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

Towards an objective measurement of sleep quality in non-human animals: using the horse as a model species for the creation of sleep quality indices

Sleep disturbance is observed across species, resulting in neurocognitive dysfunction, poor impulse control and poor regulation of negative emotion. Understanding animal sleep disturbance is thus important to understand how environmental factors influence animal sleep and day-to-day welfare. Self-reporting tools for sleep disturbance commonly used in human research to determine sleep quality cannot be transferred to non-verbal animal species research. Human research has, however, successfully used frequency of awakenings to create an objective measurement of sleep quality. The aim of this study was to use a novel sleep-quality scoring system for a non-human mammalian species. Five separate sleep quality indices calculations were developed, using frequency of awakenings, total sleep time and total time spent in different sleep states. These indices were applied to a pre-existing data set of equine sleep behaviour taken from a study investigating the effects of environmental change (lighting and bedding) on the duration of time in different sleep states. Significant treatment effects for index scores both differed and aligned with the original sleep quantity results, thus sleep quality may be a useful alternative measurement of sleep disturbance that could be used to investigate impactful (emotional, cognitive) effects on the animal.

SSAVE: A tool for analysis and visualization of sleep periods using electroencephalography data

Human sleep architecture is structured with repeated episodes of rapid-eye-movement (REM) and non-rapid-eye-movement (NREM) sleep. An overnight sleep study facilitates identification of macro and micro changes in the pattern and duration of sleep stages associated with sleep disorders and other aspects of human mental and physical health. Overnight sleep studies record, in addition to electroencephalography (EEG) and other electro-physiological signals, a sequence of sleep-stage annotations. SSAVE, introduced here, is open-source software that takes sleep-stage annotations and EEG signals as input, identifies and characterizes periods of NREM and REM sleep, and produces a hypnogram and its time-matched EEG spectrogram. SSAVE fills an important gap for the rapidly growing field of sleep medicine by providing an easy-to-use tool for sleep-period identification and visualization. SSAVE can be used as a Python package, a desktop standalone tool or through a web portal. All versions of the SSAVE tool can be found on: https://manticore.niehs.nih.gov/ssave.

A Review of Equine Sleep: Implications for Equine Welfare

Sleep is a significant biological requirement for all living mammals due to its restorative properties and its cognitive role in memory consolidation. Sleep is ubiquitous amongst all mammals but sleep profiles differ between species dependent upon a range of biological and environmental factors. Given the functional importance of sleep, it is important to understand these differences in order to ensure good physical and psychological wellbeing for domesticated animals. This review focuses specifically on the domestic horse and aims to consolidate current information on equine sleep, in relation to other species, in order to (a) identify both quantitatively and qualitatively what constitutes normal sleep in the horse, (b) identify optimal methods to measure equine sleep (logistically and in terms of accuracy), (c) determine whether changes in equine sleep quantity and quality reflect changes in the animal's welfare, and (d) recognize the primary factors that affect the quantity and quality of equine sleep. The review then discusses gaps in current knowledge and uses this information to identify and set the direction of future equine sleep research with the ultimate aim of improving equine performance and welfare. The conclusions from this review are also contextualized within the current discussions around the “social license” of horse use from a welfare perspective.

An Asymmetrical Hypothesis for the NREM-REM Sleep Alternation-What Is the NREM-REM Cycle?

Since the discovery of rapid eye movement (REM) sleep (Aserinsky and Kleitman, 1953), sleep has been described as a succession of cycles of non-REM (NREM) and REM sleep episodes. The hypothesis of short-term REM sleep homeostasis, which is currently the basis of most credible theories on sleep regulation, is built upon a positive correlation between the duration of a REM sleep episode and the duration of the interval until the next REM sleep episode (inter-REM interval): the duration of REM sleep would therefore predict the duration of this interval. However, the high variability of inter-REM intervals, especially in polyphasic sleep, argues against a simple oscillator model. A new “asymmetrical” hypothesis is presented here, where REM sleep episodes only determine the duration of a proportional post-REM refractory period (PRRP), during which REM sleep is forbidden and the only remaining options are isolated NREM episodes or waking. After the PRRP, all three options are available again (NREM, REM, and Wake). I will explain why I think this hypothesis also calls into question the notion of NREM-REM sleep cycles.

Impact of number of sleep ultradian cycles on polysomnographic parameters related to REM sleep in major depression: Implications for future sleep research in psychiatry

Given the contradictory data on REMS alterations in major depression, the aim of this study was to empirically demonstrate that based on the number of sleep ultradian cycles, it was possible to highlight different subtypes of major depression characterized by specific patterns of REMS alterations. Demographic and polysomnographic data from 211 individuals (30 healthy controls and 181 untreated major depressed individuals) recruited from the sleep laboratory database were analyzed. Major depressed individuals with sleep ultradian cycles <4 showed alterations consistent with REMS deficiency (non-shortened REM latency as well as decrease in REMS percentage, REMS duration and REMS/NREMS ratio) whereas major depressed individuals with sleep ultradian cycles >4 showed alterations consistent with REMS disinhibition (shortened REM latency as well as increase in REMS percentage, REMS duration and REMS/NREMS ratio). Regarding major depressed individuals with 4 sleep ultradian cycles, their REMS alterations were intermediate to those present in major depressed individuals with sleep ultradian cycles <4 and >4. Thus, in major depressed individuals, the highlighting of this heterogeneity of REMS alterations based on the number of sleep ultradian cycles seems to suggest the involvement of distinct pathophysiological mechanisms and could open new perspectives for future sleep research in psychiatry.

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).

Pleasure: The missing link in the regulation of sleep

Although largely unrecognized by sleep scholars, sleeping is a pleasure. This report aims first, to fill the gap: sleep, like food, water and sex, is a primary reinforcer. The levels of extracellular mesolimbic dopamine show circadian oscillations and mark the "wanting" for pro-homeostatic stimuli. Further, the dopamine levels decrease during waking and are replenished during sleep, in opposition to sleep propensity. The wanting of sleep, therefore, may explain the homeostatic and circadian regulation of sleep. Accordingly, sleep onset occurs when the displeasure of excessive waking is maximal, coinciding with the minimal levels of mesolimbic dopamine. Reciprocally, sleep ends after having replenished the limbic dopamine levels. Given the direct relation between waking and mesolimbic dopamine, sleep must serve primarily to gain an efficient waking. Pleasant sleep (i.e. emotional sleep), can only exist in animals capable of feeling emotions. Therefore, although sleep-like states have been described in invertebrates and primitive vertebrates, the association sleep-pleasure clearly marks a difference between the sleep of homeothermic vertebrates and cool blooded animals.

Hyperarousal during sleep in untreated, major depressed subjects with prodromal insomnia: A polysomnographic study

In primary insomnia, specific dynamics of hyperarousal are evident during the night. Similarly, in major depression, many elements also favor of the presence of hyperarousal. Thus, it would be interesting to investigate if hyperarousal presents the same dynamic in major depression. Polysomnographic data from 30 healthy controls, 66 patients with major depression and prodromal insomnia, and 86 primary insomnia sufferers recruited from the sleep laboratory database were studied for whole night and thirds of the night. Insomnia sufferers and patients with depression exhibit a similar polysomnographic pattern both for whole night (increased sleep latency and WASO and reduced SWS and REM) and thirds of night (increased WASO at first and last thirds, reduced SWS in first third, and reduced in REM in first and last third). No alterations were detected during the second third of the night. Just as in primary insomnia, the hyperarousal phenomenon is found mainly in major depression with prodromal insomnia during the sleep-onset period and the first and last thirds of the night, but lesser during the second third of the night. These specific dynamics of hyperarousal may aid in the understanding of the particular relationship between insomnia and depression.

Similar polysomnographic pattern in primary insomnia and major depression with objective insomnia: a sign of common pathophysiology?

BACKGROUND

Our aim is to verify empirically the existence of a major depressed subgroup with a similar polysomnographic pattern as primary insomnia, including at rapid eye movement sleep level.

METHODS

The polysomnographic data from 209 untreated individuals (30 normative, 84 primary insomnia sufferers, and 95 major depressed patients with objective insomnia) who were recruited retrospectively from the Erasme hospital database were studied for the whole night and thirds of the night.

RESULTS

Primary insomnia sufferers and major depressed patients with objective insomnia exhibit a similar polysomnographic pattern both for the whole night (excess of wake after sleep onset, deficit in slow-wave sleep/rapid eye movement sleep, and non-shortened rapid eye movement latency) and thirds of the night (excess of wake after sleep onset at first and last third, deficit in slow-wave sleep in first third, and deficit in rapid eye movement sleep in first and last third), including at rapid eye movement sleep level.

CONCLUSION

In our study, we demonstrated that major depressed patients with objective insomnia showed a similar polysomnographic pattern as primary insomnia, including at rapid eye movement sleep level, which supports the hypothesis of a common pathophysiology that could be hyperarousal. This opens new avenues for understanding the pathophysiology of major depression with objective insomnia.

Hyperarousal during sleep in untreated primary insomnia sufferers: A polysomnographic study

Because some evidence favors the hyperarousal model of insomnia, we sought to learn more about the dynamics of this phenomenon during sleep. Polysomnographic data from 30 normative subjects and 86 untreated primary insomnia sufferers recruited from the database of the sleep laboratory were studied for whole nights and in terms of thirds of the night. Untreated primary insomnia sufferers had an increased sleep latency and excess of WASO, together with a deficit in REM and NREM sleep during the entire night. In terms of thirds of the night, they presented a major excess of WASO during the first and last thirds of the night but an excess of lesser importance during the middle third. A deficit in SWS was found during the first third of the night, but for REM, the deficit was present during both the first and last thirds. Primary insomnia sufferers had no SWS or REM deficit during the second third of the night. We found that the hyperarousal phenomenon occurs mainly during the sleep-onset period of the first and last thirds of the night and is less important during the middle third. These results open new avenues for understanding the pathophysiology of primary insomnia.

Spatiotemporal Organization and Cross-Frequency Coupling of Sleep Spindles in Primate Cerebral Cortex

STUDY OBJECTIVES

The sleep spindle has been implicated in thalamic sensory gating, cortical development, and memory consolidation. These multiple functions may depend on specific spatiotemporal emergence and interactions with other spindles and other forms of brain activity. Therefore, we measured sleep spindle cortical distribution, regional heterogeneity, synchronization, and phase relationships with other electroencephalographic components in freely moving primates.

METHODS

Transcortical field potentials were recorded from Japanese monkeys via telemetry and were analyzed using the Hilbert-Huang transform.

RESULTS

Spindle (12-20 Hz) current sources were identified over a wide region of the frontoparietal cortex. Most spindles occurred independently in their own frequency, but some appeared concordant between cortical areas with frequency interdependence, particularly in nearby regions and bilaterally symmetrical regions. Spindles in the dorsolateral prefrontal cortex appeared around the surface-positive and depth-negative phase of transcortically recorded slow oscillations (< 1 Hz), whereas centroparietal spindles emerged around the opposite phase. The slow-oscillation phase reversed between the prefrontal and central regions. Gamma activities increased before spindle onset. Several regional heterogeneities in properties of human spindles were replicated in the monkeys, including frequency, density, and inter-cortical time lags, although their topographic patterns were different from those of humans. The phase-amplitude coupling between spindle and gamma activity was also replicated.

CONCLUSIONS

Spindles in widespread cortical regions are possibly driven by independent rhythm generators, but are temporally associated to spindles in other regions and to slow and gamma oscillations by corticocortical and thalamocortical pathways.

Absence of systematic relationships between REMS duration episodes and spectral power Delta and Ultra-Slow bands in contiguous NREMS episodes in healthy humans

Previous studies in animals and humans have reported correlations between the durations of rapid eye movement sleep (REMS) episodes and immediately preceding or subsequent non-REMS (NREMS) episodes. The relationship between these two types of sleep is a crucial component in understanding the regulation and neurophysiology of ultradian alternations that occur during sleep. Although the present study replicated previous studies, we also measured NREMS in terms of spectral power Delta and Ultra-Slow bands in addition to duration in examining correlations. The spectral power Delta band, also known as slow-wave activity, measures sleep quantity and is believed to reflect sleep physiology better than mere episode durations. The Ultra-Slow spectral power band was analyzed in parallel. Healthy human participants of both sexes ( n = 26, age range 15–45 yr, n = 12 female) were carefully selected to participate in two consecutive series of home polysomnograms performed after 2 nights of habituation to the equipment. In the analyses, REMS episode durations (minutes) were compared with immediately preceding and immediately subsequent NREMS episodes (Delta and Ultra-Slow power) in each sleep cycle. REMS episode duration was more strongly correlated with preceding NREMS episodes than with subsequent NREMS episodes. However, in most cases, no correlations were observed in either direction. One ultradian sleep regulation hypothesis, which is based on stronger correlations between REMS and subsequent NREMS episode durations, holds that the main purpose of REMS is to reactivate NREMS during each sleep cycle. The present results do not support that hypothesis.

Relationships between the number of ultradian cycles and key sleep variables in outpatients with major depressive disorder

The regulation of the alternation between rapid eye movement sleep (REMS) and non-rapid eye movement sleep (NREMS) is still a matter of much debate. It is also an important topic for psychiatric research, since both sleep components show anomalies in Major Depressive Disorders (MDD) and related syndromes. In previous studies on healthy controls, we showed preferential links of the number of ultradian cycles with REMS-related variables rather than with NREMS-related variables. REMS Latency (RL), for example, was shown to be inversely related to the number of cycles. The present study replicates these analyses in a group of 29 patients with MDD (age range: 23-56; 16 females), after two adaptation nights. Results showed significant correlations between the number of cycles and REMS, and between the number of cycles and RL, whereas correlations with NREMS were not significant. This indirectly supports regulation hypotheses considering REMS as the main focus of the oscillation, inhibiting and interrupting NREMS. Also, when the RL is shorter, there are more ultradian cycles than when the RL is long. This adds an interesting element in the elucidation of the physiological meaning of anomalies of RL.

Cold exposure and sleep in the rat: REM sleep homeostasis and body size

STUDY OBJECTIVES

Exposure to low ambient temperature (Ta) depresses REM sleep (REMS) occurrence. In this study, both short and long-term homeostatic aspects of REMS regulation were analyzed during cold exposure and during subsequent recovery at Ta 24 degrees C.

DESIGN

EEG activity, hypothalamic temperature, and motor activity were studied during a 24-h exposure to Tas ranging from 10 degrees C to -10 degrees C and for 4 days during recovery.

SETTING

Laboratory of Physiological Regulation during the Wake-Sleep Cycle, Department of Human and General Physiology, Alma Mater Studiorum-University of Bologna.

SUBJECTS

24 male albino rats.

INTERVENTIONS

Animals were implanted with electrodes for EEG recording and a thermistor to measure hypothalamic temperature.

MEASUREMENTS AND RESULTS

REMS occurrence decreased proportionally with cold exposure, but a fast compensatory REMS rebound occurred during the first day of recovery when the previous loss went beyond a "fast rebound" threshold corresponding to 22% of the daily REMS need. A slow REMS rebound apparently allowed the animals to fully restore the previous REMS loss during the following 3 days of recovery.

CONCLUSION

Comparing the present data on rats with data from earlier studies on cats and humans, it appears that small mammals have less tolerance for REMS loss than large ones. In small mammals, this low tolerance may be responsible on a short-term basis for the shorter wake-sleep cycle, and on long-term basis, for the higher percentage of REMS that is quickly recovered following REMS deprivation.

Comparison of sleep variables between chronic widespread musculoskeletal pain, insomnia, periodic leg movements syndrome and control subjects in a clinical sleep medicine practice

BACKGROUND

Between 50% and 89% of chronic pain patients report unrefreshing sleep. The aim of this retrospective analysis was to compare the sleep of normal subjects with the sleep of a clinical population presenting musculoskeletal chronic widespread pain (CWP), psychophysiological insomnia and restless legs syndrome/periodic limb movements during sleep (RLS/PLMS) in order to identify sleep variables that may explain the poor sleep complaints of CWP patients.

METHODS

Sleep data from 10 normal subjects and 37 patients (mean age 55+/-3 yo), matched for age and sex, were retrieved from our sleep data bank. Sub-analysis controlled for the effects of medication.

RESULTS

In comparison to normal subjects, sleep duration was shorter in CWP patients (-71 min; p<0.01); sleep efficiency was significantly lower in CWP and insomnia patients (-10.1% and -11.1%, respectively; p<0.05). CWP and PLMS patients lost one non-rapid eye movement (REM) to REM sleep cycle (p<0.04). An intermediate level of PLM was observed during the sleep of CWP patients in comparison to normal subjects (8.8/h vs. 2.0/h) and PLMS patients (33/h). Regular use of non-narcotic analgesics did not seem to interfere with sleep variables.

CONCLUSIONS

The sleep of middle-aged patients with CWP is comparable to that of insomnia patients. The moderate level of PLM during sleep suggests that such sensory motor activity needs to be evaluated in patients suffering from chronic pain.

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.

Mathematical model of network dynamics governing mouse sleep-wake behavior

Recent work in experimental neurophysiology has identified distinct neuronal populations in the rodent brain stem and hypothalamus that selectively promote wake and sleep. Mutual inhibition between these cell groups has suggested the conceptual model of a sleep-wake switch that controls transitions between wake and sleep while minimizing time spent in intermediate states. By combining wake- and sleep-active populations with populations governing transitions between different stages of sleep, a "sleep-wake network" of neuronal populations may be defined. To better understand the dynamics inherent in this network, we created a model sleep-wake network composed of coupled relaxation oscillation equations. Mathematical analysis of the deterministic model provides insight into the dynamics underlying state transitions and predicts mechanisms for each transition type. With the addition of noise, the simulated sleep-wake behavior generated by the model reproduces many qualitative and quantitative features of mouse sleep-wake behavior. In particular, the existence of simulated brief awakenings is a unique feature of the model. In addition to capturing the experimentally observed qualitative difference between brief and sustained wake bouts, the model suggests distinct network mechanisms for the two types of wakefulness. Because circadian and other factors alter the fine architecture of sleep-wake behavior, this model provides a novel framework to explore dynamical principles that may underlie normal and pathologic sleep-wake physiology.

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.

Sleep, consciousness and the spontaneous and evoked electrical activity of the brain. Is there a cortical integrating mechanism?

The physiological mechanisms that underlie consciousness and unconsciousness are the sleep/wake mechanisms. Deep sleep is a state of physiological reversible unconsciousness. The change from that state to wakefulness is mediated by the reticular activating mechanism. The reverse change from wakefulness to sleep is also an active process effected by an arousal inhibitory mechanism based on a partial blockade of the thalamus and upper brain stem, associated with thalamic sleep spindles and also with cortical sub-delta activity (<1 Hz). The deactivation of the thalamus has been demonstrated both electrically and by positron emission tomography during deep sleep. Normally, wakefulness is associated with instant awareness (defined as the ability to integrate all sensory information from the external environment and the internal environment of the body). Awareness may be a function of the thalamo-cortical network in the cerebral hemispheres, which forms the final path of the sleep/wake mechanism. Anatomical and physiological studies suggest that there may be a double thalamo-cortical network; one relating to cortical and thalamic areas with specific functions and the other global, involving all cortical areas and so-called 'non-specific' thalamic nuclei. The global system might function as a cortical integrating mechanism permitting the spread of information between the specific cortical areas and thus underlying awareness. The global system may also be responsible for much of the spontaneous and evoked electrical activity of the brain. The cognitive change between sleep and wakefulness is accompanied by changes in the autonomic system, the cerebral blood flow and cerebral metabolism. Awareness is an essential component of total consciousness (defined as continuous awareness of the external and internal environment, both past and present, together with the emotions arising from it). In addition to awareness, full consciousness requires short-term and explicit memory and intact emotional responses.