suppression of both full retinal exposures as well as the inferior retinal exposure; however, superior retinal exposure was significantly less effective in suppressing melatonin. Furthermore, suppression with superior retinal exposure was not significantly different from that of the dark control condition. The results indicate that the inferior retina contributes more to the light-induced suppression of melatonin than the superior retina at the photon dosages tested in this study. Findings suggest a greater sensitivity or denser distribution of photoreceptors in the inferior retina are involved in light detection for the retinohypothalamic tract of humans.     

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.     

Feeding rhythms in constant light and constant darkness: the role of the eyes and the effect of melatonin infusion

Exposure to constant light abolishes circadian behavioral rhythms of locomotion and feeding as well as circulating melatonin rhythms in pigeons (Columba livia). To determine if feeding rhythmicity could be maintained in pigeons exposed to constant light, periodic infusions (10h/day) of melatonin were administered to pinealectomized and bilaterally retinectomized/pinealectomized pigeons under conditions of both constant darkness and constant light. The infusions were sufficient to entrain rhythmicity in pinealectomized pigeons in constant darkness and to restore and maintain rhythmicity in bilaterally retinectomized/pinealectomized pigeons in constant darkness. On subsequent exposure to constant light, rhythmicity remained phase locked to the melatonin infusions in bilaterally retinectomized/pinealectomized pigeons but was abolished in sighted pinealectomized birds. These results suggest that while endogenous melatonin rhythms are both necessary and sufficient to maintain behavioral rhythms in DD, their effect can be overridden by constant light but only if perceived by the eyes. Thus, constant light may abolish behavioral rhythmicity in intact pigeons (and perhaps in other species) by a mechanism other than suppression of endogenous melatonin rhythmicity. Such a mechanism might involve direct stimulation of locomotor or feeding activity by retinally perceived (but not by extra-retinally perceived) light, or alternatively by suppression of a hypothalamic oscillator that receives its major light input from the retinae.Abbreviations PX pinealectomized - EX bilaterally enucleated - LD light:dark cycle - LL constant light - DD constant darkness - DD_b constant darkness before exposure to constant light - DD_a constant darkness after exposure to constant light