Plasma Melatonin Rhythm Research Articles

Locomotor, feeding and melatonin daily rhythms in sharpsnout seabream ( Diplodus puntazzo)

Sharpsnout seabream is a marine teleost of increasing interest for Mediterranean aquaculture, but there is still a lack of information regarding its circadian organization. In this study, we have investigated sharpsnout seabream locomotor activity, feeding and plasma melatonin daily rhythms under a 12:12-h LD cycle, as well as the persistence of locomotor activity circadian rhythmicity under constant light (LL) conditions. When submitted to an LD cycle, most sharpsnout seabream displayed a diurnal locomotor pattern, with an average 74% of activity recorded during daytime. However, along the experiment 40% of fish spontaneously changed their locomotor rhythm phasing and became nocturnal. Feeding behaviour, nevertheless, remained strictly diurnal in all cases, with 97% of food demands being made during the light period. Free-running locomotor rhythms were recorded in one third of the fish kept under LL. Daily plasma melatonin levels displayed a rhythmic profile, with low daytime values (111 pg/ml) and high nighttime concentrations (791 pg/ml). Taken together, these results evidence a high degree of plasticity for sharpsnout seabream activity patterns, as well as phasing independence of locomotor and feeding rhythms. Finally, the existence of a well-defined daily rhythm of plasma melatonin was found.

Spontaneous internal desynchronization of locomotor activity and body temperature rhythms from plasma melatonin rhythm in rats exposed to constant dim light

BackgroundWe have recently reported that spontaneous internal desynchronization between the locomotor activity rhythm and the melatonin rhythm may occur in rats (30% of tested animals) when they are maintained in constant dim red light (LLdim) for 60 days. Previous work has also shown that melatonin plays an important role in the modulation of the circadian rhythms of running wheel activity (Rw) and body temperature (Tb). The aim of the present study was to investigate the effect that desynchronization of the melatonin rhythm may have on the coupling and expression of circadian rhythms in Rw and Tb.MethodsRats were maintained in a temperature controlled (23–24°C) ventilated lightproof room under LLdim (red dim light 1 μW/cm2 [5 Lux], lower wavelength cutoff at 640 nm). Animals were individually housed in cages equipped with a running wheel and a magnetic sensor system to detect wheel rotation; Tb was monitored by telemetry. Tb and Rw data were recorded in 5-min bins and saved on disk. For each animal, we determined the mesor and the amplitude of the Rw and Tb rhythm using waveform analysis on 7-day segments of the data. After sixty days of LLdim exposure, blood samples (80–100 μM) were collected every 4 hours over a 24-hrs period from the tail artery, and serum melatonin levels were measured by radioimmunoassay.ResultsTwenty-one animals showed clear circadian rhythms Rw and Tb, whereas one animal was arrhythmic. Rw and Tb rhythms were always strictly associated and we did not observe desynchronization between these two rhythms. Plasma melatonin levels showed marked variations among individuals in the peak levels and in the night-to-day ratio. In six rats, the night-to-day ratio was less than 2, whereas in the rat that showed arrhythmicity in Rw and Tb melatonin levels were high and rhythmic with a large night-to-day ratio. In seven animals, serum melatonin levels peaked during the subjective day (from CT0 to CT8), thus suggesting that in these animals the circadian rhythm of serum melatonin desynchronized from the circadian rhythms of Rw and Tb. No significant correlation was observed between the amplitude (or the levels) of the melatonin profile and the amplitude and mesor of the Rw and Tb rhythms.ConclusionOur data indicate that the free-running periods (τ) and the amplitude of Rw and Tb were not different between desynchronized and non-desynchronized rats, thus suggesting that the circadian rhythm of serum melatonin plays a marginal role in the regulation of the Rw and Tb rhythms. The present study also supports the notion that in the rat the circadian rhythms of locomotor activity and body temperature are controlled by a single circadian pacemaker.

Erratum

Journal of Pineal ResearchVolume 40, Issue 2 p. 194-194 Free Access Erratum First published: 08 December 2005 https://doi.org/10.1111/j.1600-079X.2005.00292.xCitations: 1AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat In the paper by El Allali et al. [1], there was an error in the reproduction of Fig. 2. The following figure should replace Fig. 2 in the manuscript. Reference 1 El Allali K, Achaaban MR, Vivien-Roels B, Bothorel B, Tligui NS, Pévet P. Seasonal variations in the nycthemeral rhythm of plasma melatonin in the camel (Camelus dromedarius). J Pineal Res 2005; 39: 121– 128. Wiley Online LibraryCASPubMedWeb of Science®Google Scholar Citing Literature Volume40, Issue2March 2006Pages 194-194 ReferencesRelatedInformation

Maternal Fluoxetine Infusion Does Not Alter Fetal Endocrine and Biophysical Circadian Rhythms in Pregnant Sheep

Depression during pregnancy is frequently treated with the selective serotonin reuptake inhibitor (SSRI), fluoxetine (FX), commonly known as Prozac (Eli Lilly & Co, Indianapolis, IN). FX potentiates serotoninergic neurotransmission and serotonin has been implicated in the regulation of circadian rhythms. We have therefore investigated the effect of chronic administration of FX on maternal and fetal circadian rhythms in sheep. Following an initial bolus dose of 70 mg FX, an 8-day continuous infusion of FX (n = 11, 98.5 microg/kg x d) was performed. Controls (n = 13) were treated with sterile water vehicle only. Maternal and fetal plasma melatonin and prolactin concentrations were determined every 3 hours for 24 hours and then every 6 hours for 24 hours beginning on the fourth day of infusion. FX treatment did not alter either the basal or circadian rhythms of either maternal or fetal plasma melatonin and prolactin concentrations. Fetal cardiovascular and behavioral state parameters were measured continuously. While the incidence of low-voltage (LV) electrocortical (ECOG) activity was significantly reduced in fetuses in the FX group, there was no effect of FX on the diurnal rhythms in fetal arterial pressure, heart rate, breathing movements, or behavioral state. These results show that maternal FX treatment does not result in significant alterations in maternal and fetal hormonal and behavioral circadian rhythms.

Seasonal variations in the nycthemeral rhythm of plasma melatonin in the camel (Camelus dromedarius)

Seasonal changes in the pattern of plasma melatonin were investigated in two groups of camels (Camelus dromedarius): 11 adult and six young camels. Animals were subjected to the outdoor conditions of a desert environment. Blood samples were taken at regular intervals of about 3 hr (added to particular samples at 1 hr before then 30 min and 1 hr after sunset, and 1 hr before and 1 hr after sunrise) for 24 hr at both solstices and equinoxes of the year. The plasma melatonin levels steeply increased soon after sunset and remained elevated throughout all the night. Then, melatonin concentrations progressively declined shortly before sunrise and returned to daytime basal levels 1 hr later. There was no seasonal variation in the amplitude or in the offset of the melatonin peak or in the daytime basal levels. The onset of the nocturnal peak was delayed by 2 hr in June at the summer solstice (P < 0.05), which can be related to the changes in night length between the two solstices. A significant effect of age was observed in all seasons. Melatonin levels were higher in the young camel group (fall equinox: P < 0.001; spring equinox: P < 0.01; winter solstice: P < 0.01; summer solstice: P < 0.05). The pattern of melatonin secretion in the camel showed a significant seasonal variation parallel to the photoperiodic changes of the year. The observed decline of melatonin levels during an extra-light pulse in the middle of the night indicates the light control of melatonin synthesis. It is not yet known if, in this low latitude desert region, the seasonal breeding period of the camel is cued by the photoperiod. The data obtained, however, clearly demonstrate that the camel has the capacity to follow and to integrate photoperiodic changes through melatonin changes.

Influence of Light Intensity on Plasma Melatonin and Locomotor Activity Rhythms in Tench

Melatonin production by the pineal organ is influenced by light intensity, as has been described in most vertebrate species, in which melatonin is considered a synchronizer of circadian rhythms. In tench, strict nocturnal activity rhythms have been described, although the role of melatonin has not been clarified. In this study we investigated daily activity and melatonin rhythms under 12∶12 light‐dark (LD) conditions with two different light intensities (58.6 and 1,091 µW/cm2), and the effect of 1 h broad spectrum white light pulses of different intensities (3.3, 5.3, 10.5, 1,091.4 µW/cm2) applied at middarkness (MD) on nocturnal circulating melatonin. The results showed that plasma melatonin in tench under LD 12∶12 and high light conditions displayed rhythmic variation, where values at MD (255.8±65.9 pg/ml) were higher than at midlight (ML) (70.7±31.9 pg/ml). Such a difference between MD and ML values was reduced in animals exposed to LD 12∶12 and low light intensity. The application of 1 h light pulses at MD lowered plasma melatonin to 111.6±3.2 pg/ml (in the 3.3–10.5 µW/cm2 range) and to 61.8±18.3 pg/ml (with the 1,091.4 µW/cm2 light pulse) and totally suppressed nocturnal locomotor activity. These results show that melatonin rhythms persisted in tench exposed to low light intensity although the amplitude of the rhythm is affected. In addition, it was observed that light pulses applied at MD affected plasma melatonin content and locomotor activity. Such a low threshold suggests that the melatonin system is capable of transducing light even under dim conditions, which may be used by this nocturnal fish to synchronize to weak night light signals (e.g., moonlight cycles).

Effects of moonlight exposure on plasma melatonin rhythms in the seagrass rabbitfish, Siganus canaliculatus.

Influences of light-dark (LD) cycle and moonlight exposure on plasma melatonin rhythms in the seagrass rabbitfish, Siganus canaliculatus, a lunar synchronized spawner, were determined by time-resolved fluoroimmunoassay (TR-FIA). When the fish were exposed to a natural LD (12:12) cycle, plasma melatonin levels exhibited a clear daily rhythm, with higher levels at midnight and lower levels during the day. These rhythms were not evident under either constant light (LL) or constant dark (DD) conditions. Plasma melatonin levels under LL condition were low and high under DD condition. These results indicate that plasma melatonin rhythms are driven by LD cycle in this species. When the fish were exposed to the 4 lunar phases, plasma melatonin levels around the new moon were significantly higher than during the first quarter moon and the full moon. Exposure to experimental new moon and full moon conditions caused significant increases and decreases of plasma melatonin levels, respectively. The synchronous rhythmicity of melatonin levels in the plasma support the hypothesis that the seagrass rabbitfish perceives moonlight intensity and responds with secretion of melatonin into the bloodstream.

Effect of tryptophan administration on circulating levels of melatonin and phagocytic activity

Our research group has previously studied the role of melatonin in the immune system of birds and mice, finding that incubation with both pharmacological and physiological doses of melatonin augmented the activity of phagocytes from these animals, and that this activity was lowered in pinealectomized animals. Since melatonin is synthesized from the amino acid tryptophan, the aim of the present work was to determine whether the administration of tryptophan might affect the plasma levels of melatonin and the phagocytic activity of peritoneal macrophages over the course of a circadian cycle. The study animals were 14-week-old male Wistar rats. They were administered tryptophan orally in a daily single dose of 125 mg/kg at 19:00 h for 21 days. Prior to beginning this treatment, the circadian rhythms of plasma melatonin and phagocytic activity were evaluated under basal conditions over a 24-h period, taking blood and cell suspension samples each 2 hours during the light period (08:00-20:00) and each hour during the dark period (20:00-08:00), since it is during this latter period that the secretion of melatonin is maximum. The results showed that, under basal conditions, the rats' plasma melatonin levels and phagocytic activity peaked at 02:00. After the tryptophan administration, there were increases in plasma melatonin levels with respect to basal and control-group values, with a peak at 21:00, and in the phagocytic activity of the peritoneal macrophages, which peaked at 02:00. This suggests that the tryptophan administration stimulated melatonin synthesis, leading to increased and earlier peaking plasma levels of this hormone, and augmented the innate immune response carried out by the peritoneal macrophages as a result of the immunoregulatory action of melatonin.

Daily locomotor activity and melatonin rhythms in Senegal sole ( Solea senegalensis)

The daily locomotor and melatonin rhythms of the Senegal sole, a benthonic species of increasing interest in aquaculture, are still unknown, despite the fact that such knowledge is of prime importance for optimising its production. The aim of the present research was therefore to investigate the daily rhythms of locomotor activity and melatonin in the Senegal sole. For this purpose, the individual locomotor activity rhythms of fish were registered using a photocell. Plasma and ocular melatonin rhythms were studied in animals reared in circular tanks placed in earth under an LD 12:12 light regime and 16–18 °C temperature range (spring equinox). Blood and eye samples were taken every 3 h during a complete 24-h cycle. The impact of a light pulse in the middle of the dark period (MD) on plasma melatonin was also studied. Locomotor activity was mainly nocturnal, with 84.3% of the total activity occurring during darkness. The levels of plasma melatonin were higher at night (55 pg/ml) than during the day (2 pg/ml), while ocular melatonin levels appeared to be arrhythmic. Both weight and melatonin content were found to be significantly higher in the left eye in relation to the right eye. A light pulse in MD provoked a significant decrease in plasma melatonin levels. In summary, photoperiod is a key factor in synchronising locomotor activity and melatonin rhythms in the Senegal sole, whose nocturnal habits should be taken into account for their rearing by aquaculture.

Non-photic entrainment of human rest-activity cycle independent of circadian pacemaker

Entrainment of rest-activity cycles and plasma melatonin rhythms by a forced schedule was examined in humans who were held in temporal isolation with dim light conditions (<10 lx). The sleep schedule of subjects was phase-advanced by approximately 8 h from the habitual sleep time for 8 days, which was followed by a free-running session where the subjects self-determined the times of sleep and wake up for 6 days. As a result, the rest-activity cycle was advanced by 8.2 h on the first day of free-run after the forced schedule, whereas the circadian melatonin rhythm was not significantly shifted on the last day of the schedule. The findings indicate that the forced sleep schedule of 8 days entrain the rest-activity cycle but not the melatonin rhythm. Under free-running conditions, the rest-activity cycles resynchronized with the melatonin rhythm by either phase-advance or phase-delay shifts. The direction and extent of phase-shift depended on the phase relationship between the rest-activity cycle and plasma melatonin rhythm one cycle before the phase-shift. The findings indicate the existence of an oscillatory coupling between the rest-activity cycle and the plasma melatonin rhythm. It is concluded that non-photic zeitgebers can entrain the rest-activity cycle in humans, which is independent of the circadian pacemaker.

Sleep and Plasma Melatonin Rhythm in Subjects under Isolation with Dim Light: Effects of Physical Exercise

Sleep and Plasma Melatonin Rhythm in Subjects under Isolation with Dim Light: Effects of Physical Exercise

Human circadian melatonin rhythm phase delay during a fixed sleep-wake schedule interspersed with nights of sleep deprivation.

The human circadian pacemaker, with an intrinsic period between 23.9 and 24.5 hr, can be reset by low levels of light. Biomathematical models of the human clock predict that light-dark cycles consisting of only approximately 3.5 lux during 16 hr of wakefulness and 0 lux during 8 hr of sleep should entrain approximately 45% of the population. However, under real-life conditions, sleep-wake schedules and the associated light-dark exposures are often irregular. It remains unclear whether the phase of the pacemaker would remain stable under such conditions. We investigated the stability of the circadian phase in dim light by assessing the plasma melatonin rhythm during nine consecutive circadian cycles. Ten subjects were scheduled to sleep for 8 hr (0.03 lux) and to be awake for 16 hr (5-13 lux) during all days except on days 4 and 8, during which the subjects were sleep deprived for 40 hr (5-13 lux), either in a sitting/standing or supine body posture. In all subjects, the phase of the melatonin rhythm occurred at a later clock time on day 9 than on day 2 (average delay: 1.4 hr). Largest delays in the melatonin onset were observed in subjects with low amplitude melatonin rhythms. The area under the curve during active melatonin secretion was significantly reduced when subjects were sleep deprived in the 40-hr supine body posture condition compared with either the 40-hr sitting/standing sleep deprivation (SD) or the ambulatory condition under non-SD conditions. Posture differences did not significantly affect the relative phase position of the melatonin profiles. The data indicate that under conditions of reduced zeitgeber strength, the phase of the human circadian pacemaker, using plasma melatonin as a marker, can be phase delayed by one night of SD and the associated dim light exposure.

Diel changes in plasma melatonin and corticosterone concentrations in tropical Nazca boobies ( Sula granti) in relation to moon phase and age

We investigated the effects of moon phases and age on diel rhythms of plasma melatonin and corticosterone in free-living Nazca boobies ( Sula granti) on the Galápagos Islands, Ecuador. Melatonin and corticosterone secretion are regulated by the circadian system and the two hormones play a role in the control of locomotor activity and foraging, which can be influenced by moon phases. These seabirds have a long life span and in many vertebrates circadian function deteriorates with age. The functioning of the circadian system under different environmental conditions and changes related to age are poorly understood and hardly studied in wild birds. Nazca boobies had generally low plasma melatonin concentrations but showed a diel variation with higher concentrations at 00:00 and 16:00 h. The diel variations in melatonin concentrations disappeared during full moon, suggesting that natural light levels at night can suppress melatonin secretion in Nazca boobies. Maximal melatonin concentrations tended to decline in older birds (10–19 years). Birds showed a clear diel variation in basal plasma corticosterone with a peak in the early morning, before the active period begins, and low concentrations throughout the day. As with melatonin, there were no diel variations in corticosterone at full moon, which may be due to different activity patterns in response to food availability or changes in the circadian system. While other studies have found a relationship between corticosterone and melatonin, we found no such correlation in Nazca boobies. The lunar cycle appears to affect the hormone titers of Nazca boobies both directly and indirectly. First, melatonin rhythms can be directly affected by the light intensity associated with full moon. Second, prey availability may change foraging patterns and can therefore indirectly alter corticosterone secretion in Nazca boobies.

Comparison of a westward flight and an eastward flight in the circadian rhythms of plasma melatonin

We estimated the re-entrainment of plasma melatonin rhythm after an eastward flight to New York in 2000 and after a westward flight to London in 2001. Blood samples were taken at 1.5 h intervals for 24-h from eight subjects once before the flight and twice after the flight. They were exposed to natural zeitgeber outdoors. The symptoms of six subjects were moderate in London. All subjects showed the orthodromic re-entrainment, and the phase shift was small (46.3 min/day) in the westward flight. It seemed that the direction of flight was as important for jet lag symptoms as was sunlight exposure.

Separation of circadian and wake duration-dependent modulation of EEG activation during wakefulness

The separate contribution of circadian rhythmicity and elapsed time awake on electroencephalographic (EEG) activity during wakefulness was assessed. Seven men lived in an environmental scheduling facility for 4 weeks and completed fourteen 42.85-h ‘days’, each consisting of an extended (28.57-h) wake episode and a 14.28-h sleep opportunity. The circadian rhythm of plasma melatonin desynchronized from the 42.85-h day. This allowed quantification of the separate contribution of circadian phase and elapsed time awake to variation in EEG power spectra (1–32 Hz). EEG activity during standardized behavioral conditions was markedly affected by both circadian phase and elapsed time awake in an EEG frequency- and derivation-specific manner. The nadir of the circadian rhythm in alpha (8–12 Hz) activity in both fronto-central and occipito-parietal derivations occurred during the biological night, close to the crest of the melatonin rhythm. The nadir of the circadian rhythm of theta (4.5–8 Hz) and beta (20–32 Hz) activity in the fronto-central derivation was located close to the onset of melatonin secretion, i.e. during the wake maintenance zone. As time awake progressed, delta frequency (1–4.5 Hz) and beta (20–32 Hz) activity rose monotonically in frontal derivations. The interaction between the circadian and wake-dependent increase in frontal delta was such that the intrusion of delta was minimal when sustained wakefulness coincided with the biological day, but pronounced during the biological night. Our data imply that the circadian pacemaker facilitates frontal EEG activation during the wake maintenance zone, by generating an arousal signal that prevents the intrusion of low-frequency EEG components, the propensity for which increases progressively during wakefulness.

Phase-dependent effect of room light exposure in a 5-h advance of the sleep-wake cycle: implications for jet lag.

The acute disruption in sleep quality, vigilance levels, and cognitive and athletic performance observed after transmeridian flights is presumed to be the result of a transient misalignment between the endogenous circadian pacemaker and the shifted sleep schedule. Several laboratory and field experiments have demonstrated that exposure to bright artificial light can accelerate circadian entrainment to a shifted sleep-wake schedule. In the present study, the authors investigated whether the schedule of exposure to indoor room light, to which urban dwellers are typically exposed, can substantially affect circadian adaptation to a simulated eastward voyage. We enrolled 15 healthy young men in a laboratory simulation of a Montreal-to-London voyage. Subjects were exposed to 6 h of room light (mean +/- SD: 379+/-10) prior to bedtime (n = 7) or when on a progressively advancing schedule (n = 8) early in the day. The remaining 10 hours of wakefulness were spent in dim light (4+/-1 lux). Circadian assessments, performed via the constant routine procedure, evaluated the phase of the endogenous circadian rhythms of core body temperature and plasma melatonin before and after 1 week on the shifted schedule. At the end of the study, only subjects exposed to room light on the advancing schedule expressed oscillations of the endogenous circadian pacemaker in phase with the new sleep-wake cycle. In this group, a mean advance shift of the nadir of core body temperature of +5:22+/-0:15 h was observed, with parallel shifts in plasma melatonin concentration and subjective alertness. The circadian rhythms of subjects exposed to room light later in the day remained much more adjusted to the departure than to the destination time zone. These results demonstrate that the schedule of exposure to room light can substantially affect circadian adaptation to a shifted sleep-wake schedule.

Melatonin alleviates jet lag symptoms caused by an 11‐hour eastward flight

The effect of 3 mg of melatonin on the rate of re-entrainment of plasma melatonin rhythm after an 11-h eastward flight was assessed. Eight subjects participated in the study, and underwent 24-h blood samplings once before the flight and twice after the flight. Subjects were exposed to natural zeitgeber outdoors and took 3 mg of malatonin at 20:00 h local time on the days when no blood sampling was done. Antidromic re-entrainment was dominant whereby melatonin administration in the evening promoted re-entrainment. Melatonin accelerated the rate of re-entrainment by 15 min per day and alleviated the jet lag symptoms.

Seasonal Variations in Circadian Rhythms of Plasma Melatonin in Ruin Lizards

We examined melatonin profiles of ruin lizards in different seasons (spring, summer, and autumn) under light:dark (LD) and concomitant responses when transferred to continuous darkness (DD) to determine the degree to which previously reported seasonally dependent effects of pinealectomy on locomotor behavior are related to melatonin secretion. The amplitude of the melatonin rhythm and the amount of melatonin produced over 24 h varied with season. In spring, the amount of melatonin produced was greatest and the amplitude 4– 5 times that found in summer or autumn. The degree of self-sustainment of the melatonin rhythm when transferred to DD also varied with season. In DD, melatonin levels remained high but did not exhibit circadian variation in spring. In summer, the melatonin profile persisted virtually unchanged in DD, showing the existence of a circadian rhythm. Finally, in the fall there was no circadian variation in DD and levels remained low. These responses correspond closely to previously reported effects of pinealectomy on locomotor behavior where there is little or no effect of pinealectomy in spring or fall but a profound alteration of locomotor behavior in summer. These results suggest that the seasonally dependent effects of pinealectomy on locomotor behavior in ruin lizards are related to a seasonally mediated change in the degree of self-sustainment of some component of the circadian pace-making system of which melatonin plays some role.

Supplementary Dim Light Differentially Influences Sexual Maturity, Oviposition Time, and Melatonin Rhythms in Pullets

The addition of two 3-h periods of very dim light, one before and one after a normal 8-h photoperiod, advances sexual maturity in pullets by about a week. This trial tested the hypothesis that dim light given before a short day of normal intensity is linked to form a more stimulatory day length and that dim light given after it is photosexually ignored. Pullets were reared from 2 d of age on 8-h photoperiods. From 10 wk, they were continued on 8-h photoperiods, transferred to 16 h, or given an 8-h period of dim light (0.09 lx) immediately before or after the main 8-h photoperiod. The bright/dim and dim/bright groups matured at the same age, thus disproving the hypothesis tested. Both groups matured 1 wk earlier than the 8-h controls but 5 wk later than birds transferred to 16-h photoperiod. Oviposition time was similar for 8-h controls and bright/dim hens and delayed by 3 h for 16-h birds, but phase advanced by 2.4 h for dim/bright hens. Plasma melatonin rhythm was phase-advanced by about 5 h in the dim/bright hens and retarded by about 5 h in the bright/dim hens, suggesting a 13-h subjective day. However, these treatments were not regarded as fully stimulatory, as a transfer to a normal 13-h photoperiod at this age advances maturity by 5 to 6 wk. These findings show that the addition of a period of dim light to a normal nonstimulatory photoperiod differentially affects the clocks that control sexual maturation, plasma melatonin concentration, and oviposition time.

The start of the quiescent period of cortisol remains phase locked to the melatonin onset despite circadian phase alterations in humans working the night schedule

Using a 10-min blood sampling procedure, we established 24-h plasma melatonin and cortisol rhythms in 11 night workers and determined whether the extent in the shift of the melatonin onset, highly variable among night workers, was reflected in the shift of the markers of the cortisol rhythm, i.e. the quiescent period of secretion and the acrophase. In all day-active subjects, the melatonin onset occurred during low cortisol secretion, with a time lag between the start of the quiescent period and the melatonin onset of 1 h 28±27 min. In night workers, whatever the shift of the melatonin surge, the start of the quiescent period of cortisol secretion remained phase locked to the melatonin onset with a similar time lag (1 h 25±27 min). There was a significant correlation between the timing of the melatonin onset and the timing of the start of the quiescent period ( r=0.88; P=0.0072). No preserved time lag was found between the melatonin onset and the other cortisol phase markers, either with the end of the quiescent period or with the acrophase. These results settle the start of the quiescent period of cortisol and the melatonin onset as two coordinate markers, and suggest that each of them are reliable to assess circadian phase in humans.