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

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

Acute exposure to evening blue‐enriched light impacts on human sleep

Light in the short wavelength range (blue light: 446–483 nm) elicits direct effects on human melatonin secretion, alertness and cognitive performance via non‐image‐forming photoreceptors. However, the impact of blue‐enriched polychromatic light on human sleep architecture and sleep electroencephalographic activity remains fairly unknown. In this study we investigated sleep structure and sleep electroencephalographic characteristics of 30 healthy young participants (16 men, 14 women; age range 20–31 years) following 2 h of evening light exposure to polychromatic light at 6500 K, 2500 K and 3000 K. Sleep structure across the first three non‐rapid eye movement non‐rapid eye movement – rapid eye movement sleep cycles did not differ significantly with respect to the light conditions. All‐night non‐rapid eye movement sleep electroencephalographic power density indicated that exposure to light at 6500 K resulted in a tendency for less frontal non‐rapid eye movement electroencephalographic power density, compared to light at 2500 K and 3000 K. The dynamics of non‐rapid eye movement electroencephalographic slow wave activity (2.0–4.0 Hz), a functional index of homeostatic sleep pressure, were such that slow wave activity was reduced significantly during the first sleep cycle after light at 6500 K compared to light at 2500 K and 3000 K, particularly in the frontal derivation. Our data suggest that exposure to blue‐enriched polychromatic light at relatively low room light levels impacts upon homeostatic sleep regulation, as indexed by reduction in frontal slow wave activity during the first non‐rapid eye movement episode.     

Creatine supplementation reduces sleep need and homeostatic sleep pressure in rats

Sleep has been postulated to promote brain energy restoration. It is as yet unknown if increasing the energy availability within the brain reduces sleep need. The guanidine amino acid creatine (Cr) is a well‐known energy booster in cellular energy homeostasis. Oral Cr–monohydrate supplementation (CS) increases exercise performance and has been shown to have substantial effects on cognitive performance, neuroprotection and circadian rhythms. The effect of CS on cellular high‐energy molecules and sleep–wake behaviour is unclear. Here, we examined the sleep–wake behaviour and brain energy metabolism before and after 4‐week‐long oral administration of CS in the rat. CS decreased total sleep time and non‐rapid eye movement (NREM) sleep significantly during the light (inactive) but not during the dark (active) period. NREM sleep and NREM delta activity were decreased significantly in CS rats after 6 h of sleep deprivation. Biochemical analysis of brain energy metabolites showed a tendency to increase in phosphocreatine after CS, while cellular adenosine triphosphate (ATP) level decreased. Microdialysis analysis showed that the sleep deprivation‐induced increase in extracellular adenosine was attenuated after CS. These results suggest that CS reduces sleep need and homeostatic sleep pressure in rats, thereby indicating its potential in the treatment of sleep‐related disorders.     

Cognitive performance in DSWPD patients upon awakening from habitual sleep compared with forced conventional sleep

Difficult early morning awakening is one of the defining symptoms of delayed sleep–wake phase disorder. It is accompanied by low cognitive arousal and drowsiness resulting in difficulty concentrating and focusing attention upon awakening. We designed the current study to quantitate cognitive performance (i.e. omissions, commissions, reaction time [average and variability]) and cognitive domains (i.e. focused attention, sustained attention, impulsivity and vigilance) with Conners’ Continuous Performance Test II during both habitual and conventional (00:00–07:00 hr) sleep–wake schedule in young adult patients with delayed sleep–wake phase disorder (n = 20, mean age = 24.8 years, SD = 3.0) and controls (n = 16, mean age = 24.4 years, SD = 3.4). Conners’ Continuous Performance Test II was administered after awakening and in the afternoon during both habitual and conventional conditions. In‐laboratory polysomnography was performed for 2 nights. We assessed sleep, tiredness, chronotype and depression using questionnaires. Saliva was sampled for dim light melatonin onset measurements. Repeated‐measures ANOVAs were applied for the Conners’ Continuous Performance Test II measures with group (patient/control), time (afternoon/morning) and condition (habitual/conventional schedule) as fixed factors. Patients with delayed sleep–wake phase disorder had reduced reaction times, especially in the morning, greater response speed variability, and made more omission and commission errors compared with controls. Patients with delayed sleep–wake phase disorder also had reduced focused attention, especially upon forced early awakening. The short total sleep time of patients with delayed sleep–wake phase disorder could not statistically explain this outcome. In conclusion, we observed a state‐dependent reduced ability to focus attention upon early morning awakening in patients with delayed sleep–wake phase disorder. Patients also had more omissions, longer reaction time and increased RT variability after habitual sleep, suggesting a possible small cognitive trait dysfunction in delayed sleep–wake phase disorder.     

Rapid eye movement stage of sleep participates in the generation of the nocturnal meal pattern in the rat

Twenty-four hour cycles of food intake were continuously recorded in eight male rats on a $\text{12}\text{12}$ hr light/dark schedule during baseline conditions and rapid eye movement sleep deprivation. The results suggest that food intake in the rat is controlled by two different mechanisms, one serving control of immediate caloric demands during the dark part of the LD cycle and the other, activated prior to the beginning of the inactive light hours, being responsible for the intake of a few large meals covering the total energy requirement for this time period. The correct timing of the consumption of these large meals with respect to the LD cycle seems to be dependent on undisturbed rapid eye movement sleep.     

Interindividual differences in the dynamics of the homeostatic process are trait‐like and distinct for sleep versus wakefulness

The sleep homeostatic Process S reflects the build‐up of sleep pressure during waking and its dissipation during sleep. Process S is modelled as a saturating exponential function during waking and a decreasing exponential function during sleep. Slow wave activity is a physiological marker for non‐rapid eye movement (non‐REM) sleep intensity and serves as an index of Process S. There is considerable interindividual variability in the sleep homeostatic responses to sleep and sleep deprivation. The aim of this study was to investigate whether interindividual differences in Process S are trait‐like. Polysomnographic recordings of 8 nights (12‐h sleep opportunities, 22:00–10:00 hours) interspersed with three 36‐h periods of sustained wakefulness were performed in 11 healthy young adults. Empirical mean slow wave activity per non‐REM sleep episode at episode mid‐points were used for parameter estimation. Parameters of Process S were estimated using different combinations of consecutive sleep recordings, resulting in two to three sets of parameters per subject. Intraclass correlation coefficients were calculated to assess whether the parameters were stable across the study protocol and they showed trait‐like variability among individuals. We found that the group‐average time constants of the build‐up and dissipation of Process S were 19.2 and 2.7 h, respectively. Intraclass correlation coefficients ranged from 0.48 to 0.56, which reflects moderate trait variability. The time constants of the build‐up and dissipation varied independently among subjects, indicating two distinct traits. We conclude that interindividual differences in the parameters of the dynamics of the sleep homeostatic Process S are trait‐like.     

Total sleep deprivation decreases saliva ghrelin levels in adolescents

Ghrelin, a regulator of food intake and energy expenditure, has been shown to be associated with insufficient sleep. The goal of the present study was to investigate the effect of a single night of total sleep deprivation on fasting saliva ghrelin and on nocturnal variation of saliva ghrelin concentration. A further aim of the study was to investigate the influence of body mass index on changes in saliva ghrelin levels. Altogether 35 adolescents (18 boys; age: 13.8 ± 1.14 years) were studied on two subsequent days (sleep and total sleep deprivation). Saliva samples were collected during the two experimental nights at 21:00 hours, 01:00 hours and 06:00 hours. Total-ghrelin concentration showed a continuous increase from the evening until 06:00 hours. This increase was blunted significantly (p = 0.003) by total sleep deprivation. Total-ghrelin level was significantly lower (p = 0.02) during total sleep deprivation at 06:00 hours (median 403.6 pg ml⁻¹; 95% confidence interval: 343.1–468.9 pg ml⁻¹) as compared with values during the sleep condition (median 471.2 pg ml⁻¹; 95% confidence interval: 205.4–1578.7 pg ml⁻¹). Acyl-ghrelin levels did not present any change at the three time points, and were not affected by total sleep deprivation. Stratifying the study population according to body mass index (normal weight and overweight/obese groups), the blunting effect of total sleep deprivation was more pronounced in the obese/overweight group (sleep: median 428.2 pg ml⁻¹; 95% confidence interval: 331.3–606.9 pg ml⁻¹ versus total sleep deprivation: median 333.1 pg ml⁻¹; 95% confidence interval: 261.5–412.9 pg ml⁻¹; p = 0.0479). Saliva total-ghrelin concentrations gradually increased during the night, and total sleep deprivation significantly blunted this increase. This blunting effect was mainly observed in subjects with overweight/obesity. The physiological and clinical implications of the present observation are to be clarified by further studies.     

The homeostatic and circadian sleep recovery responses after total sleep deprivation in mice

Many studies on sleep deprivation effects lack data regarding the recovery period. We investigated the 2‐day homeostatic and circadian sleep recovery response to 24 h of total sleep deprivation (TSD) induced by brief rotation of an activity wheel. Eight mice were implanted with telemetry transmitters (DSI F40‐EET) that recorded simultaneously their electroencephalography (EEG), locomotor activity and temperature during 24 h of baseline (BSL), TSD and 2 days of recovery (D1 and D2). In a second experiment, two groups of five non‐implanted mice underwent TSD or ad libitum sleep, after which they were killed, adrenal glands were weighed and blood was collected for analysis of corticosterone concentration. During TSD mice were awake at least 97% of the time, with a consecutive sleep rebound during D1 that persisted during D2. This was characterized by increases of non‐rapid eye movement (NREM) sleep (44.2 ± 6.9% for D1 and 43.0 ± 7.7% for D2 versus 33.8 ± 9.2% for BSL) and the relative delta band power (179.2 ± 34.4% for D1 and 81.9 ± 11.2% for D2). Greater NREM and REM sleep amounts were observed during the ‘light’ periods. Temperature and locomotor activity characteristics were unchanged during D1 and D2 versus BSL. In non‐implanted mice, corticosterone levels as well as adrenal gland and overall body weights did not differ between TSD and ad libitum sleep groups. In conclusion, 24 h of TSD in an activity wheel without stress responses influence homeostatic sleep regulation with no effect on the circadian regulation over at least 2 days of recovery in mice.     

Interaction effect of obstructive sleep apnea and periodic limb movements during sleep on heart rate variability

We aimed at assessing cardiac autonomic function by heart rate variability during sleep in patients with obstructive sleep apnea and periodic limb movements during sleep, and to compare it with that of patients with obstructive sleep apnea only, periodic limb movements during sleep only, and controls. We also aimed at investigating the interaction effect between apnea–hypopnea index and periodic limb movement index on heart rate variability. Four groups of patients (n = 42 each, total = 168) were identified based on the presence/absence of obstructive sleep apnea and periodic limb movements during sleep: + obstructive sleep apnea/? periodic limb movements during sleep (5 ≤ apnea–hypopnea index < 30 events per hr), ? obstructive sleep apnea/+ periodic limb movements during sleep (periodic limb movement index > 15 events per hr), + obstructive sleep apnea/+ periodic limb movements during sleep, ? obstructive sleep apnea/? periodic limb movements during sleep (controls). All groups were matched for age, sex and body mass index. Time‐ and frequency‐domain heart rate variability measures were calculated over 5‐min periods of stable stage 2 non‐rapid eye movement sleep. In patients with both obstructive sleep apnea and periodic limb movements during sleep, LFnu and LF/HF ratio were higher than in those with obstructive sleep apnea only, periodic limb movements during sleep only, and controls, while HFnu was the lowest among the four groups. LFnu, HFnu and LF/HF ratio were significantly and independently associated with minimal oxygen saturation in the + obstructive sleep apnea/+ periodic limb movements during sleep group. There was a significant interaction effect between apnea–hypopnea index and periodic limb movement index on LF/HF ratio (p = 0.038) in patients with obstructive sleep apnea. Patients with elevated apnea–hypopnea index and elevated periodic limb movement index exhibited higher sympathovagal balance compared with those with high apnea–hypopnea index and low periodic limb movement index, and compared with those with low apnea–hypopnea index (regardless of periodic limb movement index). Increased sympathetic activation and decreased parasympathetic control appear to be related to the severity of oxygen desaturation. Apnea–hypopnea index and periodic limb movement index had interactive effects on increased sympathovagal balance in patients with obstructive sleep apnea.     

The effects of sleep deprivation in humans: topographical electroencephalogram changes in non‐rapid eye movement (NREM) sleep versus REM sleep

Studies on homeostatic aspects of sleep regulation have been focussed upon non‐rapid eye movement (NREM) sleep, and direct comparisons with regional changes in rapid eye movement (REM) sleep are sparse. To this end, evaluation of electroencephalogram (EEG) changes in recovery sleep after extended waking is the classical approach for increasing homeostatic need. Here, we studied a large sample of 40 healthy subjects, considering a full‐scalp EEG topography during baseline (BSL) and recovery sleep following 40 h of wakefulness (REC). In NREM sleep, the statistical maps of REC versus BSL differences revealed significant fronto‐central increases of power from 0.5 to 11 Hz and decreases from 13 to 15 Hz. In REM sleep, REC versus BSL differences pointed to significant fronto‐central increases in the 0.5–7 Hz and decreases in the 8–11 Hz bands. Moreover, the 12–15 Hz band showed a fronto‐parietal increase and that at 22–24 Hz exhibited a fronto‐central decrease. Hence, the 1–7 Hz range showed significant increases in both NREM sleep and REM sleep, with similar topography. The parallel change of NREM sleep and REM sleep EEG power is related, as confirmed by a correlational analysis, indicating that the increase in frequency of 2–7 Hz possibly subtends a state‐aspecific homeostatic response. On the contrary, sleep deprivation has opposite effects on alpha and sigma activity in both states. In particular, this analysis points to the presence of state‐specific homeostatic mechanisms for NREM sleep, limited to <2 Hz frequencies. In conclusion, REM sleep and NREM sleep seem to share some homeostatic mechanisms in response to sleep deprivation, as indicated mainly by the similar direction and topography of changes in low‐frequency activity.     

Sleep‐dependent consolidation of face recognition and its relationship to REM sleep duration,REM density and Stage 2 sleep spindles

Face recognition is a highly specialized capability that has implicit and explicit memory components. Studies show that learning tasks with facial components are dependent on rapid eye movement and non‐rapid eye movement sleep features, including rapid eye movement sleep density and fast sleep spindles. This study aimed to investigate the relationship between sleep‐dependent consolidation of memory for faces and partial rapid eye movement sleep deprivation, rapid eye movement density, and fast and slow non‐rapid eye movement sleep spindles. Fourteen healthy participants spent 1 night each in the laboratory. Prior to bed they completed a virtual reality task in which they interacted with computer‐generated characters. Half of the participants (REMD group) underwent a partial rapid eye movement sleep deprivation protocol and half (CTL group) had a normal amount of rapid eye movement sleep. Upon awakening, they completed a face recognition task that contained a mixture of previously encountered faces from the task and new faces. Rapid eye movement density and fast and slow sleep spindles were detected using in‐house software. The REMD group performed worse than the CTL group on the face recognition task; however, rapid eye movement duration and rapid eye movement density were not related to task performance. Fast and slow sleep spindles showed differential relationships to task performance, with fast spindles being positively and slow spindles negatively correlated with face recognition. The results support the notion that rapid eye movement and non‐rapid eye movement sleep characteristics play complementary roles in face memory consolidation. This study also raises the possibility that fast and slow spindles contribute in opposite ways to sleep‐dependent memory consolidation.     

Reestablishment of individual sleep structure during a single 14‐h recovery sleep episode after 58 h of wakefulness

Sleep structure is highly stable within individuals but different between individuals. The present study investigated robustness of the individual sleep structure to extended total sleep deprivation. Seventeen healthy men spent a baseline night (23:00–07:00 hours), 58 h of sleep deprivation and a 14‐h recovery night (17:00–07:00 hours) in the laboratory. Intraclass correlation coefficients showed that the agreement between baseline and recovery with respect to the proportion of the different sleep stages increased as a function of recovery sleep duration. High values were reached for most of the sleep stages at the end of 14 h of recovery sleep (intraclass correlation coefficients between 0.38 and 0.76). If sleep duration of the recovery night is extended to 14 h, sleep stage distribution resembles that of a baseline night underlining the robustness of the individual sleep structure.     

Age affects sleep microstructure more than sleep macrostructure

It is well known that the quantity and quality of physiological sleep changes across age. However, so far the effect of age on sleep microstructure has been mostly addressed in small samples. The current study examines the effect of age on several measures of sleep macro‐ and microstructure in 211 women (22–71 years old) of the ‘Sleep and Health in Women’ study for whom ambulatory polysomnography was registered. Older age was associated with significantly lower fast spindle (effect size f² = 0.32) and K‐complex density (f² = 0.19) during N2 sleep, as well as slow‐wave activity (log) in N3 sleep (f² = 0.21). Moreover, total sleep time (f² = 0.10), N3 sleep (min) (f² = 0.10), rapid eye movement sleep (min) (f² = 0.11) and sigma (log) (f² = 0.05) and slow‐wave activity (log) during non‐rapid eye movement sleep (f² = 0.09) were reduced, and N1 sleep (f² = 0.03) was increased in older age. No significant effects of age were observed on slow spindle density, rapid eye movement density and beta power (log) during non‐rapid eye movement sleep. In conclusion, effect sizes indicate that traditional sleep stage scoring may underestimate age‐related changes in sleep.     

Dynamics of cerebral responses to sustained attention performance during one night of sleep deprivation

Total sleep deprivation (TSD) is increasingly common in modern society bringing various neurobehavioural effects. Dynamic changes of behaviour performances during TSD have been reported extensively, while the cerebral activation underlying such changes have not been elucidated clearly. This study aimed to investigate dynamic changes in cerebral responses to the fastest and slowest psychomotor vigilance task (PVT) trials during TSD. Thirty‐six healthy subjects with intermediate chronotype performed the PVT while undergoing functional magnetic resonance imaging every 2 h from 22:00 hours on the first day to 06:00 hours on the second day (i.e. 22:00, 12:00, 02:00, 04:00 and 06:00 hours; a total of five imaging sessions). Behaviourally, significant time effects were found for the PVT performance. For imaging results, significant activation alterations were found in the cognitive control network and the default mode network (DMN) for the fastest and slowest PVT trials, respectively. Time–course analysis indicated that the largest differences for behavioural results and imaging results happened in session 4 and became more prominent in session 5. Our findings provide more detailed information about the process of sustained attention activation during one night of TSD and add information regarding the effect of circadian rhythmicity and homeostatic sleep pressure on regional brain responses.     

Effects of late‐night short‐sleep on in‐home polysomnography: relation to adult age and sex

Bedtime is frequently delayed by many factors in life, and a homeostatic response to the delay may compensate partly for increased time awake and shortened sleep. Because sleep becomes shorter with age and women complain of disturbed sleep more often than men, age and sex differences in the homeostatic response to a delayed bedtime may modify the homeostatic response. The purpose of the present study was to investigate the effect of late‐night short‐sleep (3 h with awakening at about 07:00 hours) on in‐home recorded sleep in men and women in two age groups (20–30 and 65–75 years). Results (N = 59) showed that late‐night short‐sleep was associated with an increase in percentage of N3 sleep and a decrease in percentage of rapid eye movement sleep, as well as decreases in several measures of sleep discontinuity and rapid eye movement density. Men showed a smaller decrease in percentage of rapid eye movement sleep than women in response to late‐night short‐sleep, as did older individuals of both sexes compared with younger. Older men showed a weaker percentage of N3 sleep in response to late‐night short‐sleep than younger men. In general, men showed a greater percentage of rapid eye movement sleep and a lower percentage of N3 sleep than women, and older individuals showed a lower percentage of N3 sleep than younger. In particular, older men showed very low levels of percentage of N3 sleep. We conclude that older males show less of a homeostatic response to late‐night short‐sleep. This may be an indication of impaired capacity for recovery in older men. Future studies should investigate if this pattern can be linked to gender‐associated differences in morbidity and mortality.     

Physiological time structure of the tibialis anterior motor activity during sleep in mice,rats and humans

The validation of rodent models for restless legs syndrome (Willis–Ekbom disease) and periodic limb movements during sleep requires knowledge of physiological limb motor activity during sleep in rodents. This study aimed to determine the physiological time structure of tibialis anterior activity during sleep in mice and rats, and compare it with that of healthy humans. Wild‐type mice (n = 9) and rats (n = 8) were instrumented with electrodes for recording the electroencephalogram and electromyogram of neck muscles and both tibialis anterior muscles. Healthy human subjects (31 ± 1 years, n = 21) underwent overnight polysomnography. An algorithm for automatic scoring of tibialis anterior electromyogram events of mice and rats during non‐rapid eye movement sleep was developed and validated. Visual scoring assisted by this algorithm had inter‐rater sensitivity of 92–95% and false‐positive rates of 13–19% in mice and rats. The distribution of the time intervals between consecutive tibialis anterior electromyogram events during non‐rapid eye movement sleep had a single peak extending up to 10 s in mice, rats and human subjects. The tibialis anterior electromyogram events separated by intervals <10 s mainly occurred in series of two‐three events, their occurrence rate in humans being lower than in mice and similar to that in rats. In conclusion, this study proposes reliable rules for scoring tibialis anterior electromyogram events during non‐rapid eye movement sleep in mice and rats, demonstrating that their physiological time structure is similar to that of healthy young human subjects. These results strengthen the basis for translational rodent models of periodic limb movements during sleep and restless legs syndrome/Willis–Ekbom disease.     

The night before night shift: Chronotype impacts total sleep and rapid eye movement sleep during a strategically delayed sleep

Transition to night shift may be improved by strategically delaying the main sleep preceding a first night shift. However, the effects of delayed timing on sleep may differ between chronotypes. Therefore, the study aim was to compare the impacts of chronotype on sleep quality and architecture during a normally timed sleep opportunity and a delayed sleep opportunity. Seventy-two (36 female, 36 male) healthy adults participated in a laboratory study. Participants were provided with a normally timed sleep opportunity (23:00–08:00) and a delayed sleep opportunity (03:00–12:00) over two consecutive nights in a sleep laboratory. Sleep was monitored by polysomnography (PSG), and chronotype was determined from dim light melatonin onset (DLMO). A tertile split of DLMO defined early (20:24 ± 0:42 h), intermediate (21:31 ± 0:12 h), and late chronotype (22:56 ± 0:54 h) categories. Although there was no main effect of chronotype on any sleep measure, early chronotypes obtained less total sleep with delayed sleep than with normally timed sleep (p = 0.044). Intermediate and late chronotypes obtained more rapid eye movement (REM) sleep with delayed sleep than with normally timed sleep (p = 0.013, p = 0.012 respectively). Wake was more elevated for all chronotypes in the later hours of the delayed sleep opportunity than at the start of the sleep opportunity. Strategically delaying the main sleep preceding a first night shift appears to benefit intermediate and late chronotypes (i.e., more REM sleep), but not early chronotypes (i.e., less total sleep). Circadian processes appear to elevate wakefulness for all chronotypes in the later stages of a delayed sleep opportunity.     

Slow oscillating transcranial direct current stimulation during sleep has a sleep‐stabilizing effect in chronic insomnia: a pilot study

Recent evidence suggests that lack of slow‐wave activity may play a fundamental role in the pathogenesis of insomnia. Pharmacological approaches and brain stimulation techniques have recently offered solutions for increasing slow‐wave activity during sleep. We used slow (0.75 Hz) oscillatory transcranial direct current stimulation during stage 2 of non‐rapid eye movement sleeping insomnia patients for resonating their brain waves to the frequency of sleep slow‐wave. Six patients diagnosed with either sleep maintenance or non‐restorative sleep insomnia entered the study. After 1 night of adaptation and 1 night of baseline polysomnography, patients randomly received sham or real stimulation on the third and fourth night of the experiment. Our preliminary results show that after termination of stimulations (sham or real), slow oscillatory transcranial direct current stimulation increased the duration of stage 3 of non‐rapid eye movement sleep by 33 ± 26 min (P = 0.026), and decreased stage 1 of non‐rapid eye movement sleep duration by 22 ± 17.7 min (P = 0.028), compared with sham. Slow oscillatory transcranial direct current stimulation decreased stage 1 of non‐rapid eye movement sleep and wake time after sleep‐onset durations, together, by 55.4 ± 51 min (P = 0.045). Slow oscillatory transcranial direct current stimulation also increased sleep efficiency by 9 ± 7% (P = 0.026), and probability of transition from stage 2 to stage 3 of non‐rapid eye movement sleep by 20 ± 17.8% (P = 0.04). Meanwhile, slow oscillatory transcranial direct current stimulation decreased transitions from stage 2 of non‐rapid eye movement sleep to wake by 12 ± 6.7% (P = 0.007). Our preliminary results suggest a sleep‐stabilizing role for the intervention, which may mimic the effect of sleep slow‐wave‐enhancing drugs.     

Visually detected NREM Stage 2 sleep spindles in kindergarten children are associated with current and future emotional and behavioural characteristics

Sleep electroencephalogram spindles are associated with efficient cortical–subcortical connectivity, and intellectual and learning abilities. In the present study, we assessed healthy preschoolers with a twofold aim: (i) to explore associations of non‐rapid eye movement S2 spindles with emotional/behavioural characteristics cross‐sectionally; and (ii) longitudinally. A total of 43 children who were 5 years old underwent objective sleep electroencephalogram monitoring in their homes. Emotional and behavioural dimensions were assessed by parents and teachers with the Strengths & Difficulties Questionnaire at baseline and at follow‐up 1 year later. Non‐rapid eye movement S2 spindles were visually scored and compared with Strengths & Difficulties Questionnaire dimensions. High non‐rapid eye movement S2 spindle density was associated with less internalizing behaviour, more prosocial behaviour and a low total problem score. In girls, high non‐rapid eye movement S2 spindle density was related to low hyperactivity, while in boys it was associated with less internalizing behaviour, more externalizing behaviour and more hyperactivity. Longitudinally, a higher number of non‐rapid eye movement S2 spindles at 5 years old predicted fewer peer problems 12 months later. In kindergarten children, high non‐rapid eye movement S2 spindle density is associated with observable current and future favourable emotional/behavioural patterns. However, gender differences were also found, as should be taken into account in future studies.     

The spectrum of the non‐rapid eye movement sleep electroencephalogram following total sleep deprivation is trait‐like

The sleep electroencephalogram (EEG) spectrum is unique to an individual and stable across multiple baseline recordings. The aim of this study was to examine whether the sleep EEG spectrum exhibits the same stable characteristics after acute total sleep deprivation. Polysomnography (PSG) was recorded in 20 healthy adults across consecutive sleep periods. Three nights of baseline sleep [12 h time in bed (TIB)] following 12 h of wakefulness were interleaved with three nights of recovery sleep (12 h TIB) following 36 h of sustained wakefulness. Spectral analysis of the non‐rapid eye movement (NREM) sleep EEG (C3LM derivation) was used to calculate power in 0.25 Hz frequency bins between 0.75 and 16.0 Hz. Intraclass correlation coefficients (ICCs) were calculated to assess stable individual differences for baseline and recovery night spectra separately and combined. ICCs were high across all frequencies for baseline and recovery and for baseline and recovery combined. These results show that the spectrum of the NREM sleep EEG is substantially different among individuals, highly stable within individuals and robust to an experimental challenge (i.e. sleep deprivation) known to have considerable impact on the NREM sleep EEG. These findings indicate that the NREM sleep EEG represents a trait.