Light-Dark Difference in Arterial Pressure Variability During REM Sleep in the Rat

We have observed mean arterial pressure (MAP) variability during rapid eye movement (REM) sleep and brain temperature (Tb) in the rat during both light and dark periods over 24 h. MAP was measured using a telemetric device with a computer data capture and analysis system. As markers of MAP variability, the maximum and coefficient of variation (CV%) of MAP during REM sleep were determined. The following results were obtained: (a) there was a light-dark difference in MAP during non-REM (NREM) sleep and Tb during both NREM and REM sleep; (b) the increase of MAP in going from NREM to REM sleep in the light period was greater than that in the dark period, whereas the increase of Tb in the light period was not different from that in the dark period; (c) the maximum and CV% for MAP during REM sleep in the light period were greater than those in the dark period; (d) there was a negative correlation between the average Tb and MAP CV% during REM sleep. We suggest that phasic fluctuation of MAP during REM sleep may be influenced, in part, by a factor independent of sleep mechanisms.     

Developmental changes in the complexity of the electrocortical activity in foetal sheep.

The foetal sheep brain develops organised sleep states from 115-120 d gestational age (dGA, term 150 dGA) alternating between REM and NREM sleep. We aimed to investigate whether maturation of REM or NREM sleep generating structures leads to the development of distinct sleep states. The electrocorticogram (ECoG) was recorded from five unanaesthetised chronically instrumented foetal sheep in utero and was analysed every 5th day between 115-130 dGA by two different non-linear methods. We calculated a non-linear prediction error which quantifies the causality of the ECoG and applied bispectral analysis which quantifies non-linear interrelations of single frequency components within the ECoG signal. The prediction error during REM sleep was significantly higher than during NREM sleep at each investigated age (P<0.0001) coincidental with poor organisation of the rhythmic pattern in the ECoG during REM sleep. At 115 dGA, organised sleep states defined behaviourally were not developed yet. The prediction error, however, showed already different states of electrocortical activity that were not detectable using power spectral analysis. The prediction error of the premature NREM sleep ECoG decreased significantly during emergence of organised sleep states between 115 and 120 dGA and continued to decrease after the emergence of distinct sleep states (P<0.05). The prediction error of the premature REM sleep ECoG did not change until 120 dGA and began to increase at 125 dGA (P<0.05). Using bispectral analysis, we showed couplings between delta waves (1.5-4 Hz) and frequencies in the range of spindle waves (4-8 and 8-12 Hz) during NREM sleep that became closer during development. The results show that maturation of ECoG synchronisation mediating structures is important for the development of organised sleep states. The further divergence of the prediction error of NREM and REM sleep after development of organised sleep states reveals continuous functional development. Thus, complementary application of non-linear ECoG analysis to power spectral analysis provide new insights in the collective behaviour of the neuronal network during the emergence of sleep states.     

Light-Dark Difference in Arterial Pressure Variability During REM Sleep in the Rat

We have observed mean arterial pressure (MAP) variability during rapid eye movement (REM) sleep and brain temperature (Tb) in the rat during both light and dark periods over 24 h. MAP was measured using a telemetric device with a computer data capture and analysis system. As markers of MAP variability, the maximum and coefficient of variation (CV%) of MAP during REM sleep were determined. The following results were obtained: (a) there was a light-dark difference in MAP during non-REM (NREM) sleep and Tb during both NREM and REM sleep; (b) the increase of MAP in going from NREM to REM sleep in the light period was greater than that in the dark period, whereas the increase of Tb in the light period was not different from that in the dark period; (c) the maximum and CV% for MAP during REM sleep in the light period were greater than those in the dark period; (d) there was a negative correlation between the average Tb and MAP CV% during REM sleep. We suggest that phasic fluctuation of MAP during REM sleep may be influenced, in part, by a factor independent of sleep mechanisms.     

Effects of selective sleep deprivation on ventilation during recovery sleep in normal humans.

To assess the effects of selective sleep loss on ventilation during recovery sleep, we deprived 10 healthy young adult humans of rapid-eye-movement (REM) sleep for 48 h and compared ventilation measured during the recovery night with that measured during the baseline night. At a later date we repeated the study using awakenings during non-rapid-eye-movement (NREM) sleep at the same frequency as in REM sleep deprivation. Neither intervention produced significant changes in average minute ventilation during presleep wakefulness, NREM sleep, or the first REM sleep period. By contrast, both interventions resulted in an increased frequency of breaths, in which ventilation was reduced below the range for tonic REM sleep, and in an increased number of longer episodes, in which ventilation was reduced during the first REM sleep period on the recovery night. The changes after REM sleep deprivation were largely due to an increase in the duration of the REM sleep period with an increase in the total phasic activity and, to a lesser extent, to changes in the relationship between ventilatory components and phasic eye movements. The changes in ventilation after partial NREM sleep deprivation were associated with more pronounced changes in the relationship between specific ventilatory components and eye movement density, whereas no change was observed in the composition of the first REM sleep period. These findings demonstrate that sleep deprivation leads to changes in ventilation during subsequent REM sleep.     

Circulating endothelin parallels arterial blood pressure during sleep in healthy subjects

OBJECTIVE: To characterize plasma endothelin 1 (ET-1) and arterial blood pressure (ABP) time courses during the first complete non-rapid eye movement (NREM)-REM sleep cycle in healthy subjects, together with plasma renin activity (PRA) and plasma atrial natriuretic peptide (ANP). METHODS: Heart rate (HR), intra-arterial blood pressure and sleep electroencephalographic activity were recorded continuously during the night in eight healthy 20-28-year-old males. Blood was sampled every 10 min during their first complete sleep cycle for simultaneous measurements of plasma ET-1, PRA and ANP. RESULTS: Circulating ET-1 demonstrated significant variations during the sleep cycle (p<0.0001) that paralleled those of ABP (p<0.05) and HR (p<0.005), with a minimum during NREM sleep and a maximum during REM sleep. ET-1 time course opposed that of PRA which increases during NREM sleep and decreases during REM sleep (p<0.0005). Plasma ANP did not demonstrate systematic variation in relation with the sleep cycle. CONCLUSION: Circulating ET-1, which parallels variations of ABP, may participate in ABP regulation during sleep in healthy subjects, in association with the renin-angiotensin system.     

Changes in upper airway muscle activation and ventilation during phasic REM sleep in normal men

Several investigators have observed that irregular breathing occurs during rapid-eye-movement (REM) sleep in healthy subjects, with ventilatory suppression being prominent during active eye movements [phasic REM (PREM) sleep] as opposed to tonic REM (TREM) sleep, when ocular activity is absent and ventilation more regular. Inasmuch as considerable data suggest that rapid eye movements are a manifestation of sleep-induced neural events that may importantly influence respiratory neurons, we hypothesized that upper airway dilator muscle activation may also be suppressed during periods of active eye movements in REM sleep. We studied six normal men during single nocturnal sleep studies. Standard sleep-staging parameters, ventilation, and genioglossus and alae nasi electromyograms (EMG) were continuously recorded during the study. There were no significant differences in minute ventilation, tidal volume, or any index of genioglossus or alae nasi EMG amplitude between non-REM (NREM) and REM sleep, when REM was analyzed as a single sleep stage. Each breath during REM sleep was scored as "phasic" or "tonic," depending on its proximity to REM deflections on the electrooculogram. Comparison of all three sleep states (NREM, PREM, and TREM) revealed that peak inspiratory genioglossus and alae nasi EMG activities were significantly decreased during PREM sleep compared with TREM sleep [genioglossus (arbitrary units): NREM 49 +/- 12 (mean +/- SE), TREM 49 +/- 5, PREM 20 +/- 5 (P less than 0.05, PREM different from TREM and NREM); alae nasi: NREM 16 +/- 4, TREM 38 +/- 7, PREM 10 +/- 4 (P less than 0.05, PREM different from TREM)]. We also observed, as have others, that ventilation, tidal volume, and mean inspiratory airflow were significantly decreased and respiratory frequency was increased during PREM sleep compared with both TREM and NREM sleep. We conclude that hypoventilation occurs in concert with reduced upper airway dilator muscle activation during PREM sleep by mechanisms that remain to be established.     

Reduced Slow-Wave Rebound during Daytime Recovery Sleep in Middle-Aged Subjects

Cortical synchronization during NREM sleep, characterized by electroencephalographic slow waves (SW <4Hz and >75 μV), is strongly related to the number of hours of wakefulness prior to sleep and to the quality of the waking experience. Whether a similar increase in wakefulness length leads to a comparable enhancement in NREM sleep cortical synchronization in young and older subjects is still a matter of debate in the literature. Here we evaluated the impact of 25-hours of wakefulness on SW during a daytime recovery sleep episode in 29 young (27y ±5), and 34 middle-aged (51y ±5) subjects. We also assessed whether age-related changes in NREM sleep cortical synchronization predicts the ability to maintain sleep during daytime recovery sleep. Compared to baseline sleep, sleep efficiency was lower during daytime recovery sleep in both age-groups but the effect was more prominent in the middle-aged than in the young subjects. In both age groups, SW density, amplitude, and slope increased whereas SW positive and negative phase duration decreased during daytime recovery sleep compared to baseline sleep, particularly in anterior brain areas. Importantly, compared to young subjects, middle-aged participants showed lower SW density rebound and SW positive phase duration enhancement after sleep deprivation during daytime recovery sleep. Furthermore, middle-aged subjects showed lower SW amplitude and slope enhancements after sleep deprivation than young subjects in frontal and prefrontal derivations only. None of the SW characteristics at baseline were associated with daytime recovery sleep efficiency. Our results support the notion that anterior brain areas elicit and may necessitate more intense recovery and that aging reduces enhancement of cortical synchronization after sleep loss, particularly in these areas. Age-related changes in the quality of wake experience may underlie age-related reduction in markers of cortical synchronization enhancement after sustained wakefulness.     

Geniohyoid muscle activity in normal men during wakefulness and sleep

Reduction in the activity of upper airway "dilator" muscles during sleep may allow the pharyngeal airway to collapse in some individuals. However, quantitative studies concerning the effect of sleep on specific upper airway muscles that may influence pharyngeal patency are sparse and inconclusive. We studied seven normal men (mean age 27, range 22-37 yr) during a single nocturnal sleep study and recorded sleep staging parameters, ventilation, and geniohyoid muscle electromyogram (EMGgh) during nasal breathing throughout the night. Anatomic landmarks for placement of intramuscular geniohyoid recording electrodes were determined from a cadaver study. These landmarks were used in percutaneous placement of wire electrodes, and raw and moving-time-averaged EMGgh activities were recorded. Sleep stage was determined using standard criteria. Stable periods of wakefulness and non-rapid-eye-movement (NREM) and rapid-eye-movement (REM) sleep were selected for analysis. The EMGgh exhibited phasic inspiratory activity during wakefulness and sleep in all subjects. In six of seven subjects, mean and peak inspiratory EMGgh activities were significant (P less than 0.05) reduced during stages 2 and 3/4 NREM sleep and REM sleep compared with wakefulness. This reduction of EMGgh activity was shown to result from a sleep-related decline in the level of tonic muscle activity. Phasic inspiratory EMGgh activity during all stages of sleep was not significantly different from that during wakefulness. Of interest, tonic, phasic, and peak EMGgh activities were not significantly reduced during REM sleep compared with any other sleep stage in any subject. In addition, the slope of onset of phasic EMGgh activity was not different during stage 2 NREM and REM sleep compared with wakefulness in these subjects.(ABSTRACT TRUNCATED AT 250 WORDS)     

Regulation of end-expiratory lung volume during sleep in premature infants

To investigate the regulation of end-expiratory lung volume (EEV) in premature infants, we recorded airflow, tidal volume, diaphragm electromyogram (EMG), and chest wall displacement during sleep. In quiet sleep, EEV during breathing was 10.8 +/- 3.6 (SD) ml greater than the minimum volume reached during unobstructed apneas. In active sleep, no decrease in EEV was observed during 28 of 35 unobstructed apneas. Breaths during quiet sleep had a variable extent of expiratory airflow retardation (braking), and inspiratory interruption occurred at substantial expiratory flow rates. During active sleep, the expiratory flow-volume curve was nearly linear, proceeding nearly to the volume axis at zero flow, and diaphragm EMG activity terminated near the peak of mechanical inspiration. Expiratory duration (TE) and inspiratory duration (TI) were significantly shortened in quiet sleep vs. active sleep although tidal volume was not significantly different. In quiet sleep, diaphragmatic braking activity and shortened TE combined to maintain EEV during breathing substantially above relaxation volume. In active sleep, reduced expiratory braking and prolongation of TE resulted in an EEV that was close to relaxation volume. We conclude that breathing strategy to regulate EEV in premature infants appears to be strongly influenced by sleep state.     

Human growth hormone release is decreased during sleep in temporal isolation (free-running)

Growth hormone (hGH) secretion was measured during sleep in 10 healthy male subjects isolated from all time cues. HGH concentrations following sleep onset were compared between scheduled sleep episodes (entrainment) and self-selected sleep episodes (free-running). Peak sleep-related hGH values were decreased significantly during free-running. The duration of the first slow wave sleep (SWS) episode and the latency to the first REM sleep episode also decreased significantly during free-running. The latencies from sleep onset to SWS and to peak hGH secretion did not differ between entrainment and free-running. These results suggest that sleep-related hGH secretion begins 'on time' during free-running, but is terminated earlier. Thus, while sleep onset facilitates hGH release, the timing of other stages of sleep such as REM may alter the magnitude of sleep-related hGH secretion.     

Heart rate variability in healthy humans during night sleep

Polysomnography has been performed and heart rate variability has been studied during night sleep in healthy young subjects of both sexes. It has been shown that, on average, a shift in autonomic support towards parasympathicotonia occurs during sleep, with a maximum at stages III and IV of NREM sleep. At stage II of NREM sleep and during REM sleep, a short-term activation of the sympathetic nervous system comparable to wakefulness is observed.     

Respiration During Sleep in Children

In 22 children (11 boys and 11 girls), aged 9 to 13 years, respiration was monitored during one night of sleep. No child had a significant history of breathing problems during sleep. Sleep was recorded using standard techniques (electroencephalography, electrooculography, electromyography), and respiration was measured with nasal thermistors and abdominal or thoracic strain gauges. Respiratory pauses (five seconds or longer) were determined for all sleep stages. Respiratory rate was scored only in the first and last sleep cycles and during ten waking minutes before sleep onset. Respiratory rate was significantly affected by wakefulness or stage of sleep: highest in wakefulness and stage 1, lowest in stage 2 of the last sleep cycle. Regularity of respiratory rate showed a similar effect. Variance of respiratory rate was significantly lower in girls than boys. Respiratory pauses during sleep were seen in every child, ranging from 3 to 40 pauses per night (average, 17.2 for boys and 18.0 for girls). Significantly greater numbers of pauses per minute were seen in stage 1 and rapid eye movement (REM) sleep than in stages 2, 3 and 4. The longest respiratory pause was 25 seconds. The conclusion is made that a small number of respiratory pauses during sleep are normal in children of this age.     

Arousal and cardiopulmonary responses to hyperoxic hypercapnia in lambs

Experiments were done to investigate the arousal and cardiopulmonary responses to hyperoxic hypercapnia in 8 lambs. Each lamb was anaesthetized and instrumented for recordings of electrocorticogram, electro-oculogram, nuchal and diaphragm electromyograms and measurements of arterial blood pressure and haemoglobin oxygen saturation. No sooner than 3 days after surgery, measurements were made in quiet sleep and active sleep during control periods when the animal was breathing 21% oxygen and during experimental periods of hyperoxic hypercapnia when the animal was breathing 10% carbon dioxide and 30% oxygen. Hyperoxic hypercapnia was terminated during each epoch by returning the inspired gas mixture to 21% oxygen once the animal aroused from sleep. Arousal occurred from both sleep states during hyperoxic hypercapnia but was delayed in active sleep compared to quiet sleep (active sleep 58 +/- 17 s; quiet sleep 21 +/- 10 s; mean +/- 1SD). There were no significant changes in heart rate or blood pressure during hyperoxic hypercapnia before arousal. However, respiratory rate and diaphragm electrical activity did increase during hyperoxic hypercapnia before arousal. Thus, our data provide evidence that hypercapnia can initiate arousal from sleep in young lambs. The mechanisms responsible for this response are yet to be determined.     

Sleep-Stage Correlates of Hippocampal Electroencephalogram in Primates

It has been demonstrated in the rodent hippocampus that rhythmic slow activity (theta) predominantly occurs during rapid eye movement (REM) sleep, while sharp waves and associated ripples occur mainly during non-REM sleep. However, evidence is lacking for correlates of sleep stages with electroencephalogram (EEG) in the hippocampus of monkeys. In the present study, we recorded hippocampal EEG from the dentate gyrus in monkeys overnight under conditions of polysomnographical monitoring. As result, the hippocampal EEG changed in a manner similar to that of the surface EEG: during wakefulness, the hippocampal EEG showed fast, desynchronized waves, which were partly replaced with slower waves of intermediate amplitudes during the shallow stages of non-REM sleep. During the deep stages of non-REM sleep, continuous, slower oscillations (0.5–8 Hz) with high amplitudes were predominant. During REM sleep, the hippocampal EEG again showed fast, desynchronized waves similar to those found during wakefulness. These results indicate that in the monkey, hippocampal rhythmic slow activity rarely occurs during REM sleep, which is in clear contrast to that of rodents. In addition, the increase in the slower oscillations of hippocampal EEG during non-REM sleep, which resembled that of the surface EEG, may at least partly reflect cortical inputs to the dentate gyrus during this behavioral state.     

The dynamics of spindles and EEG slow-wave activity in NREM sleep in mice

A quantitative analysis of spindles and spindle-related EEG activity was performed in C57BL/6 mice. The hypothesis that spindles are involved in sleep regulatory mechanisms was tested by investigating their occurrence during 24 h and after 6 h sleep deprivation (SD; n = 7). In the frontal derivation distinct spindle events were characterized as EEG oscillations with a dominant frequency approximately at 11 Hz. Spindles were most prominent during NREM sleep and increased before NREM-REM sleep transitions. Whereas spindles increased concomitantly with slow wave activity (SWA, EEG power between 0.5 and 4.0 Hz) at the beginning of the NREM sleep episode, these measures showed an opposite evolution prior to the transition to REM sleep. The 24-h time course of spindles showed a maximum at the end of the 12-h light period, and was a mirror image of SWA in NREM sleep. After 6 h SD the spindles in NREM sleep were initially suppressed, and showed a delayed rebound. In contrast, spindles occurring immediately before the transition to REM sleep were enhanced during the first 2 h of recovery. The data suggest that spindles in NREM sleep may be involved in sleep maintenance, while spindles heralding the transition to REM sleep may be related to mechanisms of REM sleep initiation.     

Migratory sleeplessness in the white-crowned sparrow (Zonotrichia leucophrys gambelii)

Twice a year, normally diurnal songbirds engage in long-distance nocturnal migrations between their wintering and breeding grounds. If and how songbirds sleep during these periods of increased activity has remained a mystery. We used a combination of electrophysiological recording and neurobehavioral testing to characterize seasonal changes in sleep and cognition in captive white-crowned sparrows (Zonotrichia leucophrys gambelii) across nonmigratory and migratory seasons. Compared to sparrows in a nonmigratory state, migratory sparrows spent approximately two-thirds less time sleeping. Despite reducing sleep during migration, accuracy and responding on a repeated-acquisition task remained at a high level in sparrows in a migratory state. This resistance to sleep loss during the prolonged migratory season is in direct contrast to the decline in accuracy and responding observed following as little as one night of experimenter-induced sleep restriction in the same birds during the nonmigratory season. Our results suggest that despite being adversely affected by sleep loss during the nonmigratory season, songbirds exhibit an unprecedented capacity to reduce sleep during migration for long periods of time without associated deficits in cognitive function. Understanding the mechanisms that mediate migratory sleeplessness may provide insights into the etiology of changes in sleep and behavior in seasonal mood disorders, as well as into the functions of sleep itself.     

Posterior cricoarytenoid muscle activity during wakefulness and sleep in normal adults

Six normal adults were studied 1) to compare respiratory-related posterior cricoarytenoid (PCA) muscle activity during wakefulness and sleep and 2) to determine the effect of upper airway occlusions during non-rapid-eye-movement (NREM) sleep on PCA activity. A new electromyographic technique was developed to implant hooked-wire electrodes into the PCA by using a nasopharyngoscope. A previously described technique was used to induce upper airway occlusions during NREM sleep (Kuna and Smickley, J. Appl. Physiol. 64: 347-353, 1988). The PCA exhibited phasic inspiratory activity during quiet breathing in wakefulness and sleep in all subjects. Discounting changes in tonic activity, peak amplitude of PCA inspiratory activity during stage 3-4 NREM sleep decreased to 77% of its value in wakefulness. Tonic activity throughout the respiratory cycle was present in all subjects during wakefulness but was absent during state 3-4 NREM sleep. In this sleep stage, PCA phasic activity abruptly terminated near the end of inspiration. During nasal airway occlusions in NREM sleep, PCA phasic activity did not increase significantly during the first or second occluded effort. The results, in combination with recent findings for vocal cord adductors in awake and sleeping adults, suggest that vocal cord position during quiet breathing in wakefulness is actively controlled by simultaneously acting antagonistic intrinsic laryngeal muscles. In contrast, the return of the vocal cords toward the midline during expiration in stage 3-4 NREM sleep appears to be a passive phenomenon.     

Occlusion pressure and ventilation during sleep in normal humans

Previous investigation in normal humans has demonstrated reduced ventilation and ventilatory responses to chemical stimuli during sleep. Most have interpreted this to be a product of decreasing central nervous system sensitivity to the normal stimuli that maintain ventilation, whereas other factors such as increasing airflow resistance could also contribute to this reduction in respiration. To improve our understanding of these events, we measured ventilation and occlusion pressures (P0.1) during unstimulated ventilation and rebreathing-induced hypercapnia during wakefulness and non-rapid-eye-movement (NREM) and rapid-eye-movement (REM) sleep. Eighteen subjects (10 males and 8 females) of whom seven were snorers (5 males and 2 females) were studied. Ventilation was reduced during both NREM and REM sleep (P less than 0.05), but this decrement in minute ventilation tended to be greater in snorers than nonsnorers. Unstimulated P0.1, on the other hand, was maintained or increased during sleep in all groups studied, with males and snorers showing the largest increase. The hypercapnic ventilatory response fell during both NREM and REM sleep and tended to be lower during REM than NREM sleep. However, the P0.1 response to hypercapnia during NREM sleep was well maintained at the waking level although the REM response was statistically reduced. These studies suggest that the mechanism of the reduction in ventilation and the hypercapnic ventilatory response seen during sleep, particularly NREM sleep, is likely to be multifactorial and not totally a product of decreasing central respiratory drive.     

Sleep EEG spectral analysis in a diurnal rodent:Eutamias sibiricus

1. Sleep was studied in the diurnal rodent Eutamias sibiricus, chronically implanted with EEG and EMG electrodes. Analysis of the distribution of wakefulness, nonrapid eye movement (NREM) sleep, and rapid eye movement (REM) sleep over the 24 h period (LD 12:12) showed that total sleep time was 27.5% of recording time during the 12 h light period and 74.4% during the 12 h dark period. Spectral analysis of the sleep EEG revealed a progressive decay in delta power density in NREM sleep during darkness. Power density of the higher frequencies increased at the end of darkness. Power density of the higher frequencies decreased and that of the lower frequencies increased during light. 2. Analysis of the distribution of vigilance states under three different photoperiods (LD 18:6; 12:12; 6:18) revealed that changes in daylength mainly resulted in a redistribution of sleep and wakefulness over light and darkness. Under long days the percentage of sleep during light was enhanced. The time course of delta power density in NREM sleep was characterized by a long rising part and a short falling part under long days, while a reversed picture emerged under short days. As a consequence, the power density during days. As a consequence, the power density during light was relatively high under long days. 3. After 24 h sleep deprivation by forced activity, no significant changes in the percentages of wakefulness and NREM were observed, whereas REM sleep was slightly enhanced. EEG power density, however, was significantly increased by ca. 50% in the 1.25-10.0 Hz range in the first 3 h of recovery sleep. This increase gradually decayed over the recovery night. 4. The same 24 h sleep deprivation technique led to a ca. 25% increase in oxygen consumption during recovery nights. While the results of the EEG spectral analysis are compatible with the hypothesis that delta power density reflects the 'intensity' of NREM sleep as enhanced by prior wakefulness and reduced by prior sleep, such enhanced sleep depth after sleep deprivation is not associated with reduced energy expenditure as might be anticipated by some energy conservation hypotheses on sleep function.     

Inter-Hemispheric Oscillations in Human Sleep

Sleep is generally categorized into discrete stages based on characteristic electroencephalogram (EEG) patterns. This traditional approach represents sleep architecture in a static way, but it cannot reflect variations in sleep across time and across the cortex. To investigate these dynamic aspects of sleep, we analyzed sleep recordings in 14 healthy volunteers with a novel, frequency-based EEG analysis. This approach enabled comparison of sleep patterns with low inter-individual variability. We then implemented a new probability dependent, automatic classification of sleep states that agreed closely with conventional manual scoring during consolidated sleep. Furthermore, this analysis revealed a previously unrecognized, interhemispheric oscillation during rapid eye movement (REM) sleep. This quantitative approach provides a new way of examining the dynamic aspects of sleep, shedding new light on the physiology of human sleep.