Statistical description and evaluation of the interrelationships of standard sleep variables for normal subjects

High stability of EEG spectral power across polysomnography and multiple sleep latency tests in good sleepers and chronic insomniacs

To assess the stability of electroencephalographic (EEG) spectral features across overnight polysomnography (PSG) and daytime multiple sleep latency tests (MSLTs) in chronic insomniacs (CIs) and normal controls (NCs). A total of 20 NCs and 22 CIs underwent standard PSG and MSLTs. Spectral analyses were performed on EEG data from PSG and MSLTs and absolute and relative power in central, frontal and occipital channels were obtained for wake (W) and non-rapid eye movement sleep stage 1 and 2 (N1, N2). Intraclass correlation coefficients (ICCs) were used to assess the stability of EEG spectral power across PSG and MSLTs for W, N1 and N2. The absolute power of all frequency bands except delta exhibited high stability across PSG and MSLTs in both NCs and CIs (ICCs ranged from 0.430 to 0.978). Although delta absolute power was stable in NCs during N1 and N2 stages (ICCs ranged from 0.571 to 0.835), it tended to be less stable in CIs during W and sleep stages (ICCs ranged from 0.042 to 0.807). We also observed lower stability of relative power compared to absolute power though the majority of relative power outcomes maintained high stability in both groups (ICCs in relative power ranged from 0.044 to 0.962). Most EEG spectral bandwidths across PSG and MSLT in W, N1 and N2 show high stability in good sleepers and chronic insomniacs. EEG signals from either an overnight PSG or a daytime MSLT may be useful for reliably exploring EEG spectral features during wakefulness or sleep.

Ultradian sleep cycles: Frequency, duration, and associations with individual and environmental factors-A retrospective study

OBJECTIVE

Sleep varies between individuals in response to sleep-wake history and various environmental factors, including light and noise. Here we report on the intranight variation of the ultradian nonrapid eye movement-rapid eye movement (NREM-REM) sleep cycle in 369 participants who have contributed to different laboratory studies from 1994 to 2020 at the Centre for Chronobiology, Basel, Switzerland.

RESULTS

We observed a large interindividual variability in sleep cycle duration, including NREM and REM sleep episodes in healthy participants who were given an 8-hour sleep opportunity at habitual bedtime in controlled laboratory settings. The median sleep cycle duration was 96 minutes out of 6064 polysomnographically-recorded cycles. The number and duration of cycles were not normally distributed, and the distribution became narrower for NREM sleep and wider for REM sleep later in the night. The first cycle was consistently shorter than subsequent cycles, and moderate presleep light or nocturnal noise exposure had no significant effects on ultradian sleep cycle duration. Age and sex significantly affected NREM and REM sleep duration, with older individuals having longer NREM and shorter REM sleep particularly in the end of the night, and females having longer NREM sleep episodes. High sleep pressure (ie, sleep deprivation) and low sleep pressure (ie, multiple naps) altered ultradian sleep cycles, with high sleep pressure leading to longer NREM sleep in the first cycle, and low sleep pressure leading to longer REM sleep episodes. Positive correlations were observed between N2 and NREM duration, and between N1 and REM duration. Weak intrasleep REM sleep homeostasis was also evident in our data set.

CONCLUSIONS

We conclude that ultradian sleep cycles are endogenous biological rhythms modulated by age, sex, and sleep homeostasis, but not directly responsive to (moderate levels of) environmental cues in healthy good sleepers.

Predictors of interindividual differences in vulnerability to neurobehavioral consequences of chronic partial sleep restriction

Interindividual differences in the neurobehavioral response to sleep loss are largely unexplained and phenotypic in nature. Numerous factors have been examined as predictors of differential response to sleep loss, but none have yielded a comprehensive view of the phenomenon. The present study examines the impact of baseline factors, habitual sleep-wake patterns, and homeostatic response to sleep loss on accrued deficits in psychomotor vigilance during chronic partial sleep restriction (SR), in a total of 306 healthy adults that participated in one of three independent laboratory studies. Findings indicate no significant impact of personality, academic intelligence, subjective reports of chronotype, sleepiness and fatigue, performance on working memory, and demographic factors such as sex, ethnicity, and body mass index, on neurobehavioral vulnerability to the negative effects of sleep loss. Only superior baseline performance on the psychomotor vigilance test and ability to sustain wakefulness on the maintenance of wakefulness test were associated with relative resilience to decrements in vigilant attention during SR. Interindividual differences in vulnerability to the effects of sleep loss were not accounted for by prior sleep history, habitual sleep patterns outside of the laboratory, baseline sleep architecture, or homeostatic sleep response during chronic partial SR. A recent theoretical model proposed that sleep-wake modulation may be influenced by competing internal and external demands which may promote wakefulness despite homeostatic and circadian signals for sleep under the right circumstances. Further research is warranted to examine the possibility of interindividual differences in the ability to prioritize external demands for wakefulness in the face of mounting pressure to sleep.

Assessing Sleep Architecture With Polysomnography During Posttraumatic Amnesia After Traumatic Brain Injury: A Pilot Study

BACKGROUND

Early-onset sleep disturbance is common following moderate to severe traumatic brain injury (TBI) and often emerges while patients are in posttraumatic amnesia (PTA). However, sleep disruptions during this subacute recovery phase are not well-defined, and research often utilizes indirect measures (actigraphy) that quantify sleep based on activity. This study aims to examine sleep macro-architecture and sleep quality directly with ambulatory polysomnography (PSG) and measure endogenous salivary melatonin levels for patients experiencing PTA following moderate to severe TBI.

METHOD

Participants were recruited from an inpatient TBI rehabilitation unit. Nighttime PSG was administered at the patient's bedside. Two saliva specimens were collected for melatonin testing on a separate evening (24:00 and 06:00 hours) using melatonin hormone profile test kits.

RESULTS

Of 27 patients in whom PSG was recorded, the minimum required monitoring time occurred in n =17 (adherence: 63%) at a median of 37.0 days (quartile 1 [Q1] to quartile 3 [Q3]: 21.5-50.5) postinjury. Median non-rapid eye movement (NREM) and REM sleep proportions were similar to normal estimates. Slow-wave sleep was reduced and absent in 35.3% of patients. Sleep periods appeared fragmented, and median sleep efficiency was reduced (63.4%; Q1-Q3: 55.1-69.2). Median melatonin levels at both timepoints were outside the normal range of values specified for this test (from Australian Clinical Labs).

CONCLUSION

This study reports that ambulatory PSG and salivary melatonin assessment are feasible for patients experiencing PTA and offers new insight into the extent of sleep disturbance. Further research is necessary to understand associations between PTA and sleep disturbance.

Perampanel effect on sleep architecture in patients with epilepsy

PURPOSE

Among patients with epilepsy, sleep disturbances can worsen seizure control. This prospective open-label study determined the effect of the antiepileptic drug perampanel on sleep architecture in patients with refractory epilepsy.

METHODS

Adult patients with refractory epilepsy received add-on perampanel, starting at 2 mg/day at bedtime, increased by 2 mg after 2 weeks and then monthly until the target dose of 4-8 mg/day was reached. The median dose of perampanel used was 6 mg (SD 1.2). Polysomnographic (PSG) recordings were scheduled 1 week before starting perampanel and the control PSG after 12 weeks under perampanel treatment and at least 4 weeks on stable perampanel dose; patients completed the Epworth Sleepiness Scale (ESS) and Pittsburgh Sleep Quality Index (PSQI) questionnaires. The main endpoints were change from baseline in the ESS and PSQI scores, and PSG variables.

RESULTS

Of 25 patients included (aged 18-65 years, 56 % female) only 17 completed the study. Perampanel caused a modest decrease from baseline in mean ESS score (n = 13 patients; p = 0.126) and PSQI score (n = 12 patients; p = 0.127). Treatment significantly improved sleep parameters (n = 17 patients) including total sleep time (p = 0.037), sleep latency (p = 0.022), sleep efficiency (p = 0.015), sleep maintenance index (p = 0.005), wake time after sleep onset (p = 0.015), and duration of N3 sleep stage (p = 0.026). Patients with altered sleep efficiency parameters at baseline showed a significant increase in sleep maintenance index (p = 0.015), and 77.8 % achieved sleep efficiency >85 % (p = 0.016 vs baseline).

CONCLUSION

Perampanel improved sleep architecture in patients with focal refractory epilepsy without worsening daytime sleepiness.

A Survey of Koreans on Sleep Habits and Sleeping Symptoms Relating to Pillow Comfort and Support

The number of people who complain of sleep disturbances is steadily increasing. An understanding of sleep-related factors is required to address sleep problems. This survey study investigated the sleep habits and sleeping symptoms relating to the comfort and support characteristics of pillows and the relationship between sleep quality and pillow design factors. The study utilized data from 332 participating Korean adults aged 20–76 years (mean age ± SD: males, 40.4 ± 15.2; females, 42.9 ± 15.4). We developed a questionnaire that evaluated sleep habits (sleep duration, bedtime, wake-up time and sleeping position); sleeping symptoms (snoring or coughing, breathing and sleepiness during waking hours) based on the Korean version of the Pittsburgh Sleep Quality Index (PSQI-K) questionnaire; and pillow-related factors (support, comfort, fatigue, height and shape) from existing pillow studies. The average sleep duration was 6.8 h, with more than half (52%) of participants sleeping in the supine position. The overall score for sleep quality was considered poor (4.84 points on a seven-point Likert scale), with some degree of sleepiness during waking hours (4.4 points on a seven-point Likert scale). Females went to bed earlier than males and were more likely to sleep in the lateral position compared to males. The number of toss and turn or waking events during sleep increased with age, and older individuals went to sleep earlier and woke up earlier. Among the symptoms of fatigue, pain, discomfort with changing position, snoring, coughing and breathing discomfort, participants reported their highest levels of discomfort due to sleepiness after waking, and they experienced the least head pain. Participants who used a regular-type pillow had poorer satisfaction on multiple comfort and support factors (support, comfort, height suitability, shape suitability) compared with those who used a functional-type pillow. Less head fatigue, less neck fatigue and less shoulder pain had significant effects on sleep quality. To reduce neck fatigue and shoulder pain, designers should consider the height for neck support in the lateral position. To reduce neck fatigue, it is desirable to use materials like latex or memory foam that provide neck support, which can improve sleep quality. The findings of this study contribute to a better understanding of sleep habits and characteristics of pillow comfort and provide practical guidelines for better pillow designs.

Sleep ultradian cycling: Statistical distribution and links with other sleep variables, depression, insomnia and sleepiness-A retrospective study on 2,312 polysomnograms

The number of alternations between Non-Rapid Eye Movement (NREM) sleep and Rapid Eye Movement (REM) sleep in humans is usually considered to consist of 4-5 cycles of about 90 minutes duration per night. Previous studies by our group showed a normal distribution on 26 healthy human subjects. The present study retrospectively analyzes the polysomnograms of 2,312 unmedicated patients who were admitted for medical and/or psychiatric reasons in the Erasme University Hospital between 2003 and 2014. The normal distribution of the Number of Cycles and Mean Cycle Duration was confirmed. Q-Q plots were very close to linearity. This distribution allows the use of these variables in parametric comparisons. The Number of Cycles per night and the Mean Cycle Duration showed predominant links with REM sleep-related variables, such as the REM Latency, REM sleep duration, the REM/NREM sleep ratio. None of these variables was associated with the diagnosis of Major Depressive Disorder, nor the intensity of Depression as measured by the Beck Depression Inventory (short version). On the other hand, the diagnosis of Major Depressive Disorder was significantly associated with the Insomnia Severity Index and correlated with the intensity of depressive symptoms (Beck Depression Inventory).

Childhood high-frequency EEG activity during sleep is associated with incident insomnia symptoms in adolescence

BACKGROUND

Insomnia has been associated in cross-sectional studies with increased beta (15-35 Hz) electroencephalogram (EEG) power during nonrapid eye movement (NREM) sleep, an index of cortical hyperarousal. However, it is unknown whether this cortical hyperarousal is present before individuals with insomnia develop the disorder. To fill this gap, we examined the association of childhood sleep high-frequency EEG activity with incident insomnia symptoms (i.e., absence of insomnia symptoms in childhood but presence in adolescence).

METHODS

We studied a case-control subsample of 45 children (6-11 years) from the Penn State Child Cohort, a population-based random sample of 421 children, who were followed up after 8 years as adolescents (13-20 years). We examined low-beta (15-25 Hz) and high-beta (25-35 Hz) relative power at central EEG derivations during NREM sleep and, in secondary analyses, during sleep onset latency, sleep onset, and REM sleep. Incident insomnia symptoms were defined as the absence of parent-reported difficulty falling and/or staying asleep during childhood and a self-report of these insomnia symptoms during adolescence.

RESULTS

Childhood high-beta power during NREM sleep was significantly increased in children who developed insomnia symptoms in adolescence (n = 25) as compared to normal sleeping controls (n = 20; p = .03). Multivariable-adjusted logistic regression models showed that increased childhood high-beta EEG power during NREM sleep was associated with a threefold increased odds (95% CI = 1.12-7.98) of incident insomnia symptoms in adolescence. No other significant relationships were observed for other sleep/wake states or EEG frequency bands.

CONCLUSIONS

Increased childhood high-frequency EEG power during NREM sleep is associated with incident insomnia symptoms in adolescence. This study indicates that cortical hyperarousal during sleep may be a premorbid neurophysiological sign of insomnia, which may mediate the increased risk of psychiatric disorders associated with insomnia.

Association Between Phase Coupling of Respiratory Sinus Arrhythmia and Slow Wave Brain Activity During Sleep

Phase coupling of respiratory sinus arrhythmia (RSA) has been proposed to be an alternative measure for evaluating autonomic nervous system (ANS) activity. The aim of this study was to analyze how phase coupling of RSA is altered during sleep, in order to explore whether this measure is a predictor of slow wave sleep (SWS). Overnight electroencephalograms (EEG), electrocardiograms (ECG), and breathing using inductance plethysmography were recorded from 30 healthy volunteers (six females, age range 21–64, 31.6 ± 14.7 years). Slow wave activity was evaluated by the envelope of the amplitude of the EEG δ-wave (0.5–4 Hz). The RSA was extracted from the change in the R-R interval (RRI) by band-pass filter, where pass band frequencies were determined from the profile of the power spectral density for respiration. The analytic signals of RSA and respiration were obtained by Hilbert transform, after which the amplitude of RSA (A_RSA) and the degree of phase coupling (λ) were quantified. Additionally, the normalized high-frequency component (HF_n) of the frequency-domain heart rate variability (HRV) was calculated. Using auto- and cross-correlation analyses, we found that overnight profiles of λ and δ-wave were correlated, with significant cross-correlation coefficients (0.461 ± 0.107). The δ-wave and HF_n were also correlated (0.426 ± 0.115). These correlations were higher than that for the relationship between δ-wave and A_RSA (0.212 ± 0.161). The variation of λ precedes the onset of the δ-wave by ~3 min, suggesting a vagal enhancement prior to the onset of SWS. Auto correlation analysis revealed that the periodicity of λ was quite similar to that of the δ-wave (88.3 ± 15.7 min vs. 88.6 ± 16.3 min, λ-cycle = 0.938 × δ-cycle + 5.77 min, r = 0.902). These results suggest that phase coupling analysis of RSA appears to be a marker for predicting SWS intervals, thereby complementing other noninvasive tools and diagnostic efforts.

The sleep EEG spectrum is a sexually dimorphic marker of general intelligence

The shape of the EEG spectrum in sleep relies on genetic and anatomical factors and forms an individual "EEG fingerprint". Spectral components of EEG were shown to be connected to mental ability both in sleep and wakefulness. EEG sleep spindle correlates of intelligence, however, exhibit a sexual dimorphism, with a more pronounced association to intelligence in females than males. In a sample of 151 healthy individuals, we investigated how intelligence is related to spectral components of full-night sleep EEG, while controlling for the effects of age. A positive linear association between intelligence and REM anterior beta power was found in females but not males. Transient, spindle-like "REM beta tufts" are described in the EEG of healthy subjects, which may reflect the functioning of a recently described cingular-prefrontal emotion and motor regulation network. REM sleep frontal high delta power was a negative correlate of intelligence. NREM alpha and sigma spectral power correlations with intelligence did not unequivocally remain significant after multiple comparisons correction, but exhibited a similar sexual dimorphism. These results suggest that the neural oscillatory correlates of intelligence in sleep are sexually dimorphic, and they are not restricted to either sleep spindles or NREM sleep.

Short- and Long-Term Sleep Stability in Insomniacs and Healthy Controls

STUDY OBJECTIVES

Assess the short- and long-term stability of sleep duration in patients with insomnia and normal-sleeping controls.

DESIGN

Observational short-term and prospective studies.

SETTING

Sleep laboratory.

PARTICIPANTS

Patients with insomnia (n = 150) and controls (n = 151) were recruited from the local community or sleep disorders clinic. A subsample of 95 men from the Penn State Adult Cohort (PSAC) were followed up 2.6 y after their initial visit.

MEASUREMENTS

Participants underwent a physical examination and 8-h polysomnography (PSG) recording for 3 consecutive nights (controls and insomniacs), or 2 single nights separated by several years (PSAC). Intraclass correlation coefficients (ICCs) assessed the stability of the variables total sleep time (TST), sleep onset latency (SOL), and wake after sleep onset (WASO). We also examined persistence of the first-night classification of "short" versus "normal" sleep duration on subsequent nights.

RESULTS

Stability of TST, SOL, and WASO based on 1 night were slight to moderate in both patients with insomnia (ICC = 0.37-0.57) and controls (ICC = 0.39-0.59), and became substantial to almost perfect when based on the average of 3 nights (ICC = 0.64-0.81). We observed similar degrees of stability for TST and WASO in the longitudinal sample, with moderate stability based on a single night and substantial stability based on both nights. In examining the persistence of "short" and "normal" sleep duration, 71.4% (controls), 74.7% (patients with insomnia), and 72.6% (longitudinal sample) of participants retained their first-night classifications over subsequent nights.

CONCLUSIONS

Sleep duration variables, particularly total sleep time based on 3 consecutive nights in both patients with insomnia and controls or two single-night recordings separated by several years, are stable and reflect a person's habitual sleep. Furthermore, a single night in the laboratory may be useful for reliably classifying one's sleep duration.

Sleep is associated with offline improvement of motor sequence skill in children

In adults, sleep is necessary for the offline improvement of certain skills, such as sequential finger tapping, but whether children show a similar effect is still debatable. Here, we tested whether sleep is associated with offline performance improvement in children. Nine- and 11-year-old children trained on an explicit sequential finger tapping task. On the night following training, their parents observed and recorded the duration of each child’s sleep. The following day, all children performed a surprise retest session on the previously trained sequence. In both 9- and 11-year-old children, skill performance was significantly improved during the first retest session relative to the end of training on the previous day, confirming the offline improvement in performance. There was a significant correlation between the degree of improvement and sleep duration the night after training, suggesting that in children, as in adults, sleep is associated with offline skill enhancement.

About sleep's role in memory

Over more than a century of research has established the fact that sleep benefits the retention of memory. In this review we aim to comprehensively cover the field of "sleep and memory" research by providing a historical perspective on concepts and a discussion of more recent key findings. Whereas initial theories posed a passive role for sleep enhancing memories by protecting them from interfering stimuli, current theories highlight an active role for sleep in which memories undergo a process of system consolidation during sleep. Whereas older research concentrated on the role of rapid-eye-movement (REM) sleep, recent work has revealed the importance of slow-wave sleep (SWS) for memory consolidation and also enlightened some of the underlying electrophysiological, neurochemical, and genetic mechanisms, as well as developmental aspects in these processes. Specifically, newer findings characterize sleep as a brain state optimizing memory consolidation, in opposition to the waking brain being optimized for encoding of memories. Consolidation originates from reactivation of recently encoded neuronal memory representations, which occur during SWS and transform respective representations for integration into long-term memory. Ensuing REM sleep may stabilize transformed memories. While elaborated with respect to hippocampus-dependent memories, the concept of an active redistribution of memory representations from networks serving as temporary store into long-term stores might hold also for non-hippocampus-dependent memory, and even for nonneuronal, i.e., immunological memories, giving rise to the idea that the offline consolidation of memory during sleep represents a principle of long-term memory formation established in quite different physiological systems.

Short-term stability of sleep and heart rate variability in good sleepers and patients with insomnia: for some measures, one night is enough

STUDY OBJECTIVES

Quantify the short-term stability of multiple indices of sleep and nocturnal physiology in good sleeper controls and primary insomnia patients.

DESIGN

Intra-class correlation coefficients (ICC) were used to quantify the short-term stability of study outcomes.

SETTING

Sleep laboratory.

PARTICIPANTS

Fifty-four adults with primary insomnia (PI) and 22 good sleeper controls (GSC).

MEASUREMENTS

Visually scored sleep outcomes included indices of sleep duration, continuity, and architecture. Quantitative EEG outcomes included power in the delta, theta, alpha, sigma, and beta bands during NREM sleep. Power spectral analysis was used to estimate high-frequency heart rate variability (HRV) and the ratio of low- to high-frequency HRV power during NREM and REM sleep.

RESULTS

With the exception of percent stage 3+4 sleep; visually scored sleep outcomes did not exhibit short-term stability across study nights. Most QEEG outcomes demonstrated short-term stability in both groups. Although power in the beta band was stable in the PI group (ICC = 0.75), it tended to be less stable in GSCs (ICC = 0.55). Both measures of cardiac autonomic tone exhibited short-term stability in GSCs and PIs during NREM and REM sleep.

CONCLUSIONS

Most QEEG bandwidths and HRV during sleep show high short-term stability in good sleepers and patients with insomnia alike. One night of data is, thus, sufficient to derive reliable estimates of these outcomes in studies focused on group differences or correlates of QEEG and/or HRV. In contrast, one night of data is unlikely to generate reliable estimates of PSG-assessed sleep duration, continuity or architecture, with the exception of slow wave sleep.

Sleep, intelligence and cognition in a developmental context: differentiation between traits and state-dependent aspects

This article addresses associations between sleep, cognition and intelligence in a developmental context and clarifies the terminology. Research must differentiate between aspects related to general underlying traits and those aspects that are characterized by state-dependent fluctuations.

The sleep EEG as a marker of intellectual ability in school age children

STUDY OBJECTIVES

To investigate the within-subject stability in the sleep EEG and the association between the sleep EEG and intellectual abilities in 9- to 12-year-old children.

DESIGN

Intellectual ability (WISC-IV, full scale, fluid, and verbal IQ, working memory, speed of processing) were examined and all-night polysomnography was performed (2 nights per subject).

SETTING

Sleep laboratory.

PARTICIPANTS

Fourteen healthy children (mean age 10.5 ± 1.0 years; 6 girls).

MEASUREMENTS AND RESULTS

Spectral analysis was performed on artifact-free NREM sleep epochs (C3/A2). To determine intra-individual stability and inter-individual variability of the sleep EEG, power spectra were used as feature vectors for the estimation of Euclidean distances, and intraclass correlation coefficients (ICC) were calculated for the 2 nights. Sleep spindle peaks were identified for each individual and individual sigma band power was determined. Trait-like aspects of the sleep EEG were observed for sleep stage variables and spectral power. Within-subject distances were smaller than between-subject distances and ICC values ranged from 0.72 to 0.96. Correlations between spectral power in individual frequency bins and intelligence scores revealed clusters of positive associations in the alpha, sigma, and beta range for full scale IQ, fluid IQ, and working memory. Similar to adults, sigma power correlated with full scale (r = 0.67) and fluid IQ (r = 0.65), but not with verbal IQ. Spindle peak frequency was negatively related to full scale IQ (r = -0.56).

CONCLUSIONS

The sleep EEG during childhood shows high within-subject stability and may be a marker for intellectual ability.

Heritability of sleep electroencephalogram

BACKGROUND

Understanding the basis of sleep-related endophenotypes might help to pinpoint factors modulating susceptibility to psychiatric disorders. However, the genetic underpinnings of sleep microarchitecture in humans remain largely unknown. Here we report on the results of a classical twin study in monozygotic (MZ) and dizygotic (DZ) twin pairs examining the genetic effect on sleep electroencephalogram (EEG) composition.

METHODS

Polysomnographic recordings were obtained in 35 pairs of MZ (26.4 +/- 5.4 years, 17-43 years, 17 male pairs, 18 female pairs) and 14 same-gender pairs of DZ twins (22.1 +/- 2.7 years, 18-26 years, 7 male pairs, 7 female pairs). The EEG power spectra were generated on the basis of Fast Fourier transformations combined with conventional sleep parameters, according to standardized criteria.

RESULTS

We tested the genetic variance contributing to the observed overall variance of the sleep measures and found that the relative contributions of the delta, theta, alpha, and sigma frequency bands at central derivations were significantly correlated to the genetic background. In these frequency bands, MZ twins also showed within-pair concordance in spectral power that was significantly higher than that of DZ twins.

CONCLUSIONS

The broad overlap of EEG frequencies during non-REM sleep and wakefulness, which shows a significant genetic variance, supports the hypothesis of common neuronal mechanisms generating EEG oscillations in humans. Our findings strongly support the suitability of the spectral composition of non-REM sleep for defining endophenotypes.

Ultradian cycles in mice: definitions and links with REMS and NREMS

Sleep can be organized in two quite different ways across homeothermic species: either in one block (monophasic), or in several bouts across the 24 h (polyphasic). Yet, the main relationships between variables, as well as regulating mechanisms, are likely to be similar. Correlations and theories on sleep regulation should thus be examined on both types of sleepers. In previous studies on monophasic humans, we have shown preferential links between the number of ultradian cycles and the rapid eye movement sleep (REMS) time, rather than with its counterpart non-rapid eye movement sleep (NREMS). Here, the sleep of 26 polyphasic mice was examined, both to better describe the NREMS distribution, which is far more complex than in humans, and to replicate the analyses performed on humans. As in humans, the strongest links with the number of cycles were with REMS. Links were not significant with NREMS taken as a whole, although positive correlations were found with the NREMS immediately preceding REMS episodes and inversely significant with the residue. This convergence between monophasic and polyphasic patterns supports the central role played by REMS in sleep alternation.

Trait interindividual differences in the sleep physiology of healthy young adults

Despite decades of sleep research by means of polysomnography (PSG), systematic interindividual differences in PSG-assessed sleep parameters have been scarcely investigated. The present study is the first to quantify interindividual variability in standard PSG-assessed variables of sleep structure in terms of stability and robustness as well as magnitude. Twenty-one carefully screened healthy young adults were studied continuously in a strictly controlled laboratory environment, where their PSGs were recorded for eight nights interspersed with three separate 36 h sleep deprivation periods. All PSG records were scored blind to subject and condition, using conventional criteria, and delta power in the non-REM sleep EEG was computed for four electrode derivations. Interindividual differences in sleep variables were examined for stability and robustness, respectively, by comparing results across equivalent nights (e.g. baseline nights) and across experimentally differentiated nights (baseline nights versus recovery nights following sleep deprivation). Among 18 sleep variables analyzed, all except slow-wave sleep (SWS) latency were found to exhibit significantly stable and robust--i.e. trait-like--interindividual differences. This was quantified by means of intraclass correlation coefficients (ICCs), which ranged from 36% to 89% across physiologic variables, and were highest for SWS (73%) and delta power in the non-REM sleep EEG (78-89%). The magnitude of the trait interindividual differences was considerable, consistently exceeding the magnitude of the group-average effect on sleep structure of 36 h total sleep deprivation. Notably, for non-REM delta power--a putative marker of sleep homeostasis--the interindividual differences were from 9.9 to 12.8 times greater than the group-average increase following sleep deprivation relative to baseline. Physiologic sleep variables did not vary among subjects in a completely independent manner--61.1% of their combined variance clustered in three trait dimensions, which appeared to represent sleep duration, sleep intensity, and sleep discontinuity. Any independent functional significance of these sleep physiologic phenotypes remains to be determined.

Individual variation in sleep and motor activity in rats

We examined individual differences in sleep and motor activity across 2 consecutive days in rats. EEG and motor activity were recorded via telemetry in Wistar rats (n=29) for 48h under well-habituated conditions. Rats were grouped based on sleep amounts and stability across days (short [SS, n=7], intermediate [IS, n=15] and long [LS, n=7] sleep) and comparisons were conducted to determine group differences for measures of sleep and motor activity. We found that correlations across recording days were significant for all selected sleep measures and motor activity counts. Rankings for 24h total sleep time and non-rapid eye movement sleep (NREM) were SS<IS<LS rats whereas amounts of rapid eye movement sleep did not differ among groups. Further analyses of NREM episode parameters found significant differences in mean episode length (SS<IS<LS) but not in the number of episodes. Total and average motor activity counts (per waking min) were greater (32-38%) in SS compared to LS rats on both recording days. The results indicate that individual differences in sleep and motor activity in Wistar rats are stable across days. Differences between SS and LS rats have parallels to those reported for short and long sleep humans.

Trait-like individual differences in the human sleep electroencephalogram

We aimed to examine whether commonly observed individual differences in sleep architecture and the sleep electroencephalogram reflect individual traits, which are amenable to a genetic investigation of human sleep. We studied intra-individual stability and inter-individual variation in sleep and sleep electroencephalogram spectra across four baseline recordings of eight healthy young men. A similarity concept based on Euclidean distances between vectors was applied. Visually scored sleep variables served as feature vector components, along with electroencephalogram power spectra in non-rapid-eye-movement and rapid-eye-movement sleep. The distributions of similarity coefficients of feature vectors revealed a clear distinction between high within-subject similarity (i.e. stability), and low between-subject similarity (i.e. variation). Moreover, a cluster analysis based on electroencephalogram spectra in both non-rapid-eye-movement and rapid-eye-movement sleep segregated all four baseline nights of each individual into a distinct cluster. To investigate whether high and low sleep pressure affects the similarity coefficients, normalized non-rapid-eye-movement sleep electroencephalogram spectra of the first and second half of the recordings were compared. Because the electroencephalogram changes systematically in the course of the night, within-subject variation no longer differed from between-subject variation. In conclusion, our data provide evidence for trait-like characteristics in the sleep electroencephalogram. Further studies may help to identify distinct phenotypes to search for genes underlying functional aspects of undisturbed human sleep.

Inverse association between Slow Wave Activity per cycle and the number of ultradian sleep cycles per night in healthy humans

OBJECTIVE

Comparisons of sleep Slow Wave Activity (SWA) during successive sleep cycles rely on the assumption that SWA in a given cycle is independent of the number of ultradian cycles present in a night. This assumption was evaluated here.

METHODS

Twenty-six healthy controls with no medical, sleep or psychiatric disorders were selected among 84 candidates and their sleep was recorded at home across 2 consecutive nights after two habituation nights.

RESULTS

In comparison with nights with less cycles, nights with more cycles showed significantly more REMS but not more NREMS. No correlation was found between the number of cycles and the integrated SWA per night (epochs visually scored as NREMS). However, inverse correlations were found between the number of cycles and the SWA per cycle. This was significant on both nights in Cycle 1 and strong trends were found for the two subsequent cycles on Night 2. Comparable results were found after removal of nights containing suspected Skipped First REMS episodes.

CONCLUSIONS

The SWA in a cycle was found to be inversely correlated to the number of cycles in the first 3 cycles in at least one of the two analyzed nights.

SIGNIFICANCE

Differences in the number of cycles per night are a potential bias in the comparisons of SWA per cycles.

First-night effect in the chronic fatigue syndrome

Since the magnitude of the first-night effect has been shown to be a function of medical conditions and of settings in which polysomnographies are performed, it is essential to evaluate the habituation phenomenon in each case in order to determine the optimal recording methodology. A first-night effect was evidenced in certain cases of chronic fatigue syndrome, but not in others. To clarify this issue, a large group of patients with chronic fatigue syndrome who had no primary sleep disorders were selected and recorded for two consecutive nights in a hospital sleep unit. Several parameters, frequently associated with the first-night effect, were found to be influenced by the recording methodology: Total Sleep Time, Sleep Efficiency, Sleep Efficiency minus Sleep Onset, Sleep Onset Latency, Wake Time, Slow Wave Sleep, Rapid Eye Movement Sleep, Rapid Eye Movement Sleep Latency and Number of Sleep Cycles. Bland and Altman plots determined that the difference scores between the nights included a systematic bias linked to the order of recordings (first-night effect). Factorial analysis grouped the difference scores into three factors. No significant difference was observed between patients with generalized anxiety comorbidity and those with no psychiatric comorbidity, or between those with and without psychiatric comorbidity. Chronic fatigue syndrome must thus be added on the list of conditions where a clinically significant habituation effect takes place.

Correlations using the NREM-REM sleep cycle frequency support distinct regulation mechanisms for REM and NREM sleep

Polysomnograms of most homeothermic species distinguish two states, rapid eye movement (REM) and non-REM (NREM) sleep. These alternate several times during the night for reasons and following rules that remain poorly understood. It is unknown whether each state has its own function and regulation or whether they represent two facets of the same process. The present study compared the mean REM/NREM sleep ratio and the mean number of NREM-REM sleep cycles across 3 consecutive nights. The rationale was that, if REM and NREM sleep are tightly associated, their ratio should be comparable whatever the cycle frequency in the night. Twenty-six healthy subjects of both sexes were recorded at their home for 4 consecutive nights. The correlation between the REM/NREM sleep ratio and the number of cycles was highly significant. Of the two sleep components, REM sleep was associated to the number of cycles, whereas NREM sleep was not. This suggests that the relationship between REM sleep and NREM sleep is rather weak within cycles, does not support the concept of NREM-REM sleep cycles as miniature units of the sleep process, and favors the concept of distinct mechanisms of regulation for the two components.

Spontaneous sleep interruptions during extended nights. Relationships with NREM and REM sleep phases and effects on REM sleep regulation

OBJECTIVES

There is no agreement in the literature as to whether sleep interruption causes rapid eye movement (REM) pressure to increase, and if so, whether this increase is expressed as shortened REM latency, increased REM density, or increased duration of REM sleep. The purpose of the present study was to examine the effect of different durations of spontaneous sleep interruptions on the regulation of REM sleep that occurs after return to sleep.

METHODS

The occurrence of spontaneous periods of wakefulness and their effects on subsequent REM sleep periods were analysed in a total sample of 1189 sleep interruptions which occurred across 364 extended nights in 13 normal subjects.

RESULTS

Compared with sleep interruptions that last less than 10 min, sleep interruptions that last longer than 10 min occur preferentially out of REM sleep. In both the short and long types of sleep interruptions, the duration of REM periods that ended in wakefulness were shorter than the duration of those that were not interrupted by wakefulness. REM densities of the REM periods that terminated in periods of wakefulness were higher than those of uninterrupted REM periods. The proportion of episodes of wakefulness following REM sleep that were long-lasting progressively increased over the course of the extended night period. The sleep episodes that followed the periods of wakefulness were characterised by a short REM latency. REM duration was increased in episodes that followed long sleep interruptions compared to those that followed short sleep interruptions. REM density did not appear to change significantly in the episodes that followed sleep interruption.

CONCLUSIONS

REM sleep mechanisms appear to be the main force controlling sleep after a spontaneous sleep interruption, presumably because during the second half of the night, where more sleep interruptions occur, the pressure for non-rapid eye movement sleep is reduced and the circadian rhythm in REM sleep propensity reaches its peak. Processes promoting REM sleep at the end of the night are consistent with the Pittendrigh and Daan dual oscillator model of the circadian pacemaker.

Shorter REM latency associated with more sleep cycles of a shorter duration in healthy humans

A significant association between rapid eye movement (REM) sleep latency and the number of non-REM/REM sleep cycles was found 15 years ago in a large retrospective study. The present prospective study further explored this intra-sleep relationship and analyzed the links between these two variables and the mean cycle duration. It was based on a carefully selected group of healthy control subjects whose sleep was polysomnographically recorded at home for 4 sequential nights. The latency of REM sleep was inversely correlated with the number of cycles and positively correlated with the mean cycle duration, both in individual nights and on means of 4 nights. The present study demonstrated that variations in the number of cycles or the mean cycle duration between the nights are far less important than the substantial differences observed between subjects. Present outcomes support the study of sleep cycle periods and frequencies in those psychiatric disorders where REM sleep latencies have been found to be shorter, and they suggest that these variables be included in sleep studies in which cycles are compared with each other.

Internight reliability and benchmark values for computer analyses of non-rapid eye movement (NREM) and REM EEG in normal young adult and elderly subjects

OBJECTIVE

To determine the reliability of computer measured non-rapid eye movement (NREM) and REM frequency bands in the 0.3-45 Hz range and to provide benchmark data for these measures in young normal (YN) and elderly normal (EN) subjects (Ss).

METHODS

Sleep EEG was recorded in 19 YN and 19 EN Ss on 4 non-consecutive baseline nights and simultaneously quantified as fast Fourier transform (FFT) power and 3 zero-cross period-amplitude (PA) measures: integrated amplitude, time in band and average wave amplitude.

RESULTS

The shapes of both the FFT and PA spectra differed among Ss but were highly consistent within individuals. Inter-night reliability of the separate frequency bands was correspondingly high. Despite substantial age effects, the reliability of computer-measured sleep EEG in the elderly equaled that of the YN Ss. Within both the YN and EN groups, the shapes of the NREM and REM spectral curves differed significantly. The NREM and REM also differed significantly in the two age groups.

CONCLUSIONS

Computer-measured sleep EEG is highly reliable across non-consecutive nights in both young and elderly normal Ss. The trait-like stability of these measures suggests they are genetically determined. This possibility is supported by twin study data that show strong heritability for FFT-measured waking EEG. The different shapes of NREM and REM spectra add further evidence that these are fundamentally different states of brain organization. The age differences in spectral shape, along with PA data for wave incidence, demonstrate that age effects on sleep EEG are not caused by changes in skull impedance or other non-cerebral factors.

EEG sleep patterns in twins

The investigation of sleep in twins represents one of the major methods for measuring the genetic contributions to sleep in humans. This paper reviews two twin studies in which the sleep EEG was recorded during three consecutive nights in young monozygotic (MZ) and dizygotic (DZ) male twins. The analyses, based on average values of repeated sleep recordings, indicate that a significant proportion of variance in stages 2, 4, and delta sleep appears to be genetically determined. Genetic influences on rapid-eye-movement sleep were found inconclusive, but this conclusion is limited by the relatively small size of the sample studied.

Evolution of delta activity within the nonREM sleep episode: a biphasic hypothesis

The time course of delta activity within nonREM (NREM) episodes is measured for 24 healthy subjects with normal REM latencies. The first two NREM episodes in particular, show two very clearly separated peaks for about 35% of the subjects. Another 25% show two less well separated peaks. These double peak patterns are also prevalent in the literature, but there has been a tendency to dismiss them as a skipped REM effect. They are, however, still evident even when the data are averaged over the 24 subjects, indicating a systematic phenomenon. These averaged data are well fitted by an analytic function given by the sum of two consecutive overlapping Gaussian curves. The well-behaved residuals also, are an indication that a biphasic model of this kind is statistically appropriate. The model proposed is simple, with parameters related to physiological phenomena, and it suggests that there may be an underlying process with delta waves emanating from two separate signal sources. Recent neurophysiological findings suggest that delta oscillations are generated both in the thalamus and in the cortex and show that excessive synchronization of slow oscillations may lead to seizures. Hence the speculation that the biphasic process may emanate from cortical and thalamic sources and be protective in the sense that it permits smaller delta amplitudes at each source while retaining the integral delta energy necessary to satisfy sleep pressure. It is significant that the two peaks are most evident in the first two NREM episodes where delta power is high.

Effects of chronic treatment with methadone and naltrexone on sleep in addicts

Previous studies have described sleep disturbance secondary to chronic opiate use and abuse. Drug-dependency insomnia is of interest because chronic sleep disturbances can promote depressive symptoms which could lead to a drug relapse. For the first time we compared the polysomnographic parameters of 10 methadone-substituted outpatients and 10 naltrexone-treated outpatients. Methadone (mu-opioid agonist) produced a marked fragmentation of the sleep architecture with frequent awakenings and a decrease in EEG arousals. In comparison with methadone and controls, the naltrexone (mu-opioid antagonist) group showed the shortest sleep latency and the longest total sleep time. These data indicate that mu-agonists and mu-antagonists have different effects on sleep. The implications, especially the involvement of opioid-dopamine interactions on sleep and movements during sleep, are discussed.

Extended sleep in humans in 14 hour nights (LD 10:14): relationship between REM density and spontaneous awakening

The sleep patterns of 8 normal subjects living in a winter-type photoperiod (10 h light and 14 h darkness; LD 10:14) for 4 weeks were characterized by the presence of periods of spontaneous wakefulness alternating with periods of spontaneous sleep. Transitions from sleep to wakefulness occurred much more frequently out of REM sleep than out of NREM sleep (P < 0.002). REM periods that terminated in wakefulness showed shorter REM durations (P < 0.0005) and higher REM densities (P < 0.0005) than REM periods that did not terminate in wakefulness. The authors discuss these results in terms of a possible relationship between REM density and arousal level. The higher REM density preceding wakefulness and the increased number of REM periods terminating in spontaneous awakenings could reflect an enhanced level of a brain arousing process, resulting from reduced sleep pressure in the extended nights.

Cluster arousal analysis in chronic pain-disturbed sleep

A new cluster arousal analysis system was developed to quantify microdisturbances in the sleep architecture. The system was evaluated in 10 healthy volunteers and the sleep of 23 outpatients with chronic low back pain was investigated before and after a special rehabilitative training. The comparison of sleep parameters before and after the treatment showed a significant reduction of the cluster disturbed sleep (CDS), whereas sleep efficiency and the number of stage shifts remained unchanged. The reduction of the CDS correlated with a reduction of depression scale. This cluster arousal analysis appears to be a sensitive tool for evaluating alterations in the sleep microstructure.

Covariation of physiological sleep parameters in coronary patients and their relationships to sleep quality

Relationships between the time series of five physiological sleep parameters were examined and related to sleep quality. Twenty-eight male coronary patients participated in the investigation. Recordings were carried out with a mobile device consisting of two Medilog 4-24 recorders and one DMI-recorder assessing five biosignals: EEG, EOG, EMG, ECG and body movements. Relationships between the time series of physiological sleep parameters were obtained by partitioning of covariance. The investigation showed that substantial relationships exist between the time series of the EEG, the number of eye movements and heart rate. Moreover, the time series of body movements showed substantial relationships to the time series of the EMG and heart rate. High correlations between the time series of the EEG and heart rate were related to a better sleep quality.

Genetic determinants of EEG sleep: a study in twins living apart

In order to investigate the genetic components of sleep and, in particular, of REM sleep, we performed 3 consecutive all-night EEG recordings in 26 pairs of normal male twins living apart (11 monozygotic and 15 dizygotic). Our results indicate that in man non-genetic rather than genetic influences substantially determine variance in stage REM, in contrast to stages 2, 4 and to delta sleep. In this sample of male twins, waking measures also showed a significant genetic component.

The intrasleep relationship between wake and stage 4 examined by transition probability analysis

The relationship between wake and stage 4 of slow-wave sleep (SWS), in particular the previously observed deficiency in SWS accompanying sleep containing long-wake periods, is examined in this study of 147 health subjects. Stage shift comportment is compared between those NREM/REM cycles with wake periods greater than 3 minutes and those with less, using the method of transition probabilities. It is shown that these long wake interruptions occur preferentially in light sleep, and systematically disrupt the regular normal descent towards SWS, but do not significantly reduce the number of SWS episodes. There is at the same time, however, a reduction in the average duration of stage 4 periods of SWS which accounts for the observed reduction in the total amount of SWS.

Latencies of REM sleep and awakening in major depression: possible indicators of cholinergic activity

REM latency and awakening latency were analyzed in a sample of 26 major depressive inpatients and 8 male controls recorded for two consecutive nights. A significant inverse relationship appeared between REM latency and awakening latency in depressed patients. The relationship was more marked in male than in female patients. No significant correlation between REM latency and awakening latency was observed in male healthy volunteers. The hypothetical cholinergic supersensitivity in major depression is proposed to explain the present relationship. These results suggest that awakening latency might also be taken into account in the evaluation of sleep disturbances in depressive illness.

EEG sleep patterns in man: a twin study

All-night EEG sleep recording was performed for 3 consecutive nights in 26 pairs of normal male twins (14 monozygotic and 12 dizygotic) in order to investigate genetic components of sleep. The analysis was based on average values of repeated sleep measures and controlled for the effect of cohabitation. Our results indicate that a significant proportion of variance in stages 2, 4 and delta sleep as well as in REM density is genetically determined in man. Genetic influences on stage 1 and REM are strongly confounded by a synchronizing effect of the cohabitational status.

Effects of daytime noise load on the sleep-wake cycle and endocrine patterns in man: I. 24 hours neurophysiological data

This study examines the influence of daytime noise load on the spontaneous EEG activity during wakefulness and sleep. Twelve healthy male subjects participated in two experimental series, each consisting of three consecutive nights and the two days in between. EEG, EOG, EMG, ECG and respiration were continuously recorded. During one series from 9:00 to 21:00 h the subjects were exposed to 85 dB(A) industrial noise. Direct effects of the noise exposure were a slight blockade of the alpha and theta activity; towards the evening, when self-estimated tiredness was high, increased alpha and beta power were found (p less than .01). Aftereffects on subsequent undisturbed night sleep were a reduction in REM sleep (p less than .001), shortened sleep cycles (p less than .01) and an increase in slow wave sleep during the 2nd sleep cycle (p less than .05). These results are interpreted as signs of strained wakefulness and intensified fatigue, as disturbance of sleep processes and as an intensified need for recovery.

Instrumentation to describe subjective sleep characteristics in healthy subjects

The purpose of this study was to develop and test the Verran and Snyder-Halpern (VSH) Sleep Scale, an instrument to subjectively measure sleep characteristics. Four major sleep factors and their associated characteristics were proposed for the Sleep Scale. Subjects completed three randomly ordered sleep questionnaires on three consecutive weekday mornings within the first two hours after arising. Scales included the VSH Sleep Scale, a sleep questionnaire and a sleep log. The VSH Sleep Scale had a reliability coefficient of .82 (theta). Construct validity was examined by factor analysis and correlations between Sleep Scale items and corresponding items on the two other study instruments. Scale validity also was assessed by the known groups method. Beginning support for the validity of the VSH Sleep Scale is provided.

Sleep movements and poor sleep in patients with non-specific somatic complaints--I. No first-night effect in poor and good sleepers

To evaluate the possible first-night effect on the nocturnal motor activity 25 poor sleepers and 12 good sleepers slept on the Static Charge Sensitive Bed (SCSB) during two consecutive nights. The frequency of body movements in poor sleepers was almost two times higher than in good sleepers. The method itself was reproducible across two nights. There were no statistically significant and systemic level differences between the nights in the movements in bed (MIB). The difference t-test did not either reveal group differences in the magnitude or direction of changes from night to night. Results are consistent with the view that the level of motor activity is one of the determinants of sleep quality. No first-night effect exists in terms of psychomotor activity.