The Light-Dosimeter: A new device to help advance research on the non-visual responses to light

This article describes the development of a device to investigate the non-visual responses to light: The Light-Dosimeter (lido). Its multidisciplinary team followed a user-centred approach throughout the project, that is, their design decisions focused on researchers' and participants' needs. Together with custom-made mountings and the software Lido Studio, the lidos provide researchers with a holistic solution to record participants' light exposure in the near-corneal plane in laboratory settings and under real-world conditions. Validation measurements with commercial equipment were deemed satisfying, as was the combining with data from other devices. The handling of the lidos and mountings and the use of the software Lido Studio during the trial period by various researchers and participants were successful. Despite some limitations, the lidos can help advance research on the non-visual responses to light over the coming years.

[Sleep and circadian rhythms in ageing]

With age, the consolidation of nocturnal sleep decreases, daytime napping increases, and sleep occurs earlier. Sleep regulation is dependent on the interaction between a circadian pacemaker (biological clock) and the sleep homeostat (sleep pressure increasing with duration of time awake). We have shown that in the healthy elderly, the amplitude of circadian rhythms (e. g. melatonin secretion) declines, as does slow wave sleep, parallel with an increase in afternoon sleepiness and a tendency to fall asleep in the early evening when younger subjects do not. Light is the major zeitgeber to stabilise the biological clock: older subjects require sufficient light exposure during daytime and in the evening, and should take no or only brief naps during the day to improve sleep.

Role of melatonin in the regulation of human circadian rhythms and sleep

The circadian rhythm of pineal melatonin is the best marker of internal time under low ambient light levels. The endogenous melatonin rhythm exhibits a close association with the endogenous circadian component of the sleep propensity rhythm. This has led to the idea that melatonin is an internal sleep "facilitator" in humans, and therefore useful in the treatment of insomnia and the readjustment of circadian rhythms. There is evidence that administration of melatonin is able: (i) to induce sleep when the homeostatic drive to sleep is insufficient; (ii) to inhibit the drive for wakefulness emanating from the circadian pacemaker; and (iii) induce phase shifts in the circadian clock such that the circadian phase of increased sleep propensity occurs at a new, desired time. Therefore, exogenous melatonin can act as soporific agent, a chronohypnotic, and/or a chronobiotic. We describe the role of melatonin in the regulation of sleep, and the use of exogenous melatonin to treat sleep or circadian rhythm disorders.

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.

Homeostatic control of slow-wave and spindle frequency activity during human sleep: effect of differential sleep pressure and brain topography

The impact of a 40 h sleep deprivation versus a 40 h multiple nap paradigm on topographic and temporal aspects of electroencephalographic (EEG) activity during the subsequent recovery sleep was investigated in 10 young volunteers in a controlled 'constant posture' protocol. The accumulation of sleep pressure with extended wakefulness could be significantly attenuated by intermittent naps. The differential sleep pressure conditions induced frequency- and topographic-specific changes in the EEG slow wave range (0.5-5 Hz) and in the low (LSFA, 12.25-13.25 Hz) and high spindle frequency activity range (HSFA, 13.75-16.5 Hz) during non-REM sleep. The observed increase of EEG slow-wave activity (SWA) after high sleep pressure was significantly more pronounced in the fronto-central (Fz, Cz) than in the parieto-occipital (Pz, Oz) derivations. Low sleep pressure after the nap paradigm decreased SWA with an occipital predominance. Spindle frequency activity showed a dissimilar homeostatic regulation: HSFA was significantly decreased after high sleep pressure and increased after low sleep pressure, exclusively in the centro-parietal brain region (Cz, Pz). LSFA was significantly enhanced after both manipulations. The data indicate that EEG activity, in particular frontal SWA and centro-parietal HSFA, are under a clear sleep-wake-dependent homeostatic control and imply a reciprocal relationship in the homeostatic regulation of SWA and HSFA, which however shows different spatio-temporal aspects.

Disturbed circadian rest-activity cycles in schizophrenia patients: an effect of drugs?

The circadian rest-activity cycle of schizophrenia patients stabilized for more than a year on monotherapy with a "classical" neuroleptic (haloperidol, flupentixol) or with the atypical neuroleptic clozapine was documented by continuous activity monitoring for 3-7 weeks. In this pilot study, the three patients treated with clozapine had remarkably highly ordered restactivity cycles, whereas the four patients on classical neuroleptics had minor to major circadian rhythm abnormalities. This is the first documentation of circadian rest-activity cycle disturbances in schizophrenia related to class of drug.

Frontal predominance of a relative increase in sleep delta and theta EEG activity after sleep loss in humans

The effect of sleep deprivation (40 h) on topographic and temporal aspects of electroencephalographic (EEG) activity during sleep was investigated by all night spectral analysis in six young volunteers. The sleep-deprivation-induced increase of EEG power density in the delta and theta frequencies (1-7 Hz) during nonREM sleep, assessed along the antero-posterior axis (midline: Fz, Cz, Pz, Oz), was significantly larger in the more frontal derivations (Fz, Cz) than in the more parietal derivations (Pz, Oz). This frequency-specific frontal predominance was already present in the first 30 min of recovery sleep, and dissipated in the course of the 8-h sleep episode. The data demonstrate that the enhancement of slow wave EEG activity during sleep following extended wakefulness is most pronounced in frontal cortical areas.

Dynamics of frontal EEG activity, sleepiness and body temperature under high and low sleep pressure

The impact of sleep deprivation (high sleep pressure) vs sleep satiation (low sleep pressure) on waking EEG dynamics, subjective sleepiness and core body temperature (CBT) was investigated in 10 young volunteers in a 40 h controlled constant posture protocol. The differential sleep pressure induced frequency-specific changes in the waking EEG from 1-7 Hz and 21-25 Hz. Frontal low EEG activity (FLA, 1-7 Hz) during sleep deprivation exhibited a prominent increase as time awake progressed, which could be significantly attenuated by sleep satiation attained with intermittent naps. Subjective sleepiness exhibited a prominent circadian regulation during sleep satiation, with virtually no homeostatic modulation. These extremely different sleep pressure conditions were not reflected in significant changes of the CBT rhythm. The data demonstrate that changes in FLA during wakefulness are to a large extent determined by the sleep-wake dependent process with little circadian modulation, and reflect differential levels of sleep pressure in the awake subject.

Cold feet and prolonged sleep-onset latency in vasospastic syndrome

People with vasospastic syndrome have cold hands and feet and abnormal vasoconstriction after local cold exposure. Normally there is a circadian rhythm of distal vasodilation, with onset in the early evening, which directly influences ability to fall asleep. We gave a sleep questionnaire to 32 patients with primary vasospastic syndrome and 31 healthy controls. People with vasospasticity had significantly prolonged sleep-onset latency both at onset of night-time sleep and after nocturnal disturbance. This prolonged latency could be associated with impaired capacity for distal vasodilation.

Early morning melatonin administration impairs psychomotor vigilance

The acute soporific effect of melatonin in humans has been demonstrated in a range of studies. How alertness and performance are changed after melatonin given in the morning is not yet known. In a double-blind, placebo-controlled study of nine healthy young men, melatonin was given at 0700 h under controlled conditions of a modified constant routine protocol lasting 56 h (2 days, 3 nights with sleep). A clear decrement in neurobehavioral functions as measured by the Psychomotor Vigilance Test lasted for 6 h after melatonin administration (particularly in the lapse domain and median of the reaction time) without any effect on a letter cancellation task. A subjective soporific effect was present but less pronounced. Thus, melatonin taken in the morning requires caution in situations where high attention is needed.

Dose-response relationship for light intensity and ocular and electroencephalographic correlates of human alertness

Light can elicit both circadian and acute physiological responses in humans. In a dose response protocol men and women were exposed to illuminances ranging from 3 to 9100 lux for 6.5 h during the early biological night after they had been exposed to <3 lux for several hours. Light exerted an acute alerting response as assessed by a reduction in the incidence of slow-eye movements, a reduction of EEG activity in the theta-alpha frequencies (power density in the 5-9 Hz range) as well as a reduction in self-reported sleepiness. This alerting response was positively correlated with the degree of melatonin suppression by light. In accordance with the dose response function for circadian resetting and melatonin suppression, the responses of all three indices of alertness to variations in illuminance were consistent with a logistic dose response curve. Half of the maximum alerting response to bright light of 9100 lux was obtained with room light of approximately 100 lux. This sensitivity to light indicates that variations in illuminance within the range of typical, ambient, room light (90-180 lux) can have a significant impact on subjective alertness and its electrophysiologic concomitants in humans during the early biological night.

Phase advance after one or three simulated dawns in humans

A specially designed apparatus that can simulate the waveform of the dawn or dusk signal at any latitude and any day of the year has been shown to phase shift the circadian pacemaker in rodents and primates at a fraction of the illuminance previously used. Until recently, it was considered that rather high illuminances or rather long exposure episodes to room light were necessary to phase shift human circadian rhythms. This experiment shows that, under controlled conditions of a modified constant routine protocol, a single dawn signal is sufficient to phase advance the timing of the onset of secretion of the pineal hormone melatonin. The significant phase advance of salivary melatonin of 20 minutes, which is enhanced to 34 minutes after three consecutive dawn signals, is small, but appears to be of sufficient magnitude to entrain the human circadian pacemaker, which has an endogenous period of about 24.2h.

EEG and subjective sleepiness during extended wakefulness in seasonal affective disorder: circadian and homeostatic influences

BACKGROUND

Seasonal affective disorder (SAD) may reflect a disturbance of circadian phase relationships or a disturbance of sleep-wake dependent processes, both of which change daytime energy and sleepiness levels.

METHODS

Under the unmasking conditions of a 40-hour constant routine protocol (CR), self-rated sleepiness and waking electroencephalogram (EEG) power density were assessed in women with SAD (n = 8) and in age-matched healthy control subjects (n = 9).

RESULTS

There was no significant effect of season or light treatment in any of the measures. The time course of subjective sleepiness was characterized by a circadian modulation and an overall increase during extended wakefulness in both SAD patients and control subjects. A prominent circadian rhythm of subjective sleepiness was not different in SAD patients and control subjects; however, the progressive buildup of sleepiness, as quantified by nonlinear regression analysis, was significantly reduced in SAD patients, mainly because they were sleepier than control subjects during the first 12 hours of the CR. The time course of waking EEG theta-alpha activity showed a more rapid increase during the first 10 hours of the CR in SAD patients. In contrast to control subjects who showed a progressive increase in the course of the 40-hour episode of extended wakefulness, EEG theta-alpha activity in SAD patients did not further increase over the remainder of the CR.

CONCLUSIONS

The data suggest that SAD patients may have a trait (rather than state) deficiency in the homeostatic buildup of sleep pressure during extended wakefulness as indexed by subjective sleepiness and EEG theta-alpha activity.

Functional link between distal vasodilation and sleep-onset latency?

Thermoregulatory processes have long been implicated in initiation of human sleep. The purpose of this study was to evaluate the role of heat loss in sleep initiation, under the controlled conditions of a constant-routine protocol modified to permit nocturnal sleep. Heat loss was indirectly measured by means of the distal-to-proximal skin temperature gradient (DPG). A stepwise regression analysis revealed that the DPG was the best predictor variable for sleep-onset latency (compared with core body temperature or its rate of change, heart rate, melatonin onset, and subjective sleepiness ratings). This study provides evidence that selective vasodilation of distal skin regions (and hence heat loss) promotes the rapid onset of sleep.

EEG and ocular correlates of circadian melatonin phase and human performance decrements during sleep loss

The aim of this study was to quantify the associations between slow eye movements (SEMs), eye blink rate, waking electroencephalogram (EEG) power density, neurobehavioral performance, and the circadian rhythm of plasma melatonin in a cohort of 10 healthy men during up to 32 h of sustained wakefulness. The time course of neurobehavioral performance was characterized by fairly stable levels throughout the first 16 h of wakefulness followed by deterioration during the phase of melatonin secretion. This deterioration was closely associated with an increase in SEMs. Frontal low-frequency EEG activity (1-7 Hz) exhibited a prominent increase with time awake and little circadian modulation. EEG alpha activity exhibited circadian modulation. The dynamics of SEMs and EEG activity were phase locked to changes in neurobehavioral performance and lagged the plasma melatonin rhythm. The data indicate that frontal areas of the brain are more susceptible to sleep loss than occipital areas. Frontal EEG activity and ocular parameters may be used to monitor and predict changes in neurobehavioral performance associated with sleep loss and circadian misalignment.

A rapid-cycling bipolar patient treated with long nights, bedrest, and light

BACKGROUND

Stabilization of rapid-cycling bipolar disorder is extremely difficult.

METHODS

A refractory bipolar I rapid-cycling patient on valproate was treated with long "nights" (extended sleep in darkness) and daytime light therapy.

RESULTS

Rapid cycling immediately stopped on initiation of a 10 hour dark/rest period. This was extended to 14 hours (plus a self-selected 1 hour midday nap) without problems. Depression gradually improved when midday light therapy was added; near-euthymia was attained after light therapy was shifted to the morning.

CONCLUSIONS

Nonpharmacological chronobiological treatments may be a means to interrupt rapid cycling.

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.

Melatonin and the circadian regulation of sleep initiation, consolidation, structure, and the sleep EEG

The endogenous circadian rhythm of melatonin, driven by the suprachiasmatic nucleus, exhibits a close association with the endogenous circadian component of the sleep propensity rhythm and the endogenous circadian component of the variation in electroencephalogram (EEG) oscillations such as sleep spindles and slow waves. This association is maintained even when the sleep-wake cycle is desynchronized from the endogenous circadian rhythm of melatonin. Administration of melatonin during the day increases daytime sleep propensity as indexed by both the latency to sleep onset and sleep consolidation. The EEG during daytime sleep after melatonin administration exhibits characteristics reminiscent of the nocturnal sleep EEG, that is, increased sleep spindle activity and reduced slow-wave sleep and slow-wave activity, as detected by quantitative EEG analysis. Administration of higher doses of melatonin (5 mg or more) prior to nocturnal sleep results in an increase in rapid eye movement (REM) sleep. These data demonstrate that melatonin exerts effects on the main characteristics of human sleep, that is, latency to sleep onset, sleep consolidation, slow waves, sleep spindles, and REM sleep. There is a need for further studies using physiological doses and delivery systems that generate physiological plasma melatonin profiles to firmly establish the role of the endogenous circadian rhythm of melatonin in the circadian regulation of sleep.

The acute soporific action of daytime melatonin administration: effects on the EEG during wakefulness and subjective alertness

Melatonin has been reported to have soporific effects; following daytime administration, it induces sleepiness and reduces sleep onset latency. However, subjective sleepiness is masked by a variety of stimuli and behaviors; thus, it is important to be able to delineate objective psychophysiological sequelae of melatonin administration. Alertness decrements during wakefulness are correlated with augmented theta/alpha power in the waking electroencephalogram (EEG). This has been validated in a constant routine protocol. In a variety of experiments with melatonin administration (5 mg), the authors have shown that the EEG changes can be measured immediately, before any subjective soporific effects are recognized. These increases in theta/alpha power occur when melatonin is administered during the day (1300 or 1800 h) but are less visible when near the endogenous melatonin rise in the evening (2040 h). Importantly, both subjective and objective measures of sleepiness are suppressed when subjects change posture from supine to standing.

A schizophrenic patient with an arrhythmic circadian rest-activity cycle

A haloperidol-treated patient with chronic schizophrenia had a near-arrhythmic circadian rest-activity cycle, whereas rhythms of 6-sulphatoxy-melatonin and core body temperature were of normal amplitude and phase-advanced. Sleep electroencephalography measured throughout a 31-h 'constant-bedrest' protocol revealed a phase-delayed sleep-wake propensity cycle, low sleep continuity (ultradian 'bouts'), and very little slow-wave sleep and slow-wave activity (0.75-4.5 Hz). Switching treatment to the atypical neuroleptic clozapine improved both the circadian organization of the rest-activity cycle and the patient's clinical state. This observation can be conceptualized in terms of the two-process model of sleep regulation. High-dose haloperidol treatment may have lowered the circadian alertness threshold, whereas clozapine augmented circadian amplitude (perhaps through its high affinity to dopamine D4 and serotonin 5HT7 receptors in the suprachiasmatic nuclei). Measurement of the circadian rest-activity cycle may be a useful non-invasive method to follow functional consequences of neuroleptic treatment.

The hypothermic effect of late evening melatonin does not block the phase delay induced by concurrent bright light in human subjects

This constant routine study (n = 9 men) compared the phase delay of the circadian system induced by a single pulse of evening light (5000 lx at 2100-2400 h) in the presence or absence of exogenous melatonin (5 mg p.o. at 2040 h). On the treatment day, light and melatonin protracted and accelerated, respectively, the evening decline in core body temperature (CBT). Subjective sleepiness ratings showed parallel shifts, the earlier the decline in CBT, the sleepier. On the post-treatment day, light induced a phase delay in the mid-range crossing time of CBT decline independent of whether melatonin was co-administered or not. Subjective sleepiness was delayed in parallel. The phase delay of the circadian system by evening light appears to be independent of an immediate hyperthermic effect and is not mediated by melatonin.

A relationship between heat loss and sleepiness: effects of postural change and melatonin administration

Both the pineal hormone melatonin (Mel) and postural changes have thermoregulatory sequelae. The purpose of the study was to evaluate their relationship to subjective sleepiness. Eight healthy young men were investigated under the unmasking conditions of a constant routine protocol. Heart rate, rectal temperature (Tre), skin temperatures (foot, Tfo; and stomach), and subjective sleepiness ratings were continuously recorded from 1000 to 1700. Mel (5 mg po) was administered at 1300, a time when Mel should not phase shift the circadian system. Both the postural change at 1000 from upright to a supine position (lying down in bed) and Mel administration at 1300 reduced Tre and increased Tfo in parallel with increased sleepiness. These findings suggest that under comfortable ambient temperature conditions, heat loss via the distal skin regions (e.g., feet) is a key mechanism for induction of sleepiness as core body temperature declines.

Early evening melatonin and S-20098 advance circadian phase and nocturnal regulation of core body temperature

The phase-shifting capacity and thermoregulatory effects of a single oral administration at 18 h of melatonin (5 mg) or S-20098, a melatonin agonist (5 or 100 mg), was investigated in eight healthy young men in a double-blind placebo crossover design. The unmasking conditions of a shortened constant-routine protocol (mini-CR) were used to collect evening phase markers of physiological parameters. In comparison to placebo, all three drug administrations induced an earlier dim-light melatonin onset (DLMO), an earlier increase in distal skin temperature, and an earlier decrease in core body temperature (CBT), heart rate, and proximal skin temperature. This indicates that administration at 18 h of both melatonin and S-20098 (more pronounced with 100 than 5 mg) induced an earlier regulation of the endogenous circadian nocturnal decline in CBT. On the posttreatment day a second mini-CR revealed persistent significantly phase-advanced circadian rhythms as estimated by DLMO, as well as by the midrange crossing time of CBT and heart rate decline. There were no significant differences between the two doses of S-20098. The data suggest that, in addition to immediate thermoregulatory changes, a phase advance of the circadian system had occurred and that the phase advance could still be measured on the posttreatment day.

Melatonin and S-20098 increase REM sleep and wake-up propensity without modifying NREM sleep homeostasis

The pineal hormone melatonin has been implicated in the circadian regulation of sleep. In a crossover design, we investigated the effect of acute administration of 5 mg melatonin and a melatonin agonist (S-20098, 5 and 100 mg) in healthy young men when given 5 h before bedtime on sleep structure and electroencephalogram (EEG) power density. Each trial comprised a baseline, a treatment, and a posttreatment sleep episode. Relative to the placebo condition, all treatments phase advanced the core body temperature rhythm [Kräuchi, K., C. Cajochen, D. Möri, C. Hetsch, and A. Wirz-Justice. Sleep Res. 24: 526, 1995; and Kräuchi, K., C. Cajochen, D. Möri, and A. Wirz-Justice. Am. J. Physiol. 272 (Regulatory Integrative Comp. Physiol. 41): R1178-1188, 1997]. Rapid eye movement (REM) sleep was increased after both melatonin and S-20098. This increase in REM sleep was most pronounced in the first REM sleep episode. On the posttreatment night after melatonin and S-20098 administration, more wakefulness was present in the latter one-half of the sleep episode. EEG power density between 0.25 and 20 Hz during either non-REM (NREM) or REM sleep did not differ from placebo. Thus a single early evening dose of melatonin or the agonist S-20098 increases REM sleep propensity and advances sleep termination while, at the same time, the EEG in NREM sleep remains unaffected.

Daytime melatonin administration enhances sleepiness and theta/alpha activity in the waking EEG

It is still controversial whether the pineal hormone melatonin can be characterized as a hypnotic. We therefore measured subjective sleepiness and waking EEG power density in the range of 0.25-20 Hz after a single dose of melatonin (5 mg). During an 8 h mini-constant routine protocol, melatonin administered in a double blind cross-over design to healthy young men at 1300 h or 1800 h increased subjective sleepiness, as rated half-hourly on three different scales (Visual Analogue Scale, Akerstedt Sleepiness Symptoms Check List, Akerstedt Sleepiness Scale) and objective fatigue as evidenced by augmented waking EEG power density in the theta/alpha range (5.25-9 Hz). The increase in subjective sleepiness reached significance 40 min and 90 min after melatonin administration (at 1300 h and 1800 h, respectively) and lasted for 3 h (at 1300 h) and 5 h (at 1800 h). The increase in the theta/alpha frequencies of the waking EEG occurred immediately after melatonin ingestion and stayed significantly higher parallel to the higher sleepiness ratings. However, the EEG changes appeared before the subjective symptoms of sleepiness became manifest. There was a significant correlation between salivary melatonin levels and the timing of increased subjective sleepiness. Melatonin had no effects on mood.

Homeostatic sleep regulation in habitual short sleepers and long sleepers

Homeostatic sleep regulation in habitual short sleepers (sleep episode < 6 h, n = 9) and long sleepers (> 9 h, n = 7) was investigated by studying their sleep structure and sleep electroencephalogram (EEG) during baseline conditions and after prolonging their habitual waking time by 24 h. In each sleep episode, total sleep time was > 3 h longer in the long sleepers than in the short sleepers. Sleep deprivation decreased sleep latency and rapid eye movement (REM) density in REM sleep more in long sleepers than in short sleepers. The enhancement of EEG slow-wave activity (SWA; spectral power density in the 0.75-4.5 Hz range) in non-REM sleep after sleep loss was larger in long sleepers (47%) than in short sleepers (19%). This difference in the SWA response was predicted by the two-process model of sleep regulation on the basis of the different sleep durations. The results indicate that short sleepers live under a higher "non-REM sleep pressure" than long sleepers. However, the two groups do not differ with respect to the homeostatic sleep regulatory mechanisms.

Power density in theta/alpha frequencies of the waking EEG progressively increases during sustained wakefulness

Electroencephalogram (EEG) power density and self-rated fatigue were assessed in nine healthy women during a 40-hour period of sustained wakefulness under constant behavioral and environmental conditions (constant routine protocol). Waking EEG recordings were performed for 4 minutes after 3, 10, 27 and 34 hours of prior wakefulness. EEG power density in the 6.25- to 9.0-Hz frequency range progressively increased across the four recordings, suggesting an endogenous homeostatic component in the regulation of the theta/alpha frequencies under constant conditions. Subjective fatigue also exhibited an increasing component in the course of the constant routine protocol, with a clear circadian modulation. Fatigue ratings and the theta/ alpha power density of the waking EEG recorded at the same four time points during the constant routine protocol correlated significantly. Our data demonstrate the presence of a homeostatic component in the control of EEG power density in the 6.25- to 9.0-Hz range.

Heart rate dynamics during human sleep

To investigate the dynamics of heart rate in the course of sleep and to relate cardiac activity to sleep intensity, the electrocardiogram was recorded concomitantly with the polysomnogram in healthy young males. Heart rate was assessed across consecutive non-REM sleep (NREMS)-REM sleep (REMS) cycles as well as within individual episodes of NREMS and REMS. Within a sleep cycle, heart rate was lower in the NREMS episode than in the subsequent REMS episode. A global declining trend was present over successive NREMS episodes and over successive REMS episodes. A rapid increase of heart rate at the NREMS-REMS transitions was followed by a slow decline that started within the REMS episodes. Heart rate variability was higher in REMS than in NREMS and showed an increasing trend over successive REMS episodes but not over successive NREMS episodes. EEG slow-wave activity (spectral power density in the 0.75-4.5 Hz band), an intensity measure of NREMS, declined across NREMS episodes and was not correlated with heart rate. The global trends and ultradian variations of heart rate may represent sleep state-dependent modulations and circadian variations of the autonomic nervous system, which are not fully reflected in the sleep EEG.

Sleep in a sitting position: effect of triazolam on sleep stages and EEG power spectra

The effect of triazolam (0.25 mg) and placebo was investigated in healthy, male subjects who slept in a sitting position. After the intake of placebo, sleep efficiency, rapid eye movement (REM) sleep and subjective sleep quality were lower than in the preceding sleep episode in bed, while stage 1 and REM sleep latency were higher. Triazolam did not prevent this impairment of sleep. However, in comparison with the placebo condition, the percentage of slow wave sleep was higher in the first third of the night, and in the morning sleep was rated as more quite. EEG power density in nonREM sleep was reduced in the frequency range of 1.25-10.0 Hz and enhanced in the range of sleep spindles (12.25-13.0 Hz). These changes were still present in the last third of the night. In REM sleep, triazolam reduced spectral activity in some frequency bins between 4.25 and 10.0 Hz. The sitting position itself affected the nonREM sleep spectra, since the placebo level in the 2.25-21.0-Hz range exceeded the baseline level. We conclude that a 0.25 mg dose of triazolam does not effectively counteract a posture-induced sleep disturbance, but induces changes in the EEG spectra which are typical for benzodiazepine receptor agonists.

Dynamics of EEG slow-wave activity and core body temperature in human sleep after exposure to bright light

In seven subjects sleep was recorded after a single 3-hour (2100-0000 hours) exposure to either bright light (BL, approx. 2,500 lux) or dim light (DL, approx. 6 lux) in a crossover design. The latency to sleep onset was increased after BL. Whereas rectal temperature before onset and during the first 4 hours of sleep was higher after BL than after DL, the time course of electroencephalographic (EEG) slow-wave activity (SWA, EEG power density in the range of 0.75-4.5 Hz) in nonrapid eye movement sleep (NREMS) differed only slightly between the conditions. After BL, SWA tended to be lower than after DL in the first NREMS-REMS cycle and was higher in the fourth cycle at the time when the rectal temperature did not differ. The differences in SWA may have been due to a minor sleep-disturbing aftereffect of BL, which was followed by a rebound. The data are not in support of a close relationship between SWA and core body temperature.

Sleep extension in humans: sleep stages, EEG power spectra and body temperature

In eight male subjects the electroencephalogram (EEG) and core body temperature (Tcore) were recorded during long sleep episodes from 0000 to 1,500 hr. EEGs were visually scored and subjected to spectral analysis by fast Fourier transform. Slow-wave sleep [SWS, i.e. stages 3 + 4 of non-rapid eye movement (NREM) sleep and slow wave activity (SWA, mean EEG power density in the range of 0.75-4.5 Hz)] in NREM sleep attained highest values in the first 3 hr of sleep and lowest values in the morning hours when rapid eye movement (REM) sleep was at its maximum. Wakefulness was significantly enhanced in the last 3 hr of the recording period. Occasional NREM episodes containing SWS were observed in the late morning and early afternoon. However, no significant increase in SWS or SWA in the last 3 hr of the sleep episode over any of the preceding 3-hr intervals was present and SWA in this interval was significantly below the values observed at the beginning of sleep. The duration of NREM episodes varied significantly over the sleep episode. Analysis of the dynamics of SWA within NREM episodes revealed that SWA gradually rose during the episode. Consequently, SWA averaged per episode was positively correlated with episode duration. Tcore dropped in the initial part of sleep, rose during the morning hours and reached values in the afternoon that were higher than at the beginning of sleep. Thus the time course of Tcore dissociated from the time course of SWA. This indicates that SWA in NREM sleep is not directly related to the variation in core body temperature.

Effect of a single 3-hour exposure to bright light on core body temperature and sleep in humans

Seven human subjects were exposed to bright light (BL, approx. 2500 lux) and dim light (DL, approx. 6 lux) during 3 h prior to nocturnal sleep, in a cross-over design. At the end of the BL exposure period core body temperature was significantly higher than at the end of the DL exposure period. The difference in core body temperature persisted during the first 4 h of sleep. The latency to sleep onset was increased after BL exposure. Rapid-eye movement sleep (REMS) and slow-wave sleep (SWS; stage 3 + 4 of non-REMS) were not significantly changed. Eight subjects were exposed to BL from 20.30 to 23.30 h while their eyes were covered or uncovered. During BL exposure with uncovered eyes, core body temperature decreased significantly less than during exposure with covered eyes. We conclude that bright light immediately affects core body temperature and that this effect is mediated via the eyes.