Behavioral and electrophysiological correlates of sleep and sleep homeostasis

A familial natural short sleep mutation promotes healthy aging and extends lifespan in Drosophila

Sleep loss typically imposes negative effects on animal health. However, humans with a rare genetic mutation in the dec2 gene ( dec2 P384R ) present an exception; these individuals sleep less without the usual effects associated with sleep deprivation. Thus, it has been suggested that the dec2 P384R mutation activates compensatory mechanisms that allows these individuals to thrive with less sleep. To test this directly, we used a Drosophila model to study the effects of the dec2 P384R mutation on animal health. Expression of human dec2 P384R in fly sleep neurons was sufficient to mimic the short sleep phenotype and, remarkably, dec2 P384R mutants lived significantly longer with improved health despite sleeping less. The improved physiological effects were enabled, in part, by enhanced mitochondrial fitness and upregulation of multiple stress response pathways. Moreover, we provide evidence that upregulation of pro-health pathways also contributes to the short sleep phenotype, and this phenomenon may extend to other pro-longevity models.

A Study on REM Sleep Homeostasis in the Day-Active Tree Shrew (Tupaia belangeri): Cold-Induced Suppression of REM Sleep Is Not Followed by a Rebound

The function and regulation of rapid-eye-movement (REM) sleep is a topic of ongoing debate. It is often assumed that REM sleep is a homeostatically regulated process and that a need for REM sleep builds up, either during prior wakefulness or during preceding slow wave sleep. In the current study, we tested this hypothesis in six diurnal tree shrews (Tupaia belangeri), small mammals closely related to primates. All animals were individually housed and kept under a 12:12 light-dark cycle with an ambient temperature of 24 °C. We recorded sleep and temperature in the tree shrews for 3 consecutive 24 h days. During the second night, we exposed the animals to a low ambient temperature of 4 °C, a procedure that is known to suppress REM sleep. Cold exposure caused a significant drop in brain temperature and body temperature and also resulted in a strong and selective suppression of REM sleep by 64.9%. However, contrary to our expectation, the loss of REM sleep was not recovered during the subsequent day and night. These findings in a diurnal mammal confirm that the expression of REM sleep is highly sensitive to environmental temperature but do not support the view that REM sleep is homeostatically regulated in this species.

[Molecular and cellular mechanisms of restorative effects of sleep]

The review article enlightens contemporary concept about a role of sleep in cellular energy metabolism, neuroplasticity and glymphatic clearance of waste products. Many researches have demonstrated that prolonged wakefulness is an energetic and a neurophysiologic issue for the brain. The article provides description of biochemical processes that are responsive for energy restoration in sleep, particularly the role of ATP, adenosine and glycogen. Energy metabolism substrates depletion leads to endoplasmic reticulum stress and unfolded protein response. At the same time the conductance of synapses increases that worsens energetic problems. Level of the glymphatic clearance during wakefulness is substantially lower in comparison with sleep, and waste products are not removed fast enough.

How Temperature Influences Sleep

Sleep is a fundamental, evolutionarily conserved, plastic behavior that is regulated by circadian and homeostatic mechanisms as well as genetic factors and environmental factors, such as light, humidity, and temperature. Among environmental cues, temperature plays an important role in the regulation of sleep. This review presents an overview of thermoreception in animals and the neural circuits that link this process to sleep. Understanding the influence of temperature on sleep can provide insight into basic physiologic processes that are required for survival and guide strategies to manage sleep disorders.

Sleep loss and addiction

Reducing sleep hours is a risk factor for developing cardiovascular, metabolic, and psychiatric disorders. Furthermore, previous studies have shown that reduction in sleep time is a factor that favors relapse in addicted patients. Additionally, animal models have demonstrated that both sleep restriction and sleep deprivation increase the preference for alcohol, methylphenidate, and the self-administration of cocaine. Therefore, the present review discusses current knowledge about the influence of sleep hours reduction on addictivebehaviors; likewise, we discuss the neuronal basis underlying the sleep reduction-addiction relationship, like the role of the orexin and dopaminergic system and neuronal plasticity (i.e., delta FosB expression). Potentially, chronic sleep restriction could increase brain vulnerability and promote addictive behavior.

Adenosine, caffeine, and sleep-wake regulation: state of the science and perspectives

For hundreds of years, mankind has been influencing its sleep and waking state through the adenosinergic system. For ~100 years now, systematic research has been performed, first started by testing the effects of different dosages of caffeine on sleep and waking behaviour. About 70 years ago, adenosine itself entered the picture as a possible ligand of the receptors where caffeine hooks on as an antagonist to reduce sleepiness. Since the scientific demonstration that this is indeed the case, progress has been fast. Today, adenosine is widely accepted as an endogenous sleep‐regulatory substance. In this review, we discuss the current state of the science in model organisms and humans on the working mechanisms of adenosine and caffeine on sleep. We critically investigate the evidence for a direct involvement in sleep homeostatic mechanisms and whether the effects of caffeine on sleep differ between acute intake and chronic consumption. In addition, we review the more recent evidence that adenosine levels may also influence the functioning of the circadian clock and address the question of whether sleep homeostasis and the circadian clock may interact through adenosinergic signalling. In the final section, we discuss the perspectives of possible clinical applications of the accumulated knowledge over the last century that may improve sleep‐related disorders. We conclude our review by highlighting some open questions that need to be answered, to better understand how adenosine and caffeine exactly regulate and influence sleep.

Behavioral and postural analyses establish sleep-like states for mosquitoes that can impact host landing and blood feeding

Sleep is an evolutionarily conserved process that has been described in different animal systems. For insects, sleep characterization has been primarily achieved using behavioral and electrophysiological correlates in a few systems. Sleep in mosquitoes, which are important vectors of disease-causing pathogens, has not been directly examined. This is surprising as circadian rhythms, which have been well studied in mosquitoes, influence sleep in other systems. In this study, we characterized sleep in mosquitoes using body posture analysis and behavioral correlates and quantified the effect of sleep deprivation on sleep rebound, host landing and blood-feeding propensity. Body and appendage position metrics revealed a clear distinction between the posture of mosquitoes in their putative sleep and awake states for multiple species, which correlate with a reduction in responsiveness to host cues. Sleep assessment informed by these posture analyses indicated significantly more sleep during periods of low activity. Nighttime and daytime sleep deprivation resulting from the delivery of vibration stimuli induced sleep rebound in the subsequent phase in day and night active mosquitoes, respectively. Lastly, sleep deprivation suppressed host landing in both laboratory and field settings, and impaired blood feeding of a human host when mosquitoes would normally be active. These results suggest that quantifiable sleep states occur in mosquitoes and highlight the potential epidemiological importance of mosquito sleep.

Sleep timing and the circadian clock in mammals: Past, present and the road ahead

Nearly all mammals display robust daily rhythms of physiology and behavior. These approximately 24-h cycles, known as circadian rhythms, are driven by a master clock in the suprachiasmatic nucleus (SCN) of the hypothalamus and affect biological processes ranging from metabolism to immune function. Perhaps the most overt output of the circadian clock is the sleep-wake cycle, the integrity of which is critical for health and homeostasis of the organism. In this review, we summarize our current understanding of the circadian regulation of sleep. We discuss the neural circuitry and molecular mechanisms underlying daily sleep timing, and the trajectory of circadian regulation of sleep across development. We conclude by proposing future research priorities for the field that will significantly advance our mechanistic understanding of the circadian regulation of sleep.

The European starling (Sturnus vulgaris) shows signs of NREM sleep homeostasis but has very little REM sleep and no REM sleep homeostasis

Most of our knowledge about the regulation and function of sleep is based on studies in a restricted number of mammalian species, particularly nocturnal rodents. Hence, there is still much to learn from comparative studies in other species. Birds are interesting because they appear to share key aspects of sleep with mammals, including the presence of two different forms of sleep, i.e. non-rapid eye movement (NREM) and rapid eye movement (REM) sleep. We examined sleep architecture and sleep homeostasis in the European starling, using miniature dataloggers for electroencephalogram (EEG) recordings. Under controlled laboratory conditions with a 12:12 h light-dark cycle, the birds displayed a pronounced daily rhythm in sleep and wakefulness with most sleep occurring during the dark phase. Sleep mainly consisted of NREM sleep. In fact, the amount of REM sleep added up to only 1~2% of total sleep time. Animals were subjected to 4 or 8 h sleep deprivation to assess sleep homeostatic responses. Sleep deprivation induced changes in subsequent NREM sleep EEG spectral qualities for several hours, with increased spectral power from 1.17 Hz up to at least 25 Hz. In contrast, power below 1.17 Hz was decreased after sleep deprivation. Sleep deprivation also resulted in a small compensatory increase in NREM sleep time the next day. Changes in EEG spectral power and sleep time were largely similar after 4 and 8 h sleep deprivation. REM sleep was not noticeably compensated after sleep deprivation. In conclusion, starlings display signs of NREM sleep homeostasis but the results do not support the notion of important REM sleep functions.

Prolonged Exposure to Social Stress Impairs Homeostatic Sleep Regulation

Stress and sleep are tightly regulated as a result of the substantial overlap in neurotransmitter signaling and regulatory pathways between the neural centers that modulate mood and sleep-wake cycle. The chronicity of the stressor and variability in coping with it are major determinants of the psychiatric outcomes and subsequent effect on sleep. The regulation of sleep is mediated by the interaction of a homeostatic and a circadian process according to the two-process model. Chronic stress induces stress-related disorders which are associated with deficient sleep homeostasis. However, little is known about how chronic stress affects sleep homeostasis and whether the differences in adaptation to stress distinctively influence sleep. Therefore, we assessed sleep homeostasis in C57BL6/J mice following exposure to 15-d of chronic social defeat stress. We implemented wake:sleep ratio as a behavioral correlate of sleep pressure. Both stress-resilient and stress-susceptible mice displayed deficient sleep homeostasis in post-stress baseline sleep. This was due to poor temporal correlation between frontal slow wave activity (SWA) power and sleep pressure in the dark/active phase. Moreover, the buildup rate of sleep pressure in the dark was lower in susceptible mice in comparison to stress-naïve mice. Additionally, 4-h SD in the dark caused a deficient sleep recovery response in susceptible mice characterized by non-rapid eye movement (NREM) sleep loss. Our findings provide evidence of deficient homeostatic sleep process (S) in baseline sleep in stress-exposed mice, while impaired sleep recovery following a mild enforced wakefulness experienced during the dark was only detected in stress-susceptible mice. This alludes to the differential homeostatic adaptation to stress between susceptible and resilient mice and its effect on sleep regulation.

The Rostromedial Tegmental Nucleus: Anatomical Studies and Roles in Sleep and Substance Addictions in Rats and Mice

The rostromedial tegmental nucleus (RMTg), a brake of the dopamine system, is specifically activated by aversive stimuli, such as foot shock. It is principally composed of gamma-aminobutyric acid neurons. However, there is no exact location of the RMTg on the brain stereotaxic atlas. The RMTg can be defined by c-Fos staining elicited by psychostimulants, the position of retrograde-labeled neurons stained by injections into the ventral tegmental area (VTA), the terminal field formed by axons from the lateral habenula, and some molecular markers identified as specifically expressed in the RMTg such as FoxP1. The RMTg receives a broad range of inputs and produces diverse outputs, which indicates that the RMTg has multiple functions. First, the RMTg plays an essential role for non-rapid eye movement sleep. Additionally, the RMTg serves a vital role in response to addiction. Opiates increase the firing rates of dopaminergic neurons in the VTA by acting on μ-opioid receptors on RMTg neurons and their terminals inside the VTA. In this review, we summarize the recent research advances on the anatomical location of the RMTg in rats and mice, its projections, and its regulation of sleep-wake behavior and addiction.

Effects of chronic caffeine consumption on sleep and the sleep electroencephalogram in mice

BACKGROUND

Caffeine is one of the most widely consumed psychostimulants, and it impacts sleep and circadian physiology.

AIM

Caffeine is generally used chronically on a daily basis. Therefore, in the current study, we investigated the chronic effect of caffeine on sleep in mice.

METHODS

We recorded the electroencephalogram and electromyogram on a control day, on the first day of caffeine consumption (acute), and following two weeks of continuous caffeine consumption (chronic). In the latter condition, a period of six-hour sleep deprivation was conducted during the light period. Control mice, which received normal drinking water, were also recorded and sleep deprived.

RESULTS

We found that caffeine induced differential effects following acute and chronic consumption. Over 24 h, waking increased following acute caffeine whereas no changes were found in the chronic condition. The daily amplitude of sleep-wake states increased in both acute and chronic conditions, with the highest amplitude in the chronic condition, showing an increase in sleep during the light and an increase in waking during the dark. Furthermore, electroencephalogram slow-wave-activity in non-rapid eye-movement sleep was increased, compared with both control conditions, during the first half of the light period in the chronic condition. It was particularly challenging to keep the animals awake during the sleep deprivation period under chronic caffeine.

CONCLUSIONS

Together the data suggest an increased sleep pressure under chronic caffeine. In contrast to the traditional conception on the impact on sleep, chronic caffeine intake seems to increase the daily sleep-wake cycle amplitude and increase sleep pressure in mice.

Sleep homeostasis and the circadian clock: Do the circadian pacemaker and the sleep homeostat influence each other's functioning?

Sleep is regulated by a homeostatic and a circadian process. Together these two processes determine most aspects of sleep and related variables like sleepiness and alertness. The two processes are known to be able to work independently, but also to both influence sleep and sleep related variables in an additive or more complex manner. The question remains whether the two processes are directly influencing each other. The present review summarizes evidence from behavioural and electroencephalographic determined sleep, electrophysiology, gene knock out mouse models, and mathematical modelling to explore whether sleep homeostasis can influence circadian clock functioning and vice versa. There is a multitude of data available showing parallel action or influence of sleep homeostatic mechanisms and the circadian clock on several objective and subjective variables related to sleep and alertness. However, the evidence of a direct influence of the circadian clock on sleep homeostatic mechanisms is sparse and more research is needed, particularly applying longer sleep deprivations that include a second night. The strongest evidence of an influence of sleep homeostatic mechanisms on clock functioning comes from sleep deprivation experiments, demonstrating an attenuation of phase shifts of the circadian rhythm to light pulses when sleep homeostatic pressure is increased. The data suggest that the circadian clock is less susceptible to light when sleep pressure is high. The available data indicate that a strong central clock will induce periods of deep sleep, which in turn will strengthen clock function. Both are therefore important for health and wellbeing. Weakening of one will also hamper functioning of the other. Shift work and jet lag are situations where one tries to adapt to zeitgebers in a condition where sleep is compromised. Adaptation to zeitgebers may be improved by introducing nap schedules to reduce sleep pressure, and through that increasing clock susceptibility to light.

Sleep research goes wild: new methods and approaches to investigate the ecology, evolution and functions of sleep

Despite being a prominent aspect of animal life, sleep and its functions remain poorly understood. As with any biological process, the functions of sleep can only be fully understood when examined in the ecological context in which they evolved. Owing to technological constraints, until recently, sleep has primarily been examined in the artificial laboratory environment. However, new tools are enabling researchers to study sleep behaviour and neurophysiology in the wild. Here, we summarize the various methods that have enabled sleep researchers to go wild, their strengths and weaknesses, and the discoveries resulting from these first steps outside the laboratory. The initial studies to 'go wild' have revealed a wealth of interindividual variation in sleep, and shown that sleep duration is not even fixed within an individual, but instead varies in response to an assortment of ecological demands. Determining the costs and benefits of this inter- and intraindividual variation in sleep may reveal clues to the functions of sleep. Perhaps the greatest surprise from these initial studies is that the reduction in neurobehavioural performance resulting from sleep loss demonstrated in the laboratory is not an obligatory outcome of reduced sleep in the wild.This article is part of the themed issue 'Wild clocks: integrating chronobiology and ecology to understand timekeeping in free-living animals'.

The rostromedial tegmental nucleus is essential for non-rapid eye movement sleep

The rostromedial tegmental nucleus (RMTg), also called the GABAergic tail of the ventral tegmental area, projects to the midbrain dopaminergic system, dorsal raphe nucleus, locus coeruleus, and other regions. Whether the RMTg is involved in sleep-wake regulation is unknown. In the present study, pharmacogenetic activation of rat RMTg neurons promoted non-rapid eye movement (NREM) sleep with increased slow-wave activity (SWA). Conversely, rats after neurotoxic lesions of 8 or 16 days showed decreased NREM sleep with reduced SWA at lights on. The reduced SWA persisted at least 25 days after lesions. Similarly, pharmacological and pharmacogenetic inactivation of rat RMTg neurons decreased NREM sleep. Electrophysiological experiments combined with optogenetics showed a direct inhibitory connection between the terminals of RMTg neurons and midbrain dopaminergic neurons. The bidirectional effects of the RMTg on the sleep-wake cycle were mimicked by the modulation of ventral tegmental area (VTA)/substantia nigra compacta (SNc) dopaminergic neuronal activity using a pharmacogenetic approach. Furthermore, during the 2-hour recovery period following 6-hour sleep deprivation, the amount of NREM sleep in both the lesion and control rats was significantly increased compared with baseline levels; however, only the control rats showed a significant increase in SWA compared with baseline levels. Collectively, our findings reveal an essential role of the RMTg in the promotion of NREM sleep and homeostatic regulation.

The Sleep-Wake Cycle in the Nicotinic Alpha-9 Acetylcholine Receptor Subunit Knock-Out Mice

There is a neural matrix controlling the sleep-wake cycle (SWC) embedded within high ranking integrative mechanisms in the central nervous system. Nicotinic alpha-9 acetylcholine receptor subunit (alpha-9 nAChR) participate in physiological processes occurring in sensory, endocrine and immune systems. There is a relationship between the SWC architecture, body homeostasis and sensory afferents so that disruption of afferent signaling is expected to affect the temporal organization of sleep and wake states. The analysis of the SWC of 9 nAChR knock-out animals may help to reveal the contribution of alpha-9 nAChR to sleep chronobiological determinants. Here we explore the polysomnogram in chronically implanted alpha-9 nAChR knock-out (KO) and wild-type (WT) individuals of the hybrid CBA/Sv129 mouse strain. Records were obtained in isolation chambers under a stable 12:12 light:dark cycle (LD). To unmask the 24-h modulation of the SWC a skeleton photoperiod (SP) protocol was performed. Under LD the daily quota (in %) of wakefulness (W), NREM sleep and REM sleep obtained in KO and WT animals were 45, 48 and 7, and 46, 46 and 8 respectively. Both groups exhibit nocturnal phase preference of W as well as diurnal and unimodal phase preference of NREM and REM sleep. The acrophase mean angles of KO vs. WT genotypes were not different (Zeitgeber Time: 6.5 vs. 14.9 for W, 4.3 vs. 2.8 for NREM sleep and 5.3 vs. 3.4 for REM sleep, respectively). Transference to SP do not affect daily state quotas, phase preferences and acrophases among genotypes. Unmasking phenomena of the SWC such as wake increment during the rest phase under SP was evident only among WT mice suggesting the involvement of retinal structures containing alpha-9 nAChR in masking processes. Furthermore, KO animals exhibit longer NREM and REM sleep episodes that is independent of illumination conditions. Consolidated diurnal NREM sleep contributed to obtain higher values of NREM sleep delta-EEG activity among KO mice during rest phase. In conclusion, circadian and sleep homeostatic aspects of the SWC are operative among alpha-9 nAChR KO animals. We propose that alpha-9 nAChR participate in retinal signaling processes responsible of the positive masking of sleep by light.

Molecular Mechanisms of Sleep Homeostasis in Flies and Mammals

Sleep is homeostatically regulated with sleep pressure accumulating with the increasing duration of prior wakefulness. Yet, a clear understanding of the molecular components of the homeostat, as well as the molecular and cellular processes they sense and control to regulate sleep intensity and duration, remain a mystery. Here, we will discuss the cellular and molecular basis of sleep homeostasis, first focusing on the best homeostatic sleep marker in vertebrates, slow wave activity; second, moving to the molecular genetic analysis of sleep homeostasis in the fruit fly Drosophila; and, finally, discussing more systemic aspects of sleep homeostasis.

Differences in electroencephalographic non-rapid-eye movement sleep slow-wave characteristics between young and old mice

Changes in sleep pattern are typical for the normal aging process. However, aged mice show an increase in the amount of sleep, whereas humans show a decrease when aging. Mice are considered an important model in aging studies, and this divergence warrants further investigation. Recently, insights into the network dynamics of cortical activity during sleep were obtained by investigating characteristics of individual electroencephalogram (EEG) slow waves in young and elderly humans. In this study, we investigated, for the first time, the parameters of EEG slow waves, including their incidence, amplitude, duration and slopes, in young (6 months) and older (18-24 months) C57BL/6J mice during undisturbed 24 h, and after a 6-h sleep deprivation (SD). As expected, older mice slept more but, in contrast to humans, absolute NREM sleep EEG slow-wave activity (SWA, spectral power density between 0.5-4 Hz) was higher in the older mice, as compared to the young controls. Furthermore, slow waves in the older mice were characterized by increased amplitude, steeper slopes and fewer multipeak waves, indicating increased synchronization of cortical neurons in aging, opposite to what was found in humans. Our results suggest that older mice, in contrast to elderly humans, live under a high sleep pressure.

Activation of the ventral tegmental area increased wakefulness in mice

The ventral tegmental area (VTA) is crucial for brain functions, such as voluntary movement and cognition; however, the role of VTA in sleep-wake regulation when directly activated or inhibited remains unknown. In this study, we investigated the effects of activation or inhibition of VTA neurons on sleep-wake behavior using the pharmacogenetic “designer receptors exclusively activated by designer drugs (DREADD)” approach. Immunohistochemistry staining was performed to confirm the microinjection sites, and combined with electrophysiological experiments, to determine whether the VTA neurons were activated or inhibited. The hM3Dq-expressing VTA neurons were excited confirmed by clozapine-N-oxide (CNO)-driven c-Fos expression and firing in patch-clamp recordings; whereas the hM4Di-expressing VTA neurons inhibited by reduction of firing. Compared with controls, the activation of VTA neurons at 9:00 (inactive period) produced a 120.1% increase in the total wakefulness amount for 5 h, whereas NREM and REM sleep were decreased by 62.5 and 92.2%, respectively. Similarly, when VTA neurons were excited at 21:00 (active period), the total wakefulness amount increased 81.5%, while NREM and REM sleep decreased 64.6 and 93.8%, respectively, for 8 h. No difference of the amount and EEG power density of the NREM sleep was observed following the arousal effects of CNO. The inhibition of VTA neurons during active or inactive periods gave rise to no change in the time spent in the wakefulness, REM, and NREM sleep compared with control. The results indicated that VTA neurons activated pharmacogentically played important roles in promoting wakefulness.

Global and local complexity of intracranial EEG decreases during NREM sleep

Key to understanding the neuronal basis of consciousness is the characterization of the neural signatures of changes in level of consciousness during sleep. Here we analysed three measures of dynamical complexity on spontaneous depth electrode recordings from 10 epilepsy patients during wakeful rest (WR) and different stages of sleep: (i) Lempel-Ziv complexity, which is derived from how compressible the data are; (ii) amplitude coalition entropy, which measures the variability over time of the set of channels active above a threshold; (iii) synchrony coalition entropy, which measures the variability over time of the set of synchronous channels. When computed across sets of channels that are broadly distributed across multiple brain regions, all three measures decreased substantially in all participants during early-night non-rapid eye movement (NREM) sleep. This decrease was partially reversed during late-night NREM sleep, while the measures scored similar to WR during rapid eye movement (REM) sleep. This global pattern was in almost all cases mirrored at the local level by groups of channels located in a single region. In testing for differences between regions, we found elevated signal complexity in the frontal lobe. These differences could not be attributed solely to changes in spectral power between conditions. Our results provide further evidence that the level of consciousness correlates with neural dynamical complexity.

Nocturia: The circadian voiding disorder

Nocturia is a prevalent condition of waking to void during the night. The concept of nocturia has evolved from being a symptomatic aspect of disease associated with the prostate or bladder to a form of lower urinary tract disorder. However, recent advances in circadian biology and sleep science suggest that it might be important to consider nocturia as a form of circadian dysfunction. In the current review, nocturia is reexamined with an introduction to sleep disorders and recent findings in circadian biology in an attempt to highlight the importance of rediscovering nocturia as a problem of chronobiology.

Local Slow Waves in Superficial Layers of Primary Cortical Areas during REM Sleep

Sleep is traditionally constituted of two global behavioral states, non-rapid eye movement (NREM) and rapid eye movement (REM), characterized by quiescence and reduced responsiveness to sensory stimuli [1]. NREM sleep is distinguished by slow waves and spindles throughout the cerebral cortex and REM sleep by an "activated," low-voltage fast electroencephalogram (EEG) paradoxically similar to that of wake, accompanied by rapid eye movements and muscle atonia. However, recent evidence has shown that cortical activity patterns during wake and NREM sleep are not as global as previously thought. Local slow waves can appear in various cortical regions in both awake humans [2] and rodents [3-5]. Intracranial recordings in humans [6] and rodents [4, 7] have shown that NREM sleep slow waves most often involve only a subset of brain regions that varies from wave to wave rather than occurring near synchronously across all cortical areas. Moreover, some cortical areas can transiently "wake up" [8] in an otherwise sleeping brain. Yet until now, cortical activity during REM sleep was thought to be homogenously wake-like. We show here, using local laminar recordings in freely moving mice, that slow waves occur regularly during REM sleep, but only in primary sensory and motor areas and mostly in layer 4, the main target of relay thalamic inputs, and layer 3. This finding may help explain why, during REM sleep, we remain disconnected from the environment even though the bulk of the cortex shows wake-like, paradoxical activation.

The two-process model of sleep regulation: a reappraisal

In the last three decades the two-process model of sleep regulation has served as a major conceptual framework in sleep research. It has been applied widely in studies on fatigue and performance and to dissect individual differences in sleep regulation. The model posits that a homeostatic process (Process S) interacts with a process controlled by the circadian pacemaker (Process C), with time-courses derived from physiological and behavioural variables. The model simulates successfully the timing and intensity of sleep in diverse experimental protocols. Electrophysiological recordings from the suprachiasmatic nuclei (SCN) suggest that S and C interact continuously. Oscillators outside the SCN that are linked to energy metabolism are evident in SCN-lesioned arrhythmic animals subjected to restricted feeding or methamphetamine administration, as well as in human subjects during internal desynchronization. In intact animals these peripheral oscillators may dissociate from the central pacemaker rhythm. A sleep/fast and wake/feed phase segregate antagonistic anabolic and catabolic metabolic processes in peripheral tissues. A deficiency of Process S was proposed to account for both depressive sleep disturbances and the antidepressant effect of sleep deprivation. The model supported the development of novel non-pharmacological treatment paradigms in psychiatry, based on manipulating circadian phase, sleep and light exposure. In conclusion, the model remains conceptually useful for promoting the integration of sleep and circadian rhythm research. Sleep appears to have not only a short-term, use-dependent function; it also serves to enforce rest and fasting, thereby supporting the optimization of metabolic processes at the appropriate phase of the 24-h cycle.

Identification of Neurons with a Privileged Role in Sleep Homeostasis in Drosophila melanogaster

Sleep is thought to be controlled by two main processes: a circadian clock that primarily regulates sleep timing and a homeostatic mechanism that detects and responds to sleep need. Whereas abundant experimental evidence suggests that sleep need increases with time spent awake, the contributions of different brain arousal systems have not been assessed independently of each other to determine whether certain neural circuits, rather than waking per se, selectively contribute to sleep homeostasis. Using the fruit fly, Drosophila melanogaster, we found that sustained thermogenetic activation of three independent neurotransmitter systems promoted nighttime wakefulness. However, only sleep deprivation resulting from activation of cholinergic neurons was sufficient to elicit subsequent homeostatic recovery sleep, as assessed by multiple behavioral criteria. In contrast, sleep deprivation resulting from activation of octopaminergic neurons suppressed homeostatic recovery sleep, indicating that wakefulness can be dissociated from accrual of sleep need. Neurons that promote sleep homeostasis were found to innervate the central brain and motor control regions of the thoracic ganglion. Blocking activity of these neurons suppressed recovery sleep but did not alter baseline sleep, further differentiating between neural control of sleep homeostasis and daily fluctuations in the sleep/wake cycle. Importantly, selective activation of wake-promoting neurons without engaging the sleep homeostat impaired subsequent short-term memory, thus providing evidence that neural circuits that regulate sleep homeostasis are important for behavioral plasticity. Together, our data suggest a neural circuit model involving distinct populations of wake-promoting neurons, some of which are involved in homeostatic control of sleep and cognition.

Lempel-Ziv complexity of cortical activity during sleep and waking in rats

Understanding the dynamics of brain activity manifested in the EEG, local field potentials (LFP), and neuronal spiking is essential for explaining their underlying mechanisms and physiological significance. Much has been learned about sleep regulation using conventional EEG power spectrum, coherence, and period-amplitude analyses, which focus primarily on frequency and amplitude characteristics of the signals and on their spatio-temporal synchronicity. However, little is known about the effects of ongoing brain state or preceding sleep-wake history on the nonlinear dynamics of brain activity. Recent advances in developing novel mathematical approaches for investigating temporal structure of brain activity based on such measures, as Lempel-Ziv complexity (LZC) can provide insights that go beyond those obtained with conventional techniques of signal analysis. Here, we used extensive data sets obtained in spontaneously awake and sleeping adult male laboratory rats, as well as during and after sleep deprivation, to perform a detailed analysis of cortical LFP and neuronal activity with LZC approach. We found that activated brain states—waking and rapid eye movement (REM) sleep are characterized by higher LZC compared with non-rapid eye movement (NREM) sleep. Notably, LZC values derived from the LFP were especially low during early NREM sleep after sleep deprivation and toward the middle of individual NREM sleep episodes. We conclude that LZC is an important and yet largely unexplored measure with a high potential for investigating neurophysiological mechanisms of brain activity in health and disease.

Sleep, recovery, and metaregulation: explaining the benefits of sleep

A commonly held view is that extended wakefulness is causal for a broad spectrum of deleterious effects at molecular, cellular, network, physiological, psychological, and behavioral levels. Consequently, it is often presumed that sleep plays an active role in providing renormalization of the changes incurred during preceding waking. Not surprisingly, unequivocal empirical evidence supporting such a simple bi-directional interaction between waking and sleep is often limited or controversial. One difficulty is that, invariably, a constellation of many intricately interrelated factors, including the time of day, specific activities or behaviors during preceding waking, metabolic status and stress are present at the time of measurement, shaping the overall effect observed. In addition to this, although insufficient or disrupted sleep is thought to prevent efficient recovery of specific physiological variables, it is also often difficult to attribute specific changes to the lack of sleep proper. Furthermore, sleep is a complex phenomenon characterized by a multitude of processes, whose unique and distinct contributions to the purported functions of sleep are difficult to determine, because they are interrelated. Intensive research effort over the last decades has greatly progressed current understanding of the cellular and physiological processes underlying the regulation of vigilance states. Notably, it also highlighted the infinite complexity within both waking and sleep, and revealed a number of fundamental conceptual and technical obstacles that need to be overcome in order to fully understand these processes. A promising approach could be to view sleep not as an entity, which has specific function(s) and is subject to direct regulation, but as a manifestation of the process of metaregulation, which enables efficient moment-to-moment integration between internal and external factors, preceding history and current homeostatic needs.

NREM and REM Sleep: Complementary Roles in Recovery after Wakefulness

The overall function of sleep is hypothesized to provide "recovery" after preceding waking activities, thereby ensuring optimal functioning during subsequent wakefulness. However, the functional significance of the temporal dynamics of sleep, manifested in the slow homeostatic process and the alternation between non-rapid eye movement (NREM) and REM sleep remains unclear. We propose that NREM and REM sleep have distinct and complementary contributions to the overall function of sleep. Specifically, we suggest that cortical slow oscillations, occurring within specific functionally interconnected neuronal networks during NREM sleep, enable information processing, synaptic plasticity, and prophylactic cellular maintenance ("recovery process"). In turn, periodic excursions into an activated brain state-REM sleep-appear to be ideally placed to perform "selection" of brain networks, which have benefited from the process of "recovery," based on their offline performance. Such two-stage modus operandi of the sleep process would ensure that its functions are fulfilled according to the current need and in the shortest time possible. Our hypothesis accounts for the overall architecture of normal sleep and opens up new perspectives for understanding pathological conditions associated with abnormal sleep patterns.

The role of sleep in human declarative memory consolidation

Through a variety of methods, researchers have begun unraveling the mystery of why humans spend one-third of their lives asleep. Though sleep likely serves multiple functions, it has become clear that the sleeping brain offers an ideal environment for solidifying newly learned information in the brain. Sleep , which comprises a complex collection of brain states, supports the consolidation of many different types of information. It not only promotes learning and memory stabilization, but also memory reorganization that can lead to various forms of insightful behavior. As this chapter will describe, research provides ample support for these crucial cognitive functions of sleep . Focusing on the declarative memory system in humans, we review the literature regarding the benefits of sleep for both neutral and emotionally salient declarative memory. Finally, we discuss the literature regarding the impact of sleep on emotion regulation.

Sleep and synaptic homeostasis

In the last decades a substantial knowledge about sleep mechanisms has been accumulated. However, the function of sleep still remains elusive. The difficulty with unraveling sleep's function may arise from the lack of understanding of how the multitude of processes associated with waking and sleep-from gene expression and single neuron activity to the whole brain dynamics and behavior-functionally and mechanistically relate to each other. Therefore, novel conceptual frameworks, which integrate and take into account the variety of phenomena occurring during waking and sleep at different levels, will likely lead to advances in our understanding of the function of sleep, above and beyond what merely descriptive or correlative approaches can provide. One such framework, the synaptic homeostasis hypothesis, focuses on wake- and sleep-dependent changes in synaptic strength. The core claim of this hypothesis is that learning and experience during wakefulness are associated with a net increase in synaptic strength. In turn, the proposed function of sleep is to provide synaptic renormalization, which has important implications with respect to energy needs, intracranial space, metabolic supplies, and, importantly, enables further plastic changes. In this article we review the empirical evidence for this hypothesis, which was obtained at several levels-from gene expression and cellular excitability to structural synaptic modifications and behavioral outcomes. We conclude that although the mechanisms behind the proposed role of sleep in synaptic homeostasis are undoubtedly complex, this conceptual framework offers a unique opportunity to provide mechanistic and functional explanation for many previously disparate observations, and define future research strategies.