Molecular Mechanisms of Sleep Homeostasis in Flies and Mammals

Genetic determinants of daytime napping and effects on cardiometabolic health

Daytime napping is a common, heritable behavior, but its genetic basis and causal relationship with cardiometabolic health remain unclear. Here, we perform a genome-wide association study of self-reported daytime napping in the UK Biobank (n = 452,633) and identify 123 loci of which 61 replicate in the 23andMe research cohort (n = 541,333). Findings include missense variants in established drug targets for sleep disorders (HCRTR1, HCRTR2), genes with roles in arousal (TRPC6, PNOC), and genes suggesting an obesity-hypersomnolence pathway (PNOC, PATJ). Association signals are concordant with accelerometer-measured daytime inactivity duration and 33 loci colocalize with loci for other sleep phenotypes. Cluster analysis identifies three distinct clusters of nap-promoting mechanisms with heterogeneous associations with cardiometabolic outcomes. Mendelian randomization shows potential causal links between more frequent daytime napping and higher blood pressure and waist circumference.

Feeding state functionally reconfigures a sensory circuit to drive thermosensory behavioral plasticity

Internal state alters sensory behaviors to optimize survival strategies. The neuronal mechanisms underlying hunger-dependent behavioral plasticity are not fully characterized. Here we show that feeding state alters C. elegans thermotaxis behavior by engaging a modulatory circuit whose activity gates the output of the core thermotaxis network. Feeding state does not alter the activity of the core thermotaxis circuit comprised of AFD thermosensory and AIY interneurons. Instead, prolonged food deprivation potentiates temperature responses in the AWC sensory neurons, which inhibit the postsynaptic AIA interneurons to override and disrupt AFD-driven thermotaxis behavior. Acute inhibition and activation of AWC and AIA, respectively, restores negative thermotaxis in starved animals. We find that state-dependent modulation of AWC-AIA temperature responses requires INS-1 insulin-like peptide signaling from the gut and DAF-16/FOXO function in AWC. Our results describe a mechanism by which functional reconfiguration of a sensory network via gut-brain signaling drives state-dependent behavioral flexibility.

Sleep and Sex: A Review of the Interrelationship of Sleep and Sexuality Disorders in the Female Population, Through the Lens of Sleeping Beauty Syndrome

INTRODUCTION

For many, sleep and sex are crucial for physical, emotional, and mental well-being. Poor sleep quality is linked to a myriad of ailments from coronary artery disease to major depressive disorder. Likewise, a decrease in the frequency of sexual activity is associated with a decrease in self-rated health status. Kleine-Levin syndrome (KLS) is a rare sleep disorder that provides a unique lens to examine the intricate interplay between sleep and sex as it is one of the few sleep disorders defined by concomitant sexual dysfunction.

OBJECTIVES

This study reviews the literature on links between sleep disorders and sexuality with a focus on women's health followed by a case study of unusual patient with KLS with persistent genital arousal disorder.

METHODS

Literature searches were conducted for English language publications, including foreign language publications with English abstracts with ninety-five articles reviewed. The literature review is followed by a case report.

RESULTS

We review the known literature linking sleep and women's sexual health with a focus on insomnia, circadian rhythm sleep disorder, obstructive sleep apnea, restless leg syndrome, sexsomnia, and KLS. We then present a case of KLS-associated persistent genital arousal disorder, which was amenable to treatment with a multimodal approach aimed at symptomatic relief with intravaginal diazepam suppositories, topical clitoral lidocaine, and duloxetine.

CONCLUSION

This case highlights that hypersexuality and persistent arousal cannot effectively be treated in isolation but rather must be contextualized within a patient's broader medical history and diagnoses. Specifically, sleep quality and potential sleep disorders should be assessed for those presenting with sexual health complaints (and vice versa). Zwerling B, Keymeulen S, Krychman ML. Sleep and Sex: A Review of the Interrelationship of Sleep and Sexuality Disorders in the Female Population, Through the Lens of Sleeping Beauty Syndrome. Sex Med Rev 2021;9:221-229.

The effect of the endosymbiont Wolbachia on the behavior of insect hosts

As one of the most successful intracellular symbiotic bacteria, Wolbachia can infect many arthropods and nematodes. Wolbachia infection usually affects the reproduction of their hosts to promote their own proliferation and transmission. Currently, most of the studies focus on the mechanisms of Wolbachia interactions with host reproduction. However, in addition to distribution in the reproductive tissues, Wolbachia also infect various somatic tissues of their hosts, including the brain. This raises the potential that Wolbachia may influence some somatic processes, such as behaviors in their hosts. So far, information about the effects of Wolbachia infection on host behavior is still very limited. The present review presents the current literature on different aspects of the influence of Wolbachia on various behaviors, including sleep, learning and memory, mating, feeding and aggression in their insect hosts. We then highlight ongoing scientific efforts in the field that need addressing to advance this field, which can have significant implications for further developing Wolbachia as environmentally friendly biocontrol agents to control insect-borne diseases and agricultural pests.

Regulation of sleep homeostasis mediator adenosine by basal forebrain glutamatergic neurons

Sleep and wakefulness are homeostatically regulated by a variety of factors, including adenosine. However, how neural activity underlying the sleep-wake cycle controls adenosine release in the brain remains unclear. Using a newly developed genetically encoded adenosine sensor, we found an activity-dependent rapid increase in the concentration of extracellular adenosine in mouse basal forebrain (BF), a critical region controlling sleep and wakefulness. Although the activity of both BF cholinergic and glutamatergic neurons correlated with changes in the concentration of adenosine, optogenetic activation of these neurons at physiological firing frequencies showed that glutamatergic neurons contributed much more to the adenosine increase. Mice with selective ablation of BF glutamatergic neurons exhibited a reduced adenosine increase and impaired sleep homeostasis regulation. Thus, cell type–specific neural activity in the BF dynamically controls sleep homeostasis.

Regulation of rhythmic behaviors by astrocytes

Glial astrocytes of vertebrates and invertebrates are important modulators of nervous system development, physiology, and behavior. In all species examined, astrocytes of the adult brain contain conserved circadian clocks, and multiple studies have shown that these glial cells participate in the regulation of circadian behavior and sleep. This short review summarizes recent work, using fruit fly (Drosophila) and mouse models, that document participation of astrocytes and their endogenous circadian clocks in the control of rhythmic behavior. This article is categorized under: Gene Expression and Transcriptional Hierarchies > Regulatory Mechanisms Nervous System Development > Flies.

Sleep Loss Can Cause Death through Accumulation of Reactive Oxygen Species in the Gut

The view that sleep is essential for survival is supported by the ubiquity of this behavior, the apparent existence of sleep-like states in the earliest animals, and the fact that severe sleep loss can be lethal. The cause of this lethality is unknown. Here we show, using flies and mice, that sleep deprivation leads to accumulation of reactive oxygen species (ROS) and consequent oxidative stress, specifically in the gut. ROS are not just correlates of sleep deprivation but drivers of death: their neutralization prevents oxidative stress and allows flies to have a normal lifespan with little to no sleep. The rescue can be achieved with oral antioxidant compounds or with gut-targeted transgenic expression of antioxidant enzymes. We conclude that death upon severe sleep restriction can be caused by oxidative stress, that the gut is central in this process, and that survival without sleep is possible when ROS accumulation is prevented. VIDEO ABSTRACT.

Circadian and Sleep Metabolomics Across Species

Under normal circadian function, metabolic control is temporally coordinated across tissues and behaviors with a 24-h period. However, circadian disruption results in negative consequences for metabolic homeostasis including energy or redox imbalances. Yet, circadian disruption has become increasingly prevalent within today's society due to many factors including sleep loss. Metabolic consequences of both have been revealed by metabolomics analyses of circadian biology and sleep. Specifically, two primary analytical platforms, mass spectrometry and nuclear magnetic resonance spectroscopy, have been used to study molecular clock and sleep influences on overall metabolic rhythmicity. For example, human studies have demonstrated the prevalence of metabolic rhythms in human biology, as well as pan-metabolome consequences of sleep disruption. However, human studies are limited to peripheral metabolic readouts primarily through minimally invasive procedures. For further tissue- and organism-specific investigations, a number of model systems have been studied, based upon the conserved nature of both the molecular clock and sleep across species. Here we summarize human studies as well as key findings from metabolomics studies using mice, Drosophila, and zebrafish. While informative, a limitation in existing literature is a lack of interpretation regarding dynamic synthesis or catabolism within metabolite pools. To this extent, future work incorporating isotope tracers, specific metabolite reporters, and single-cell metabolomics may provide a means of exploring dynamic activity in pathways of interest.

Disrupting flight increases sleep and identifies a novel sleep-promoting pathway in Drosophila

Sleep is plastic and is influenced by ecological factors and environmental changes. The mechanisms underlying sleep plasticity are not well understood. We show that manipulations that impair flight in Drosophila increase sleep as a form of sleep plasticity. We disrupted flight by blocking the wing-expansion program, genetically disrupting flight, and by mechanical wing perturbations. We defined a new sleep regulatory circuit starting with specific wing sensory neurons, their target projection neurons in the ventral nerve cord, and the neurons they connect to in the central brain. In addition, we identified a critical neuropeptide (burs) and its receptor (rickets) that link wing expansion and sleep. Disrupting flight activates these sleep-promoting projection neurons, as indicated by increased cytosolic calcium levels, and stably increases the number of synapses in their axonal projections. These data reveal an unexpected role for flight in regulating sleep and provide new insight into how sensory processing controls sleep need.

Unique transcriptional signatures of sleep loss across independently evolved cavefish populations

Animals respond to sleep loss with compensatory rebound sleep, and this is thought to be critical for the maintenance of physiological homeostasis. Sleep duration varies dramatically across animal species, but it is not known whether evolutionary differences in sleep duration are associated with differences in sleep homeostasis. The Mexican cavefish, Astyanax mexicanus, has emerged as a powerful model for studying the evolution of sleep. While eyed surface populations of A. mexicanus sleep approximately 8 hr each day, multiple blind cavefish populations have converged on sleep patterns that total as little as 2 hr each day, providing the opportunity to examine whether the evolution of sleep loss is accompanied by changes in sleep homeostasis. Here, we examine the behavioral and molecular response to sleep deprivation across four independent populations of A. mexicanus. Our behavioral analysis indicates that surface fish and all three cavefish populations display robust recovery sleep during the day following nighttime sleep deprivation, suggesting sleep homeostasis remains intact in cavefish. We profiled transcriptome-wide changes associated with sleep deprivation in surface fish and cavefish. While the total number of differentially expressed genes was not greater for the surface population, the surface population exhibited the highest number of uniquely differentially expressed genes than any other population. Strikingly, a majority of the differentially expressed genes are unique to individual cave populations, suggesting unique expression responses are exhibited across independently evolved cavefish populations. Together, these findings suggest sleep homeostasis is intact in cavefish despite a dramatic reduction in overall sleep duration.

Pumilio Regulates Sleep Homeostasis in Response to Chronic Sleep Deprivation in Drosophila melanogaster

Recent studies have identified the Drosophila brain circuits involved in the sleep/wake switch and have pointed to the modulation of neuronal excitability as one of the underlying mechanisms triggering sleep need. In this study we aimed to explore the link between the homeostatic regulation of neuronal excitability and sleep behavior in the circadian circuit. For this purpose, we selected Pumilio (Pum), whose main function is to repress protein translation and has been linked to modulation of neuronal excitability during chronic patterns of altered neuronal activity. Here we explore the effects of Pum on sleep homeostasis in Drosophila melanogaster, which shares most of the major features of mammalian sleep homeostasis. Our evidence indicates that Pum is necessary for sleep rebound and that its effect is more pronounced during chronic sleep deprivation (84 h) than acute deprivation (12 h). Knockdown of pum, results in a reduction of sleep rebound during acute sleep deprivation and the complete abolishment of sleep rebound during chronic sleep deprivation. Based on these findings, we propose that Pum is a critical regulator of sleep homeostasis through neural adaptations triggered during sleep deprivation.

Identification of a molecular basis for the juvenile sleep state

Across species, sleep in young animals is critical for normal brain maturation. The molecular determinants of early life sleep remain unknown. Through an RNAi-based screen, we identified a gene, pdm3, required for sleep maturation in Drosophila. Pdm3, a transcription factor, coordinates an early developmental program that prepares the brain to later execute high levels of juvenile adult sleep. PDM3 controls the wiring of wake-promoting dopaminergic (DA) neurites to a sleep-promoting region, and loss of PDM3 prematurely increases DA inhibition of the sleep center, abolishing the juvenile sleep state. RNA-Seq/ChIP-Seq and a subsequent modifier screen reveal that pdm3 represses expression of the synaptogenesis gene Msp300 to establish the appropriate window for DA innervation. These studies define the molecular cues governing sleep behavioral and circuit development, and suggest sleep disorders may be of neurodevelopmental origin.

Drosophila insulin-like peptide 2 mediates dietary regulation of sleep intensity

Sleep is a nearly universal behavior that is regulated by diverse environmental stimuli and physiological states. A defining feature of sleep is a homeostatic rebound following deprivation, where animals compensate for lost sleep by increasing sleep duration and/or sleep depth. The fruit fly, Drosophila melanogaster, exhibits robust recovery sleep following deprivation and represents a powerful model to study neural circuits regulating sleep homeostasis. Numerous neuronal populations have been identified in modulating sleep homeostasis as well as depth, raising the possibility that the duration and quality of recovery sleep is dependent on the environmental or physiological processes that induce sleep deprivation. Here, we find that unlike most pharmacological and environmental manipulations commonly used to restrict sleep, starvation potently induces sleep loss without a subsequent rebound in sleep duration or depth. Both starvation and a sucrose-only diet result in increased sleep depth, suggesting that dietary protein is essential for normal sleep depth and homeostasis. Finally, we find that Drosophila insulin like peptide 2 (Dilp2) is acutely required for starvation-induced changes in sleep depth without regulating the duration of sleep. Flies lacking Dilp2 exhibit a compensatory sleep rebound following starvation-induced sleep deprivation, suggesting Dilp2 promotes resiliency to sleep loss. Together, these findings reveal innate resilience to starvation-induced sleep loss and identify distinct mechanisms that underlie starvation-induced changes in sleep duration and depth.

Sleep is nearly universal throughout the animal kingdom and homeostatic regulation represents a defining feature of sleep, where animals compensate for lost sleep by increasing sleep over subsequent time periods. Despite the robustness of this feature, the neural mechanisms regulating recovery from different types of sleep deprivation are not fully understood. Fruit flies provide a powerful model for investigating the genetic regulation of sleep, and like mammals, display robust recovery sleep following deprivation. Here, we find that unlike most stimuli that suppress sleep, sleep deprivation by starvation does not require a homeostatic rebound. These findings are likely due to flies engaging in deeper sleep during the period of partial sleep deprivation, suggesting a natural resilience to starvation-induced sleep loss. This unique resilience to starvation-induced sleep loss is dependent on Drosophila insulin-like peptide 2, revealing a critical role for insulin signaling in regulating interactions between diet and sleep homeostasis.

Presynaptic Active Zone Plasticity Encodes Sleep Need in Drosophila

Sleep is universal across species and essential for quality of life and health, as evidenced by the consequences of sleep loss. Sleep might homeostatically normalize synaptic gains made over wake states in order to reset information processing and storage and support learning, and sleep-associated synaptic (ultra)structural changes have been demonstrated recently. However, causal relationships between the molecular and (ultra)structural status of synapses, sleep homeostatic regulation, and learning processes have yet to be established. We show here that the status of the presynaptic active zone can directly control sleep in Drosophila. Short sleep mutants showed a brain-wide upregulation of core presynaptic scaffold proteins and release factors. Increasing the gene copy number of ELKS-family scaffold master organizer Bruchpilot (BRP) not only mimicked changes in the active zone scaffold and release proteins but importantly provoked sleep in a dosage-dependent manner, qualitatively and quantitatively reminiscent of sleep deprivation effects. Conversely, reducing the brp copy number decreased sleep in short sleep mutant backgrounds, suggesting a specific role of the active zone plasticity in homeostatic sleep regulation. Finally, elimination of BRP specifically in the sleep-promoting R2 neurons of 4xBRP animals partially restored sleep patterns and rescued learning deficits. Our results suggest that the presynaptic active zone plasticity driven by BRP operates as a sleep homeostatic actuator that also restricts periods of effective learning.

Healthy Sleepers Can Worsen Their Sleep by Wanting to Do so: The Effects of Intention on Objective and Subjective Sleep Parameters

PURPOSE

Sleep is regulated by homeostatic and circadian factors. In addition, psychological factors have a strong modulatory impact on our sleep, but the exact underlying mechanisms are still largely unknown. Here, we examined the role of intentions on subjective and objective sleep parameters. Young healthy sleepers were instructed to voluntarily either worsen or improve their sleep. We predicted that participants would be capable of worsening, but not improving, their sleep compared to a regular sleep condition. In addition, we predicted that the instruction to alter sleep would lead to a higher discrepancy between subjective and objective sleep variables.

PARTICIPANTS AND METHODS

Twenty-two healthy students participated in one adaptation and three experimental nights. Polysomnography and subjective sleep parameters were measured during all four nights. Participants were instructed to sleep regularly ("neutral"), better ("good") or worse ("bad") than normal, in a counterbalanced order.

RESULTS

The instruction to sleep "bad" increased objective sleep onset latency and the number of awakings during the night. The effects were stronger on subjective sleep variables, resulting in a higher sleep misperception in the "bad" condition as compared to the other two conditions. The instruction to sleep "good" did not improve sleep nor did it affect sleep misperception.

CONCLUSION

We conclude that intention is sufficient to impair (but not improve) subjective and objective sleep quality and to increase sleep misperception in healthy young sleepers. Our results have important implications for the understanding of the impact of psychological factors on our sleep.

Sleep and Cellular Stress

Sleep is a universal phenomenon occurring in all species studied thus far. Sleep loss results in adverse physiological effects at both the organismal and cellular levels suggesting an adaptive role for sleep in the maintenance of overall health. This review examines the bidirectional relationship between sleep and cellular stress. Cellular stress in this review refers to a shift in cellular homeostasis in response to an external stressor. Studies that illustrate the fact that sleep loss induces cellular stress and those that provide evidence that cellular stress in turn promotes sleep will be discussed.

Therapeutic Approaches for the Management of Sleep Disorders in Geriatric Population

OBJECTIVE

Aging is a natural biological phenomenon that occurs in human beings. With increasing of age, there is an appearance of deleterious changes related to progression onto pathological conditions, including hypertension, heart disease, diabetes, hearing and vision impairments, as well as sleep disorders. It is important to recognize that some sleep disturbances reported by aged subjects include insomnia, obstructive sleep apnea, restless legs syndrome, among others. Moreover, accumulating evidence indicates that coexistence of medical issues with sleep disorders constitutes clinical challenges for treatment of comorbidities in elderly. Here, we have attempted to review and summarize the available literature that assesses the sleep disturbances in aging. In addition, we highlight the management of sleep disorders associated with aging. Due to the particular health condition of aged adults, the development of effective pharmacological interventions for sleep disorders treatment in aging is warranted.

METHODS

Review of studies retrieved from the PubMed.

RESULTS

The sleep-wake cycle includes abnormalities classified as sleep disorders. Comorbidity between sleep disturbances and aging-related health issues will represent a public health challenge to be addressed in the near future. Moreover, this scenario will suggest an area that requires further drug investigation and design of new pharmacological and pharmaceutical strategies to treat sleep disorders in the elderly population.

CONCLUSION

The review highlights the sleep disturbances in aging. We focus on current knowledge in medicinal chemistry and further design of new treatments tools for managing sleep disturbances in the aged population.

Network-Specific Synchronization of Electrical Slow-Wave Oscillations Regulates Sleep Drive in Drosophila

Slow-wave rhythms characteristic of deep sleep oscillate in the delta band (0.5-4 Hz) and can be found across various brain regions in vertebrates. Across phyla, however, an understanding of the mechanisms underlying oscillations and how these link to behavior remains limited. Here, we discover compound delta oscillations in the sleep-regulating R5 network of Drosophila. We find that the power of these slow-wave oscillations increases with sleep need and is subject to diurnal variation. Optical multi-unit voltage recordings reveal that single R5 neurons get synchronized by activating circadian input pathways. We show that this synchronization depends on NMDA receptor (NMDAR) coincidence detector function, and that an interplay of cholinergic and glutamatergic inputs regulates oscillatory frequency. Genetically targeting the coincidence detector function of NMDARs in R5, and thus the uncovered mechanism underlying synchronization, abolished network-specific compound slow-wave oscillations. It also disrupted sleep and facilitated light-induced wakening, establishing a role for slow-wave oscillations in regulating sleep and sensory gating. We therefore propose that the synchronization-based increase in oscillatory power likely represents an evolutionarily conserved, potentially "optimal," strategy for constructing sleep-regulating sensory gates.

A sleep-inducing gene, nemuri, links sleep and immune function in Drosophila

Sleep remains a major mystery of biology. In particular, little is known about the mechanisms that account for the drive to sleep. In an unbiased screen of more than 12,000 Drosophila lines, we identified a single gene, nemuri, that induces sleep. The NEMURI protein is an antimicrobial peptide that can be secreted ectopically to drive prolonged sleep (with resistance to arousal) and to promote survival after infection. Loss of nemuri increased arousability during daily sleep and attenuated the acute increase in sleep induced by sleep deprivation or bacterial infection. Conditions that increase sleep drive induced expression of nemuri in a small number of fly brain neurons and targeted it to the sleep-promoting, dorsal fan-shaped body. We propose that NEMURI is a bona fide sleep homeostasis factor that is particularly important under conditions of high sleep need; because these conditions include sickness, our findings provide a link between sleep and immune function.

A Secreted Ig-Domain Protein Required in Both Astrocytes and Neurons for Regulation of Drosophila Night Sleep

Endogenous rhythmic behaviors are evolutionarily conserved and essential for life. In mammalian and invertebrate models, well-characterized neuronal circuits and evolutionarily conserved mechanisms regulate circadian behavior and sleep [1-4]. In Drosophila, neuronal populations located in multiple brain regions mediate arousal, sleep drive, and homeostasis (reviewed in [3, 5-7]). Similar to mammals [8], there is also evidence that fly glial cells modulate the neuronal circuits controlling rhythmic behaviors, including sleep [1]. Here, we describe a novel gene (CG14141; aka Nkt) that is required for normal sleep. NKT is a 162-amino-acid protein with a single IgC2 immunoglobulin (Ig) domain and a high-quality signal peptide [9], and we show evidence that it is secreted, similar to its C. elegans ortholog (OIG-4) [10]. We demonstrate that Nkt-null flies or those with selective knockdown in either neurons or glia have decreased and fragmented night sleep, indicative of a non-redundant requirement in both cell types. We show that Nkt is required in fly astrocytes and in a specific set of wake-promoting neurons-the mushroom body (MB) α'β' cells that link sleep to memory consolidation [11]. Importantly, Nkt gene expression is required in the adult nervous system for normal sleep, consistent with a physiological rather than developmental function for the Ig-domain protein.

Smith-Magenis Syndrome: Molecular Basis of a Genetic-Driven Melatonin Circadian Secretion Disorder

Smith-Magenis syndrome (SMS), linked to Retinoic Acid Induced (RAI1) haploinsufficiency, is a unique model of the inversion of circadian melatonin secretion. In this regard, this model is a formidable approach to better understand circadian melatonin secretion cycle disorders and the role of the RAI1 gene in this cycle. Sleep-wake cycle disorders in SMS include sleep maintenance disorders with a phase advance and intense sleepiness around noon. These disorders have been linked to a general disturbance of sleep-wake rhythm and coexist with inverted secretion of melatonin. The exact mechanism underlying the inversion of circadian melatonin secretion in SMS has rarely been discussed. We suggest three hypotheses that could account for the inversion of circadian melatonin secretion and discuss them. First, inversion of the circadian melatonin secretion rhythm could be linked to alterations in light signal transduction. Second, this inversion could imply global misalignment of the circadian system. Third, the inversion is not linked to a global circadian clock shift but rather to a specific impairment in the melatonin secretion pathway between the suprachiasmatic nuclei (SCN) and pinealocytes. The development of diurnal SMS animal models that produce melatonin appears to be an indispensable step to further understand the molecular basis of the circadian melatonin secretion rhythm.

Sleep-promoting effects of threonine link amino acid metabolism in Drosophila neuron to GABAergic control of sleep drive

Emerging evidence indicates the role of amino acid metabolism in sleep regulation. Here we demonstrate sleep-promoting effects of dietary threonine (SPET) in Drosophila. Dietary threonine markedly increased daily sleep amount and decreased the latency to sleep onset in a dose-dependent manner. High levels of synaptic GABA or pharmacological activation of metabotropic GABA receptors (GABA_B-R) suppressed SPET. By contrast, synaptic blockade of GABAergic neurons or transgenic depletion of GABA_B-R in the ellipsoid body R2 neurons enhanced sleep drive non-additively with SPET. Dietary threonine reduced GABA levels, weakened metabotropic GABA responses in R2 neurons, and ameliorated memory deficits in plasticity mutants. Moreover, genetic elevation of neuronal threonine levels was sufficient for facilitating sleep onset. Taken together, these data define threonine as a physiologically relevant, sleep-promoting molecule that may intimately link neuronal metabolism of amino acids to GABAergic control of sleep drive via the neuronal substrate of sleep homeostasis.

Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

On the cause of sleep: Protein fragments, the concept of sentinels, and links to epilepsy

The molecular-level cause of sleep is unknown. In 2012, we suggested that the cause of sleep stems from cumulative effects of numerous intracellular and extracellular protein fragments. According to the fragment generation (FG) hypothesis, protein fragments (which are continually produced through nonprocessive cleavages by intracellular, intramembrane, and extracellular proteases) can be beneficial but toxic as well, and some fragments are eliminated slowly during wakefulness. We consider the FG hypothesis and propose that, during wakefulness, the degradation of accumulating fragments is delayed within natural protein aggregates such as postsynaptic densities (PSDs) in excitatory synapses and in other dense protein meshworks, owing to an impeded diffusion of the ∼3,000-kDa 26S proteasome. We also propose that a major function of sleep involves a partial and reversible expansion of PSDs, allowing an accelerated destruction of PSD-localized fragments by the ubiquitin/proteasome system. Expansion of PSDs would alter electrochemistry of synapses, thereby contributing to a decreased neuronal firing during sleep. If so, the loss of consciousness, a feature of sleep, would be the consequence of molecular processes (expansions of protein meshworks) that are required for degradation of protein fragments. We consider the concept of FG sentinels, which signal to sleep-regulating circuits that the levels of fragments are going up. Also discussed is the possibility that protein fragments, which are known to be overproduced during an epileptic seizure, may contribute to postictal sleep and termination of seizures. These and related suggestions, described in the paper, are compatible with current evidence about sleep and lead to testable predictions.

Cortical region-specific sleep homeostasis in mice: effects of time of day and waking experience

Sleep–wake history, wake behaviors, lighting conditions, and circadian time influence sleep, but neither their relative contribution nor the underlying mechanisms are fully understood. The dynamics of electroencephalogram (EEG) slow-wave activity (SWA) during sleep can be described using the two-process model, whereby the parameters of homeostatic Process S are estimated using empirical EEG SWA (0.5–4 Hz) in nonrapid eye movement sleep (NREMS), and the 24 hr distribution of vigilance states. We hypothesized that the influence of extrinsic factors on sleep homeostasis, such as the time of day or wake behavior, would manifest in systematic deviations between empirical SWA and model predictions. To test this hypothesis, we performed parameter estimation and tested model predictions using NREMS SWA derived from continuous EEG recordings from the frontal and occipital cortex in mice. The animals showed prolonged wake periods, followed by consolidated sleep, both during the dark and light phases, and wakefulness primarily consisted of voluntary wheel running, learning a new motor skill or novel object exploration. Simulated SWA matched empirical levels well across conditions, and neither waking experience nor time of day had a significant influence on the fit between data and simulation. However, we consistently observed that Process S declined during sleep significantly faster in the frontal than in the occipital area of the neocortex. The striking resilience of the model to specific wake behaviors, lighting conditions, and time of day suggests that intrinsic factors underpinning the dynamics of Process S are robust to extrinsic influences, despite their major role in shaping the overall amount and distribution of vigilance states across 24 hr.

Genome-wide association study identifies genetic loci for self-reported habitual sleep duration supported by accelerometer-derived estimates

Sleep is an essential state of decreased activity and alertness but molecular factors regulating sleep duration remain unknown. Through genome-wide association analysis in 446,118 adults of European ancestry from the UK Biobank, we identify 78 loci for self-reported habitual sleep duration (p < 5 × 10⁻⁸; 43 loci at p < 6 × 10⁻⁹). Replication is observed for PAX8, VRK2, and FBXL12/UBL5/PIN1 loci in the CHARGE study (n = 47,180; p < 6.3 × 10⁻⁴), and 55 signals show sign-concordant effects. The 78 loci further associate with accelerometer-derived sleep duration, daytime inactivity, sleep efficiency and number of sleep bouts in secondary analysis (n = 85,499). Loci are enriched for pathways including striatum and subpallium development, mechanosensory response, dopamine binding, synaptic neurotransmission and plasticity, among others. Genetic correlation indicates shared links with anthropometric, cognitive, metabolic, and psychiatric traits and two-sample Mendelian randomization highlights a bidirectional causal link with schizophrenia. This work provides insights into the genetic basis for inter-individual variation in sleep duration implicating multiple biological pathways.

Sleep is essential for homeostasis and insufficient or excessive sleep are associated with adverse outcomes. Here, the authors perform GWAS for self-reported habitual sleep duration in adults, supported by accelerometer-derived measures, and identify genetic correlation with psychiatric and metabolic traits

Biological and clinical insights from genetics of insomnia symptoms

Insomnia is a common disorder linked with adverse long-term medical and psychiatric outcomes. The underlying pathophysiological processes and causal relationships of insomnia with disease are poorly understood. Here we identified 57 loci for self-reported insomnia symptoms in the UK Biobank (n = 453,379) and confirmed their effects on self-reported insomnia symptoms in the HUNT Study (n = 14,923 cases and 47,610 controls), physician-diagnosed insomnia in the Partners Biobank (n = 2,217 cases and 14,240 controls), and accelerometer-derived measures of sleep efficiency and sleep duration in the UK Biobank (n = 83,726). Our results suggest enrichment of genes involved in ubiquitin-mediated proteolysis and of genes expressed in multiple brain regions, skeletal muscle, and adrenal glands. Evidence of shared genetic factors was found between frequent insomnia symptoms and restless legs syndrome, aging, and cardiometabolic, behavioral, psychiatric, and reproductive traits. Evidence was found for a possible causal link between insomnia symptoms and coronary artery disease, depressive symptoms, and subjective well-being.

Differential regulation of the Drosophila sleep homeostat by circadian and arousal inputs

One output arm of the sleep homeostat in Drosophila appears to be a group of neurons with projections to the dorsal fan-shaped body (dFB neurons) of the central complex in the brain. However, neurons that regulate the sleep homeostat remain poorly understood. Using neurogenetic approaches combined with Ca²⁺ imaging, we characterized synaptic connections between dFB neurons and distinct sets of upstream sleep-regulatory neurons. One group of the sleep-promoting upstream neurons is a set of circadian pacemaker neurons that activates dFB neurons via direct glutaminergic excitatory synaptic connections. Opposing this population, a group of arousal-promoting neurons downregulates dFB axonal output with dopamine. Co-activating these two inputs leads to frequent shifts between sleep and wake states. We also show that dFB neurons release the neurotransmitter GABA and inhibit octopaminergic arousal neurons. We propose that dFB neurons integrate synaptic inputs from distinct sets of upstream sleep-promoting circadian clock neurons, and arousal neurons.

Daytime Central Thalamic Deep Brain Stimulation Modulates Sleep Dynamics in the Severely Injured Brain: Mechanistic Insights and a Novel Framework for Alpha-Delta Sleep Generation

Loss of organized sleep electrophysiology is a characteristic finding following severe brain injury. The return of structured elements of sleep architecture has been associated with positive prognosis across injury etiologies, suggesting a role for sleep dynamics as biomarkers of wakeful neuronal circuit function. In a continuing study of one minimally conscious state patient studied over the course of ~8½ years, we sought to investigate whether changes in daytime brain activation induced by central thalamic deep brain stimulation (CT-DBS) influenced sleep electrophysiology. In this patient subject, we previously reported significant improvements in sleep electrophysiology during 5½ years of CT-DBS treatment, including increased sleep spindle frequency and SWS delta power. We now present novel findings that many of these improvements in sleep electrophysiology regress following CT-DBS discontinuation; these regressions in sleep features correlate with a significant decrease in behavioral responsiveness. We also observe the re-emergence of alpha-delta sleep, which had been previously suppressed by daytime CT-DBS in this patient subject. Importantly, CT-DBS was only active during the daytime and has been proposed to mediate recovery of consciousness by driving synaptic activity across frontostriatal systems through the enhancement of thalamocortical output. Accordingly, the improvement of sleep dynamics during daytime CT-DBS and their subsequent regression following CT-DBS discontinuation implicates wakeful synaptic activity as a robust modulator of sleep electrophysiology. We interpret these findings in the context of the “synaptic homeostasis hypothesis,” whereby we propose that daytime upregulation of thalamocortical output in the severely injured brain may facilitate organized frontocortical circuit activation and yield net synaptic potentiation during wakefulness, providing a homeostatic drive that reconstitutes sleep dynamics over time. Furthermore, we consider common large-scale network dynamics across several neuropsychiatric disorders in which alpha-delta sleep has been documented, allowing us to formulate a novel mechanistic framework for alpha-delta sleep generation. We conclude that the bi-directional modulation of sleep electrophysiology by daytime thalamocortical activity in the severely injured brain: (1) emphasizes the cyclical carry-over effects of state-dependent circuit activation on large-scale brain dynamics, and (2) further implicates sleep electrophysiology as a sensitive indicator of wakeful brain activation and covert functional recovery in the severely injured brain.

BMAL1 knockout macaque monkeys display reduced sleep and psychiatric disorders

Circadian disruption is a risk factor for metabolic, psychiatric and age-related disorders, and non-human primate models could help to develop therapeutic treatments. Here, we report the generation of BMAL1 knockout cynomolgus monkeys for circadian-related disorders by CRISPR/Cas9 editing of monkey embryos. These monkeys showed higher nocturnal locomotion and reduced sleep, which was further exacerbated by a constant light regimen. Physiological circadian disruption was reflected by the markedly dampened and arrhythmic blood hormonal levels. Furthermore, BMAL1-deficient monkeys exhibited anxiety and depression, consistent with their stably elevated blood cortisol, and defective sensory processing in auditory oddball tests found in schizophrenia patients. Ablation of BMAL1 up-regulated transcriptional programs toward inflammatory and stress responses, with transcription networks associated with human sleep deprivation, major depressive disorders, and aging. Thus, BMAL1 knockout monkeys are potentially useful for studying the physiological consequences of circadian disturbance, and for developing therapies for circadian and psychiatric disorders.

Neuroendocrine Control of Sleep

Sleep is a phenomenon in animal behavior as enigmatic as it is ubiquitous, and one deeply tied to endocrine function. Though there are still many unanswered questions about the neurochemical basis of sleep and its functions, extensive interactions have been identified between sleep and the endocrine system, in both the endocrine system's effect on sleep and sleep's effect on the endocrine system. Unfortunately, until recent years, much research on sleep behavior largely disregarded its connections with the endocrine system. Use of both clinical studies and rodent models to investigate interactions between neuroendocrine function, including biological sex, and sleep therefore presents a promising area of further exploration. Further investigation of the neurobiological and neuroendocrine basis of sleep could have wide impact on a number of clinical and basic science fields. In this review, we summarize the state of basic sleep biology and its connections to the field of neuroendocrine biology, as well as suggest key future directions for the neuroendocrine regulation of sleep that may significantly impact new therapies for sleep disorders in women and men.

Gap Junctions and NCA Cation Channels Are Critical for Developmentally Timed Sleep and Arousal in Caenorhabditis elegans

An essential characteristic of sleep is heightened arousal threshold, with decreased behavioral response to external stimuli. The molecular and cellular mechanisms underlying arousal threshold changes during sleep are not fully understood. We report that loss of UNC-7 or UNC-9 innexin function dramatically reduced sleep and decreased arousal threshold during developmentally timed sleep in Caenorhabditis elegans UNC-7 function was required in premotor interneurons and UNC-9 function was required in motor neurons in this paradigm. Simultaneous transient overexpression of UNC-7 and UNC-9 was sufficient to induce anachronistic sleep in adult animals. Moreover, loss of UNC-7 or UNC-9 suppressed the increased sleep of EGL-4 gain-of-function animals, which have increased cyclic-GMP-dependent protein kinase activity. These results suggest C. elegans gap junctions may act downstream of previously identified sleep regulators. In other paradigms, the NCA cation channels act upstream of gap junctions. Consistent with this, diminished NCA channel activity in C. elegans robustly increased arousal thresholds during sleep bouts in L4-to-adult developmentally timed sleep. Total time in sleep bouts was only modestly increased in animals lacking NCA channel auxiliary subunit UNC-79, whereas increased channel activity dramatically decreased sleep. Loss of EGL-4 or innexin proteins suppressed UNC-79 loss-of-function sleep and arousal defects. In Drosophila, the ion channel narrow abdomen, an ortholog of the C. elegans NCA channels, drive the pigment dispersing factor (PDF) neuropeptide release, regulating circadian behavior. However, in C. elegans, we found that loss of the PDF receptor PDFR-1 did not suppress gain-of-function sleep defects, suggesting an alternative downstream pathway. This study emphasizes the conservation and importance of neuronal activity modulation during sleep, and unequivocally demonstrates that gap junction function is critical for normal sleep.

A bidirectional relationship between sleep and oxidative stress in Drosophila

Although sleep appears to be broadly conserved in animals, the physiological functions of sleep remain unclear. In this study, we sought to identify a physiological defect common to a diverse group of short-sleeping Drosophila mutants, which might provide insight into the function and regulation of sleep. We found that these short-sleeping mutants share a common phenotype of sensitivity to acute oxidative stress, exhibiting shorter survival times than controls. We further showed that increasing sleep in wild-type flies using genetic or pharmacological approaches increases survival after oxidative challenge. Moreover, reducing oxidative stress in the neurons of wild-type flies by overexpression of antioxidant genes reduces the amount of sleep. Together, these results support the hypothesis that a key function of sleep is to defend against oxidative stress and also point to a reciprocal role for reactive oxygen species (ROS) in neurons in the regulation of sleep.

Wolbachia affects sleep behavior in Drosophila melanogaster

Wolbachia are endosymbiotic bacteria present in a wide range of insects. Although their dramatic effects on host reproductive biology have been well studied, the effects of Wolbachia on sleep behavior of insect hosts are not well documented. In this study, we report that Wolbachia infection caused an increase of total sleep time in both male and female Drosophila melanogaster. The increase in sleep was associated with an increase in the number of nighttime sleep bouts or episodes, but not in sleep bout duration. Correspondingly, Wolbachia infection also reduced the arousal threshold of their fly hosts. However, neither circadian rhythm nor sleep rebound following deprivation was influenced by Wolbachia infection. Transcriptional analysis of the dopamine biosynthesis pathway revealed that two essential genes, Pale and Ddc, were significantly upregulated in Wolbachia-infected flies. Together, these results indicate that Wolbachia mediates the expression of dopamine related genes, and decreases the sleep quality of their insect hosts. Our findings help better understand the host-endosymbiont interactions and in particular the Wolbachia's impact on behaviors, and thus on ecology and evolution in insect hosts.

Sexual dimorphism of sleep regulated by juvenile hormone signaling in Drosophila

Sexually dimorphic phenotypes are a universal phenomenon in animals. In the model animal fruit fly Drosophila, males and females exhibit long- and short-sleep phenotypes, respectively. However, the mechanism is still a mystery. In this study, we showed that juvenile hormone (JH) is involved in regulation of sexually dimorphic sleep in Drosophila, in which gain of JH function enlarges differences of the dimorphic sleep phenotype with higher sleep in males and lower sleep in females, while loss of JH function blurs these differences and results in feminization of male sleep and masculinization of female sleep. Further studies indicate that germ cell-expressed (GCE), one of the JH receptors, mediates the response in the JH pathway because the sexually dimorphic sleep phenotypes cannot be rescued by JH hormone in a gce deletion mutant. The JH-GCE regulated sleep dimorphism is generated through the sex differentiation-related genes -fruitless (fru) and doublesex (dsx) in males and sex-lethal (sxl), transformer (tra) and doublesex (dsx) in females. These are the “switch” genes that separately control the sleep pattern in males and females. Moreover, analysis of sleep deprivation and circadian behaviors showed that the sexually dimorphic sleep induced by JH signals is a change of sleep drive and independent of the circadian clock. Furthermore, we found that JH seems to also play an unanticipated role in antagonism of an aging-induced sleep decrease in male flies. Taken together, these results indicate that the JH signal pathway is critical for maintenance of sexually dimorphic sleep by regulating sex-relevant genes.

Sleep is a very important biological behavior in all animals and takes up around one third of the lifespan in many animals. In both insects and mammals (including humans), sleep differences between male and female (sexually dimorphic sleep) have been described over the past decades. However, its internal regulation mechanism is still unclear. The fruit fly Drosophila melanogaster, sharing most sleep characteristics with humans, has been used for sleep studies as a powerful model for genetic analysis. In this study, we reported that Juvenile hormone (JH) induces completely different sleep effects between males and females with higher sleep in males and lower sleep in females, while loss of JH function blurs these differences and results in feminization of male sleep and masculinization of female sleep. Further studies indicate that the sexual dimorphism of sleep is generated through the sex differentiation-related genes regulated by JH and its receptor GCE (germ cell-expressed) signaling. Furthermore, we found that JH seems to also play an unanticipated role in aging-induced sleep changes.

Neuronal Mechanisms for Sleep/Wake Regulation and Modulatory Drive

Humans have been fascinated by sleep for millennia. After almost a century of scientific interrogation, significant progress has been made in understanding the neuronal regulation and functions of sleep. The application of new methods in neuroscience that enable the analysis of genetically defined neuronal circuits with unprecedented specificity and precision has been paramount in this endeavor. In this review, we first discuss electrophysiological and behavioral features of sleep/wake states and the principal neuronal populations involved in their regulation. Next, we describe the main modulatory drives of sleep and wakefulness, including homeostatic, circadian, and motivational processes. Finally, we describe a revised integrative model for sleep/wake regulation.

Valerian/Cascade mixture promotes sleep by increasing non-rapid eye movement (NREM) in rodent model

The aim of this study was to investigate the beneficial effect of Valerian/Cascade mixture on sleeping in mammal models. In pentobarbital-induced sleep model, Valerian, Cascade, and Valerian/Cascade mixture significantly reduced the latency time for sleeping, and total sleeping time effectively increased in these sample groups compared with the control. Valerian/Cascade mixture increased sleep duration by 37%. The mixture significantly increased the non-rapid eye movement (NREM) sleep time by 53% compared with the control, while REM sleeping time was decreased by 33% with Valerian/Cascade mixture, in Electroencephalography (EEG) analysis, resulting in the increase of total sleep time and the decrease of awakening. This sleep-promoting effect was obvious in caffeine-induced awakening model; Valerian, Cascade, and the mixture significantly enhanced NREM and total sleep time, which were reduced by caffeine. Caffeine-induced increase of awakening was effectively deceased to the normal level by these three samples. In particular, delta wave responsible for deep sleep in NREM was greatly increased by the mixture in both normal and caffeine-induced awake models. This sleep-promoting effect of Valerian/Cascade mixture was shown to be due to the upregulation of gamma-aminobutyric acid A receptor (GABA_AR). Valerian/Cascade mixture showed 91% binding capacity to GABA_A-BZD receptor. Two compounds, Valerenic acid and Xanthohumol, were shown to significantly contribute to the binding activity of Valerian/Cascade mixture on the GABA receptor.

Identification of Genes that Maintain Behavioral and Structural Plasticity during Sleep Loss

Although patients with primary insomnia experience sleep disruption, they are able to maintain normal performance on a variety of cognitive tasks. This observation suggests that insomnia may be a condition where predisposing factors simultaneously increase the risk for insomnia and also mitigate against the deleterious consequences of waking. To gain insight into processes that might regulate sleep and buffer neuronal circuits during sleep loss, we manipulated three genes, fat facet (faf), highwire (hiw) and the GABA receptor Resistance to dieldrin (Rdl), that were differentially modulated in a Drosophila model of insomnia. Our results indicate that increasing faf and decreasing hiw or Rdl within wake-promoting large ventral lateral clock neurons (lLNvs) induces sleep loss. As expected, sleep loss induced by decreasing hiw in the lLNvs results in deficits in short-term memory and increases of synaptic growth. However, sleep loss induced by knocking down Rdl in the lLNvs protects flies from sleep-loss induced deficits in short-term memory and increases in synaptic markers. Surprisingly, decreasing hiw and Rdl within the Mushroom Bodies (MBs) protects against the negative effects of sleep deprivation (SD) as indicated by the absence of a subsequent homeostatic response, or deficits in short-term memory. Together these results indicate that specific genes are able to disrupt sleep and protect against the negative consequences of waking in a circuit dependent manner.