Acute finger temperature changes preceding sleep onsets over a 45-h period

A resurgence in the field of sleep initiation and thermoregulation has seen a number of investigators reporting relatively gradual increases in distal skin temperatures of the hands and feet prior to sleep onset at typical bedtimes. The present study extends upon prior knowledge by investigating whether: (1) this is a change of distal skin temperature triggered specifically by the attempt to fall asleep and (2) whether this relationship holds for various phases of the circadian rhythm whenever sleep is attempted. Fourteen healthy good sleepers participated in a modified 45-h constant routine (CR) with multiple sleep onset latency tests (MSLT) conducted every half hour. After a brief decrease of finger temperature associated with small postural adjustments at the beginning of each sleep latency test, finger temperature showed rapid (0.8 degrees C min-1) increases of 1-3 degrees C leading up to the onset of sleep. This rapid increase of finger temperature was relatively consistent across the 45-h CR, despite very significant circadian variation of the pre-MSLT baseline finger temperature and homeostatic decrease of sleep latency.     

Lengthening of the photoperiod influences sleep characteristics before and during total sleep deprivation in rat

The photoperiod has been evidenced to influence sleep regulation in the rat. Nevertheless, lengthening of the photoperiod beyond 30 days seems to have little effect on the 24‐hr baseline level of sleep and the response to total sleep deprivation. We studied the effects of 12:12 (habitual) and 16:8 (long) light–dark photoperiods on sleep, locomotor activity and body core temperature, before and after 24 hr of total sleep deprivation. Eight rats were submitted for 14 days to light–dark 12:12 (lights on: 08:00 hours–20:00 hours) followed by total sleep deprivation, and then for 14 days to light–dark 16:8 (light extended to 24:00 hours) followed by total sleep deprivation. Rats were simultaneously recorded for electroencephalogram, locomotor activity and body core temperature for 24 hr before and after total sleep deprivation. At baseline before total sleep deprivation, total sleep time and non‐rapid eye movement sleep per 24 hr and during extended light hours (20:00 hours–24:00 hours) were higher (13% for total sleep time) after light–dark exposure compared with habitual photoperiod, while percentage delta power in non‐rapid eye movements and rapid eye movements were unchanged. Locomotor activity and body core temperature were lower, particularly during extended light hours (20:00 hours–24:00 hours). Following total sleep deprivation, total sleep time and non‐rapid eye movements were significantly lower after long photoperiod between 20:00 hours and 24:00 hours, and between 10:00 hours and 12:00 hours, and unchanged per 24 hr. The percentage delta power in non‐rapid eye movements was lower between 08:00 hours and 11:00 hours. Total sleep deprivation decreased locomotor activity and body core temperature after habitual photoperiod exposure only. Fourteen days under long photoperiod (light–dark 16:8) increased non‐rapid eye movements sleep, and decreased sleep rebound related to total sleep deprivation (lower non‐rapid eye movements duration and delta power). This may create a model of sleep extension for the rat that has been found to favour anabolism in the brain and the periphery.     

Plasma melatonin rhythms in young and older humans during sleep, sleep deprivation, and wake

STUDY OBJECTIVES: To determine the effects of sleep and sleep deprivation on plasma melatonin concentrations in humans and whether these effects are age-dependent. DESIGN: At least 2 weeks of regular at-home, sleep/wake schedule followed by 3 baseline days in the laboratory and at least one constant routine (sleep deprivation). SETTING: General Clinical Research Center (GCRC), Brigham and Women's Hospital, Boston, MA. PARTICIPANTS: In Study 1, one group (<10 lux when awake) of 19 young men (18-30 y) plus a second group (<2 lux when awake) of 15 young men (20-28 y) and 10 young women (19-27 y); in Study 2, 90 young men (18-30 y), 18 older women (65-81 y), and 11 older men (64-75 y). All participants were in good health, as determined by medical and psychological screening. INTERVENTIONS: One to three constant routines with interspersed inversion of the sleep/wake cycle in those with multiple constant routines. MEASUREMENTS AND RESULTS: Examination of plasma melatonin concentrations and core body temperature. Study 1. There was a small, but significant effect of sleep deprivation of up to 50 hours on melatonin concentrations (increase of 9.81 +/- 3.73%, P <0.05, compared to normally timed melatonin). There was also an effect of circadian phase angle with the prior sleep episode, such that if melatonin onset occurred <8 hours after wake time, the amplitude was significantly lower (22.4% +/- 4.79%, P <0.001). Study 2. In comparing melatonin concentrations during sleep to the same hours during constant wakefulness, in young men, melatonin amplitude was 6.7% +/- 2.1% higher(P <0.001) during the sleep episode. In older men, melatonin amplitude was 37.0% +/- 12.5% lower (P <0.05) during the sleep episode and in older women, melatonin amplitude was non-significantly 10.9% +/- 8.38% lower (P = 0.13) during the sleep episode. CONCLUSIONS: Both sleep and sleep deprivation likely influence melatonin amplitude, and the effect of sleep on melatonin appears to be age dependent.     

The effect of 40 h constant wakefulness on task-switching efficiency

This study investigated efficiency of switching between different tasks in 12 male participants (19–30 years) during 40 h of constant wakefulness. As index of task-switching efficiency, switch costs in reaction time were assessed every 3 h under controlled behavioural and environmental conditions. Overall reaction times and switch costs showed a temporal pattern consistent with the assumption of a combined influence of a sleep homeostatic and a circadian process. An additional analysis indicated that the variation in switch costs could not be attributed to interference of the current task with persisting activation from preceding tasks. We therefore conclude that sleep loss and the circadian system affect the ability to prepare the current task rather than automatic processing of irrelevant stimulus information.     

Pattern of desynchronized sleep during deprivation and recovery induced in the rat by changes in ambient temperature

The pattern of desynchronized sleep (DS) occurrence in the rat was studied during exposure to an ambient temperature (Ta) of 0 degrees C for 48 h and during a 12 h recovery period at laboratory Ta (23 degrees C) following the first and second 24 h of cold exposure. The exposure to low Ta induces a DS deprivation which is followed, during recovery, by a clear DS rebound. Both the decrease and the following increase in the amount of DS are due to changes in the frequency rather than in the duration of DS episodes. The frequency distribution of the intervals between the end of one DS episode and the beginning of the next (DS interval) has shown that two populations of DS intervals exist, i.e. short DS intervals (3 min). On the basis of this, two types of DS episodes have been identified: the 'single DS episode', which is both preceded and followed by a long DS interval, and the 'sequential DS episode', which is a DS episode occurring within a cluster or a sequence of DS episodes and is characteristically separated by short DS intervals. The occurrence of such sequential DS episodes in a 'DS cluster', allows a high amount of DS to occur without increasing the duration of the DS episode. DS clusters are repressed during cold exposure, when the DS drive is counteracted by the need to thermoregulate, and enhanced during recovery, when the DS drive is unrestrained. In contrast, the occurrence of single DS episodes is much less affected by such different experimental conditions.     

The effect of the timing, quality and quantity of sleep upon the depression (masking) of body temperature on an irregular sleep/wake schedule

SUMMARY  Eight subjects were studied on an irregular sleep/wake schedule which was designed so that prior wake time and the time of day when 4-h sleep periods were taken were both balanced. Rectal temperature and the sleep EEG were measured throughout the 8-d protocol. A purification method was used to estimate the depression of rectal temperature (masking) caused by the first and second halves of each 4-h sleep period. On average this was 0.187 ± 0.015°C during the first 2h and 0.262 ± 0.017°C during the second 2h. Masking increases beyond the first 2 h of sleep, but any effects due to the phase of the rectal temperature when sleep is taken are very weak. However, masking did increase (that is, the temperature was depressed more) when the amount of prior wake time was greater than 4 h. When the effects of sleep content variables were considered also, masking was still greater in the second half of sleep and tended to increase with the amount of slow-wave sleep, but it decreased with increasing amounts of time awake during a sleep period (sleep latency or sleep discontinuity). Some implications of these results for the mechanism of sleep-induced masking are considered.     

The effect of 60-h sleep deprivation on cardiovascular regulation and body temperature

This study examined cardiovascular regulation and body temperature (BT) during 60 h of sleep deprivation in 20 young healthy cadets. Heart rate variability was measured during an active orthostatic test (AOT). Measurements were performed each day in the morning and evening after 2, 14, 26, 38, 50 and 60 h of sleep deprivation. In AOT, in the sitting and standing positions, heart rate decreased (P < 0.001), while high frequency and low frequency power increased (P < 0.05–0.001) during sleep deprivation. Body temperature also decreased (P < 0.001), but no changes were detected in blood pressure. In conclusion, the accumulation of 60 h of sleep loss resulted in increased vagal outflow, as evidenced by decreased heart rate. In addition, BT decreased during sleep deprivation. Thus, sleep deprivation causes alterations in autonomic regulation of the heart, and in thermoregulation.     

Effects of prolonged wakefulness on cyclic alternating pattern (CAP) during sleep recovery at different circadian phases

Two separate groups of healthy subjects aged between 20 and 30 years underwent a random sequence of two non-consecutive polysomnographic recordings under standard conditions (night basal sleep) and after continuous sleep deprivation (recovery sleep). In the first group of 6 subjects (3 males and 3 females) recovery sleep occurred in the morning (after 24 h of prior waking); in the second group of 6 subjects (3 males and 3 females) recovery sleep occurred in the night (after 36 h of prior waking). In all cases the recording time was restricted to 500 minutes. Scoring was accomplished on conventional sleep variables and on Cyclic Alternating Pattern (CAP) parameters, while statistical analysis was based on a 2 x 2 ANOVA test. Compared to the night basal conditions, total sleep time and total NREM sleep were significantly longer in night recovery sleep and shorter in morning recovery sleep, respectively. No significant differences were found for sleep latency, intra-sleep awakenings, stage 2, REM sleep, NREM stages and slow-wave sleep. Total CAP time, CAP time in slow-wave sleep and all CAP rates were significantly higher in morning recovery sleep and lower in night recovery sleep. The enhanced amounts of CAP time and CAP rates during morning recovery sleep may be the outcome of two opposite forces, i.e. high sleep pressure versus maximum wake propensity. In contrast, the lower values of CAP during night recovery sleep suggest an in-phase associations between strong sleep pressure and the circadian clock.     

Individual differences in the phase and amplitude of the human circadian temperature rhythm: with an emphasis on morningness-eveningness

We studied the relationship between the phase and the amplitude of the circadian temperature rhythm using questionnaires that measure individual differences in personality variables, variables that relate to circadian rhythms, age and sex. The ambulatory core body temperature of 101 young men and 71 young women was recorded continuously over 6 days. The temperature minimum (Tmin) and amplitude (Tamp) were derived by fitting a complex cosine curve to each day's data for each subject. Participants completed the Horne-Ostberg Morningness-Eveningness Questionnaire (MEQ), the Circadian Type Inventory (CTI) and the MMPI-2, scored for the Psychopathology-5 (PSY-5) personality variables. We found that the average Tmin occurred at 03.50 h for morning-types (M-types), 05.02 h for the neither-types and 06.01 h for evening-types (E-types). Figures were presented that could provide an estimate of Tmin given an individual's morningness-eveningness score or weekend wake time. The Tmin occurred at approximately the middle of the 8-h sleep period, but it occurred closer to wake in subjects with later Tmin values and increasing eveningness. In other words, E-types slept on an earlier part of their temperature cycle than M-types. This difference in the phase-relationship between temperature and sleep may explain why E-types are more alert at bedtime and sleepier after waking than M-types. The Tmin occurred about a half-hour later for men than women. Another interesting finding included an association between circadian rhythm temperature phase and amplitude, in that subjects with more delayed phases had larger amplitudes. The greater amplitude was due to lower nocturnal temperature.     

The light/dark difference in meal size in the laboratory rat on a standard diet is abolished during REM sleep deprivation

Using a modified flowerpot technique, by which it was possible to use test animals as their own controls, the meal patterns of eight adult male rats were recorded before, during and after rapid eye movement sleep (REMs) deprivation. The results demonstrated that the prominent light/dark (LD) difference in meal size, typical of the baseline meal pattern, was abolished during REMs deprivation. This equalization was entirely due to reduction in meal size during the dark hours. After termination of REMs deprivation the size of the dark meals increased almost to the baseline level. However, the LD difference was not restored since the size of the light meals was also significantly increased. Changes in meal frequency were confined to the period of REMs deprivation. The possible role of REMs as a modulator of meal size in relation to the LD cycle of illumination is discussed.     

Mechanisms underling the effects of adaptogens on erythropoiesis during paradoxical sleep deprivation

We studied the effects of adaptogens extracts of Siberian ginseng, Rhodiola rosea, bergenia, and ginseng and pantohematogen, on erythropoiesis after paradoxical sleep deprivation. Adaptogens stimulated bone marrow erythropoiesis in the early stage, but decreased the count of bone marrow erythrokaryocytes 3-7 days after treatment. The effect of adaptogens on erythropoiesis is associated with modulation of the state of brain neurotransmitter systems followed by changes in functional activity of cells in the hemopoiesis-inducing microenvironment.     

Effects of playing a computer game using a bright display on presleep physiological variables, sleep latency, slow wave sleep and REM sleep

Epidemiological studies have shown that playing a computer game at night delays bedtime and shortens sleeping hours, but the effects on sleep architecture and quality have remained unclear. In the present study, the effects of playing a computer game and using a bright display on nocturnal sleep were examined in a laboratory. Seven male adults (24.7+/-5.6 years old) played exciting computer games with a bright display (game-BD) and a dark display (game-DD) and performed simple tasks with low mental load as a control condition in front of a BD (control-BD) and DD (control-DD) between 23:00 and 1:45 hours in randomized order and then went to bed at 2:00 hours and slept until 8:00 hours. Rectal temperature, electroencephalogram (EEG), heart rate and subjective sleepiness were recorded before sleep and a polysomnogram was recorded during sleep. Heart rate was significantly higher after playing games than after the control conditions, and it was also significantly higher after using the BD than after using the DD. Subjective sleepiness and relative theta power of EEG were significantly lower after playing games than after the control conditions. Sleep latency was significantly longer after playing games than after the control conditions. REM sleep was significantly shorter after the playing games than after the control conditions. No significant effects of either computer games or BD were found on slow-wave sleep. These results suggest that playing an exciting computer game affects sleep latency and REM sleep but that a bright display does not affect sleep variables.     

Effect of prolonged wakefulness on electroencephalographic oscillatory activity during sleep

The human sleep electroencephalogram (EEG) is characterized by the occurrence of distinct oscillatory events such as delta waves, sleep spindles and alpha activity. We applied a previously proposed algorithm for the detection of such events and investigated their incidence and frequency in baseline and recovery sleep after 40 h of sustained wakefulness in 27 healthy young subjects. The changes in oscillatory events induced by sleep deprivation were compared to the corresponding spectral changes. Both approaches revealed, on average, an increase in low frequency activity and a decrease in spindle activity after sleep deprivation. However, the increase of oscillatory events in the delta range and decrease in the sigma range occurred in a more restricted frequency range compared to spectral changes. The mean relative power spectra showed a significant increase in theta and alpha activity after sleep deprivation while, on average, the event analysis showed only a weak effect in the theta band. The reason for this discrepancy is that the spectral analysis does not distinguish between diffuse activity and clearly visible temporally localized oscillations, while the event analysis would detect only the latter. Additionally, only a few individuals clearly showed activity in the theta or alpha frequency bands. Conversely, event analysis revealed that some individuals showed an increased rate of sleep spindles after sleep deprivation, a fact that was not evident in the relative power spectra due to a decrease in background activity. The two methods complement each other and facilitate the interpretation of distinct changes induced by prolonged wakefulness in sleep EEG.     

Body temperature,activity and melatonin profiles in adults with attention‐deficit/hyperactivity disorder and delayed sleep: a case–control study

Irregular sleep–wake patterns and delayed sleep times are common in adults with attention‐deficit/hyperactivity disorder, but mechanisms underlying these problems are unknown. The present case–control study examined whether circadian abnormalities underlie these sleep problems in a naturalistic home setting. We included 12 medication‐naïve patients with attention‐deficit/hyperactivity disorder and delayed sleep phase syndrome, and 12 matched healthy controls. We examined associations between sleep/wake rhythm in attention‐deficit/hyperactivity disorder and circadian parameters (i.e. salivary melatonin concentrations, core and skin temperatures, and activity patterns) of the patients and controls during five consecutive days and nights. Daily bedtimes were more variable within patients compared with controls (F = 8.19, P < 0.001), but melatonin profiles were equally stable within individuals. Dim‐light melatonin onset was about 1.5 h later in the patient group (U = 771, Z = ?4.63, P < 0.001). Patients slept about 1 h less on nights before work days compared with controls (F = 11.21, P = 0.002). The interval between dim‐light melatonin onset and sleep onset was on average 1 h longer in patients compared with controls (U = 1117, Z = ?2.62, P = 0.009). This interval was even longer in patients with extremely late chronotype. Melatonin, activity and body temperatures were delayed to comparable degrees in patients. Overall temperatures were lower in patients than controls. Sleep‐onset difficulties correlated with greater distal–proximal temperature gradient (DPG; i.e. colder hands, r² = ?0.32, P = 0.028) in patients. Observed day‐to‐day bedtime variability of individuals with attention‐deficit/hyperactivity disorder and delayed sleep phase syndrome were not reflected in their melatonin profiles. Irregular sleep–wake patterns and delayed sleep in individuals with attention‐deficit/hyperactivity disorder and delayed sleep phase syndrome are associated with delays and dysregulations of the core and skin temperatures.     

Daily rhythm of locomotor activity is abolished during rapid eye movement sleep deprivation in the rat

Applying a modified flowerpot technique, which made it possible to use a test animal as its own control, twenty-four hour cycles of locomotor activity were recorded in eight juvenile male rats on 12/12 hr light/dark (LD) schedule during six days of rapid eye movement (REM) sleep deprivation. It was found that the LD difference in locomotor activity unrelated to feeding was instantaneously abolished during REM sleep deprivation. The daily rhythm of food-directed activity, however, was only gradually attenuated. Due to this equalisation in the light and dark activity the rats gave an impression of hyperactivity during the light hours although the total daily motor output after an initial increase returned close to the baseline value.     

Effect of the diurnal rhythm and 24 h of sleep deprivation on dichotic temporal order judgment

The present study examined the impact of mild (24 h) sleep deprivation and of the circadian rhythm on auditory temporal resolution, measured by dichotic temporal order judgment (TOJ). The rationale for the present study was based on several areas of research. First, the 'sleep-based neuropsychological perspective' hypothesis posits that sleep reduction initially impacts the functions associated with intact prefrontal cortical activity, e.g. language tasks. Secondly, recent studies indicate the importance of the role of auditory temporal resolution in speech comprehension. Thirdly, there is accumulating evidence of the involvement of prefrontal cortical structures in auditory temporal resolution. We hypothesized that mild to moderate sleep deprivation would affect dichotic TOJ negatively. The results showed that: (1) 24 h of sleep deprivation significantly reduced the overall level of accuracy in dichotic TOJ and increased dichotic TOJ threshold from 57.61 ms to 73.93 ms, a reduction in temporal resolution of 28.3%; (2) dichotic TOJ was subject to a small, but significant diurnal rhythm having a nadir in early to mid afternoon. As auditory temporal resolution of speech and non-speech sounds seems to be dependent on intact functioning of the left inferior and left dorso-lateral prefrontal cortex (PFC), these data strengthen the argument that even mild to moderate sleep deprivation can impact negatively on PFC-dependent functions. Furthermore, based on these findings, we suggest that the deficit in auditory temporal resolution in individuals suffering from sleep loss may also affect language comprehension.     

Event-related activity and phase locking during a psychomotor vigilance task over the course of sleep deprivation

There is profound knowledge that sleep restriction increases tonic (event‐unrelated) electroencephalographic (EEG) activity. In the present study we focused on time‐locked activity by means of phasic (event‐related) EEG analysis during a psychomotor vigilance task (PVT) over the course of sleep deprivation. Twenty healthy subjects (10 male; mean age ± SD: 23.45 ± 1.97 years) underwent sleep deprivation for 24 h. Subjects had to rate their sleepiness hourly (Karolinska Sleepiness Scale) and to perform a PVT while EEG was recorded simultaneously. Tonic EEG changes in the δ (1–4 Hz), θ (4–8 Hz) and α (8–12 Hz) frequency range were investigated by power spectral analyses. Single‐trial (phase‐locking index, PLI) and event‐related potential (ERP) analyses (P1, N1) were used to examine event‐related changes in EEG activity. Subjective sleepiness, PVT reaction times and tonic EEG activity (delta and theta spectral power) significantly increased over the night. In contrast, event‐related EEG parameters decreased throughout sleep deprivation. Specifically, the ERP component P1 diminished in amplitude, and delta and theta PLI estimates decreased progressively over the night. It is suggested that event‐related EEG measures (such as the amplitude of the P1 and especially delta/theta phase‐locking) serve as a complimentary method to track the deterioration of attention and performance during sleep loss. As these measures actually reflect the impaired response to specific events rather than tonic changes during sleep deprivation they are a promising tool for future sleep research.     

Effects of amphetamine and modafinil on the sleep/wake cycle during experimental hypersomnia induced by sleep deprivation in the cat

Modafinil is a newly discovered waking substance now being used in the treatment of hypersomnia and narcolepsy. We have shown previously in the cat that, unlike amphetamine, modafinil induces long-lasting wakefulness (W) without behavioral excitation and subsequent sleep rebound, and that its waking effect does not depend on endogenous catecholamines. To further characterize the awakening properties of modafinil and current psychostimulants in experimental models of hypersomnia, we examined the effect of oral administration of placebo, modafinil (5 mg kg-1) or amphetamine (1 mg kg-1) on the sleep/wake cycle and power spectral density (PSD) in cats after an 18-h water-tank sleep deprivation period. We found that the placebo had no effect on the dynamics of sleep recovery, while both modafinil and amphetamine induced suppression of cortical slow activity and a waking state lasting 6-8 h. After the amphetamine-induced waking period, both deep slow wave sleep (SWS2) and paradoxical sleep (PS) occurred in greater amounts than after placebo and the PSD during SWS was also increased. Thus, the cumulative time spent in W during a 48-h period was similar to that with placebo, indicating enhanced sleep rebound. In contrast, after the modafinil-induced W, the occurrence and evolution of SWS2 or PS, as well as the PSD during SWS, were similar to those seen with placebo during the same period, so that the total time spent in W in a 48-h period remained significantly higher than the control level, indicating no additional sleep rebound. These results indicate that modafinil is effective against somnolence and hypersomnia and does not produce a subsequent increase in sleep and suggest that the pharmacological profile of modafinil is different from that of amphetamine.     

Some developmental observations on the effects of prolonged deprivation of low-voltage fast-wave sleep in the deermouse, Peromyscus maniculatus bairdi

Previous electrophysiological work with adult deermice (P. M. Bairdi) (Mitler & Levine, 1970) indicated that: three states of arousal could be reliably distinguished: low-voltage, fast-wave sleep (LVF), high-voltage, slow-wave sleep (HVS), and waking (W);low-electromyograph (EMG) activity was concomitant with LVF; and time in LVF was radically reduced in mice perched above a shock-grid on a pedestal too small to permit total loss of muscle tonus. A preliminary aim of the present research was to gather electrophysiological data on juvenile deermice to construct some estimate of developmental sleep changes. Electrocorticograph (ECoG) and (EMG) records, from three 20-day-old mice while motionless with eyes closed, also reflected LVF, HVS, and W, but in proportions different from those reported by Mitler and Levine. Juvenile mice showed 1/3 more LVF than did adults. The main goal of this research was to explore the effects of long-term LVF-deprivation on juvenile and adult mice. Litters were selected at either 20 or 53 days of age. Littermates were assigned to one of four 14-day treatment conditions in an Age Treatment design with 3 subjects per cell. The 1st treatment, LVF-deprivation, consisted of perching on a pedestal over a shock-grid. The remaining 3 conditions ran simultaneously with the former. Mice in the yoked control condition had approximately equal free movement volume as those mice restricted to the pedestal, and were “yoked” to those animals with respect to shock. Mice in the sleep deprivation condition were confined to a treadmill in motion 19 of each 24 hr. Finally, mice in the isolated condition were caged alone. After treatment, animals were placed in activity recording cages for 21 days. Dependent variables included body weight before treatment, after treatment, and after procedure, brain weight, brain/body weight ratios, activity, and regularity of activity for the 21 days posttreatment. Major findings indicated that animals LVF-deprived from 20 to 34 days of age were more active than controls while animals LVF-deprived from 53 to 67 days of age were less active than controls. There were no interpretable differences among treatments in regularity of activity, body weight, or brain weight. Results were discussed in terms of input requirements for CNS development and/or maintenance. Directions for future research were suggested.     

Evening administration of melatonin and bright light: Interactions on the EEG during sleep and wakefulness

Both the pineal hormone melatonin and light exposure are considered to play a major role in the circadian regulation of sleep. In a placebo- controlled balanced cross-over design, we investigated the acute effects of exogenous melatonin (5 mg p.o. at 20.40 hours) with or without a 3-h bright light exposure (5000 lux from 21.00 hours–24.00 hours) on subjective sleepiness, internal sleep structure and EEG power density during sleep and wakefulness in healthy young men. The acute effects of melatonin, bright light and their interaction were measured on the first day (treatment day), possible circadian phase shifts were assessed on the post-treatment day. On the treatment day, the evening rise in subjective sleepiness was accelerated after melatonin and protracted during bright light exposure. These effects were also reflected in specific changes of EEG power density in the theta/alpha range during wakefulness. Melatonin shortened and bright light increased sleep latency. REMS latency was reduced after melatonin administration but bright light had no effect. Slow-wave sleep and slow-wave activity during the first non-rapid eye movement (NREMS) episode were suppressed after melatonin administration and rebounded in the second NREMS episode, independent of whether light was co-administered or not. Self rated sleep quality was better after melatonin administration whereas the awakening process was rated as more difficult after bright light. On the post-treatment day after evening bright light, the rise in sleepiness and the onset of sleep were delayed, independent of whether melatonin was co-administered or not. Thus, although acute bright light and melatonin administration affected subjective sleepiness, internal sleep structure and EEG power density during sleep and wakefulness in a additive manner, the phase shifting effect of a single evening bright light exposure could not be blocked by exogenous melatonin