Energy conservation and sleep

Is sleep in animals affected by prior waking experiences?

Methods to assess changes in the mental state of animals in response to their environment can be used to provide information to enhance animal welfare. One of the most profound changes of mental state observable in mammals is the change between wakefulness and sleep. Sleeping mammals have characteristics that are similar to one another and are measurable, such as specific behaviours, changes in responsiveness to external stimuli and changes in electrophysiology and neurochemistry. Although sleep is a ubiquitous behaviour in the life of mammals, there has been relatively little research on this topic in domesticated animals. All animals are motivated to sleep and this motivation increases after a prolonged period of wakefulness. In humans, sleep can be affected by what has occurred in the prior period of wakefulness and this has also been demonstrated in some non-human mammals. An important aspect of human sleep medicine is the association between stress and subsequent sleep disturbances. Studying changes in amount, bout length, distribution or type of sleep after exposure to potentially stressful events, could help us understand how animals respond to changes in their environment. It is possible that different types of stressors could affect sleep characteristics in different ways and that monitoring and identifying these changes could be useful in providing an additional way of identifying management procedures that have the potential to affect welfare. Sleep measurement is a potentially valuable tool in studies to assess animal welfare.

The ecological relevance of sleep: the trade-off between sleep, memory and energy conservation

All animals in which sleep has been studied express signs of sleep-like behaviour, suggesting that sleep must have some fundamental functions that are sustained by natural selection. Those functions, however, are still not clear. Here, we examine the ecological relevance of sleep from the perspective of behavioural trade-offs that might affect fitness. Specifically, we highlight the advantage of using food-caching animals as a system in which a conflict might occur between engaging in sleep for memory/learning and hypothermia/torpor to conserve energy. We briefly review the evidence for the importance of sleep for memory, the importance of memory for food-caching animals and the conflicts that might occur between sleep and energy conservation in these animals. We suggest that the food-caching paradigm represents a naturalistic and experimentally practical system that provides the opportunity for a new direction in sleep research that will expand our understanding of sleep, especially within the context of ecological and evolutionary processes.

A role for sleep in brain plasticity

The idea that sleep might be involved in brain plasticity has been investigated for many years through a large number of animal and human studies, but evidence remains fragmentary. Large amounts of sleep in early life suggest that sleep may play a role in brain maturation. In particular, the influence of sleep in developing the visual system has been highlighted. The current data suggest that both Rapid Eye Movement (REM) and non-REM sleep states would be important for brain development. Such findings stress the need for optimal paediatric sleep management. In the adult brain, the role of sleep in learning and memory is emphasized by studies at behavioural, systems, cellular and molecular levels. First, sleep amounts are reported to increase following a learning task and sleep deprivation impairs task acquisition and consolidation. At the systems level, neurophysiological studies suggest possible mechanisms for the consolidation of memory traces. These imply both thalamocortical and hippocampo-neocortical networks. Similarly, neuroimaging techniques demonstrated the experience-dependent changes in cerebral activity during sleep. Finally, recent works show the modulation during sleep of cerebral protein synthesis and expression of genes involved in neuronal plasticity.

Understanding the neurogenetics of sleep: progress from Drosophila

Most behaviors manifest themselves through interactions with environments. Sleep, however, is characterized by immobility and reduced responsiveness. Although nearly all animals sleep, the purpose of sleep remains an enduring puzzle. Drosophila melanogaster exhibits all the behavioral characteristics of mammalian sleep, enabling the use of powerful genetic approaches to dissect conserved fundamental neurogenetic aspects of sleep. Drosophila studies over the past four years have identified novel genes and pathways modulating sleep, such as Shaker and sleepless, and candidate brain regions known to function in circadian regulation and learning and memory. Advances in systems genetics coupled with the ability to target specific brain regions enable the characterization of transcriptional networks and neural circuits contributing to phenotypic variation in sleep.

Parasite resistance and the adaptive significance of sleep

Background

Sleep is a biological enigma. Despite occupying much of an animal's life, and having been scrutinized by numerous experimental studies, there is still no consensus on its function. Similarly, no hypothesis has yet explained why species have evolved such marked variation in their sleep requirements (from 3 to 20 hours a day in mammals). One intriguing but untested idea is that sleep has evolved by playing an important role in protecting animals from parasitic infection. This theory stems, in part, from clinical observations of intimate physiological links between sleep and the immune system. Here, we test this hypothesis by conducting comparative analyses of mammalian sleep, immune system parameters, and parasitism.

Results

We found that evolutionary increases in mammalian sleep durations are strongly associated with an enhancement of immune defences as measured by the number of immune cells circulating in peripheral blood. This appeared to be a generalized relationship that could be independently detected in 4 of the 5 immune cell types and in both of the main sleep phases. Importantly, no comparable relationships occur in related physiological systems that do not serve an immune function. Consistent with an influence of sleep on immune investment, mammalian species that sleep for longer periods also had substantially reduced levels of parasitic infection.

Conclusion

These relationships suggest that parasite resistance has played an important role in the evolution of mammalian sleep. Species that have evolved longer sleep durations appear to be able to increase investment in their immune systems and be better protected from parasites. These results are neither predicted nor explained by conventional theories of sleep evolution, and suggest that sleep has a much wider role in disease resistance than is currently appreciated.

Phylogenetic analysis of the ecology and evolution of mammalian sleep

The amount of time asleep varies greatly in mammals, from 3 h in the donkey to 20 h in the armadillo. Previous comparative studies have suggested several functional explanations for interspecific variation in both the total time spent asleep and in rapid-eye movement (REM) or "quiet" (non-REM) sleep. In support of specific functional benefits of sleep, these studies reported correlations between time in specific sleep states (NREM or REM) and brain size, metabolic rate, and developmental variables. Here we show that estimates of sleep duration are significantly influenced by the laboratory conditions under which data are collected and that, when analyses are limited to data collected under more standardized procedures, traditional functional explanations for interspecific variation in sleep durations are no longer supported. Specifically, we find that basal metabolic rate correlates negatively rather than positively with sleep quotas, and that neither adult nor neonatal brain mass correlates positively with REM or NREM sleep times. These results contradict hypotheses that invoke energy conservation, cognition, and development as drivers of sleep variation. Instead, the negative correlations of both sleep states with basal metabolic rate and diet are consistent with trade-offs between sleep and foraging time. In terms of predation risk, both REM and NREM sleep quotas are reduced when animals sleep in more exposed sites, whereas species that sleep socially sleep less. Together with the fact that REM and NREM sleep quotas correlate strongly with each other, these results suggest that variation in sleep primarily reflects ecological constraints acting on total sleep time, rather than the independent responses of each sleep state to specific selection pressures. We propose that, within this ecological framework, interspecific variation in sleep duration might be compensated by variation in the physiological intensity of sleep.

Quantitative genetic analysis of sleep in Drosophila melanogaster

Although intensively studied, the biological purpose of sleep is not known. To identify candidate genes affecting sleep, we assayed 136 isogenic P-element insertion lines of Drosophila melanogaster. Since sleep has been negatively correlated with energy reserves across taxa, we measured energy stores (whole-body protein, glycogen, and triglycerides) in these lines as well. Twenty-one insertions with known effects on physiology, development, and behavior affect 24-hr sleep time. Thirty-two candidate insertions significantly impact energy stores. Mutational genetic correlations among sleep parameters revealed that the genetic basis of the transition between sleep and waking states in males and females may be different. Furthermore, sleep bout number can be decoupled from waking activity in males, but not in females. Significant genetic correlations are present between sleep phenotypes and glycogen stores in males, while sleep phenotypes are correlated with triglycerides in females. Differences observed in male and female sleep behavior in flies may therefore be related to sex-specific differences in metabolic needs. Sleep thus emerges as a complex trait that exhibits extensive pleiotropy and sex specificity. The large mutational target that we observed implicates genes functioning in a variety of biological processes, suggesting that sleep may serve a number of different functions rather than a single purpose.

Proteomic changes in rat hippocampus and adrenals following short-term sleep deprivation

Background

To identify the biochemical changes induced by sleep deprivation at a proteomic level, we compared the hippocampal proteome of rats either after 4 hours of sleep or sleep deprivation obtained by gentle handling. Because sleep deprivation might induce some stress, we also analyzed proteomic changes in rat adrenals in the same conditions. After sleep deprivation, proteins from both tissues were extracted and subjected to 2D-DIGE analysis followed by protein identification through mass spectrometry and database search.

Results

In the hippocampus, 87 spots showed significant variation between sleep and sleep deprivation, with more proteins showing higher abundance in the latter case. Of these, 16 proteins were present in sufficient amount for a sequencing attempt and among the 12 identified proteins, inferred affected cellular functions include cell metabolism, energy pathways, transport and vesicle trafficking, cytoskeleton and protein processing. Although we did not observe classical, macroscopic effect of stress in sleep-deprived rats, 47 protein spots showed significant variation in adrenal tissue between sleep and sleep deprivation, with more proteins showing higher abundance following sleep. Of these, 16 proteins were also present in sufficient amount for a sequencing attempt and among the 13 identified proteins, the most relevant cellular function that was affected was cell metabolism.

Conclusion

At a proteomic level, short term sleep deprivation is characterized by a higher expression of some proteins in the hippocampus and a lower abundance of other proteins in the adrenals (compared to normal sleep control). Altogether, this could indicate a general activation of a number of cellular mechanisms involved in the maintenance of wakefulness and in increased energy expenditure during sleep deprivation. These findings are relevant to suggested functions of sleep like energy repletion and the restoration of molecular stocks or a more global homeostasis of synaptic processes.

Characterization of sleep in zebrafish and insomnia in hypocretin receptor mutants

Sleep is a fundamental biological process conserved across the animal kingdom. The study of how sleep regulatory networks are conserved is needed to better understand sleep across evolution. We present a detailed description of a sleep state in adult zebrafish characterized by reversible periods of immobility, increased arousal threshold, and place preference. Rest deprivation using gentle electrical stimulation is followed by a sleep rebound, indicating homeostatic regulation. In contrast to mammals and similarly to birds, light suppresses sleep in zebrafish, with no evidence for a sleep rebound. We also identify a null mutation in the sole receptor for the wake-promoting neuropeptide hypocretin (orexin) in zebrafish. Fish lacking this receptor demonstrate short and fragmented sleep in the dark, in striking contrast to the excessive sleepiness and cataplexy of narcolepsy in mammals. Consistent with this observation, we find that the hypocretin receptor does not colocalize with known major wake-promoting monoaminergic and cholinergic cell groups in the zebrafish. Instead, it colocalizes with large populations of GABAergic neurons, including a subpopulation of Adra2a-positive GABAergic cells in the anterior hypothalamic area, neurons that could assume a sleep modulatory role. Our study validates the use of zebrafish for the study of sleep and indicates molecular diversity in sleep regulatory networks across vertebrates.

Sleep disorders are common and poorly understood. Further, how and why the brain generates sleep is the object of intense speculations. In this study, we demonstrate that a bony fish used for genetic studies sleeps and that a molecule, hypocretin, involved in causing narcolepsy, is conserved. In humans, narcolepsy is a sleep disorder associated with sleepiness, abnormal dreaming, and paralysis and insomnia. We generated a mutant fish in which the hypocretin system was disrupted. Intriguingly, this fish sleep mutant does not display sleepiness or paralysis but has a 30% reduction of its sleep time at night and a 60% decrease in sleep bout length compared with non-mutant fish. We also studied the relationships between the hypocretin system and other sleep regulatory brain systems in zebrafish and found differences in expression patterns in the brain that may explain the differences in behavior. Our study illustrates how a sleep regulatory system may have evolved across vertebrate phylogeny. Zebrafish, a powerful genetic model that has the advantage of transparency to study neuronal networks in vivo, can be used to study sleep.

Zebrafish sleep, and have the receptor for the wake-inducing molecule hypocretin. While mutation in this receptor causes narcolepsy in mammals, in fish, sleep is fragmented, demonstrating differences in sleep control in vertebrates.

Phylogenetics and the correlates of mammalian sleep: a reappraisal

The correlates of mammalian sleep have been investigated previously in at least eight comparative studies in an effort to illuminate the functions of sleep. However, all of these univariate analyses treated each species, or taxonomic Family, as a statistically independent unit, which is invalid due to the phylogenetic relationships among species. Here, we reassess these influential correlates of mammalian sleep using the formal phylogenetic framework of independent contrasts. After controlling for phylogeny using this procedure, the interpretation of many of the correlates changed. For instance, and contrary to previous studies, we found interspecific support for a neurophysiological role for rapid-eye-movement sleep, such as memory consolidation. Also in contrast to previous studies, we did not find comparative support for an energy conservation function for slow-wave sleep. Thus, the incorporation of a phylogenetic control into comparative analyses of sleep yields meaningful differences that affect our understanding of why we sleep.

Sleep complaints in community-living older persons: a multifactorial geriatric syndrome

In older persons, sleep complaints in the form of insomnia and daytime drowsiness are highly prevalent and are associated with adverse outcomes. The underlying mechanisms are linked to age-related declines in physiology (normal aging) and age-related increases in disease prevalence (usual aging). This article describes how normal aging leads to less-restorative sleep, characterized by reductions in homeostatic and circadian sleep, and to phase advancement of the sleep-wake cycle, characterized by older persons being more alert in the early morning but drowsier in the early evening. It also describes how usual aging leads to sleep complaints through reductions in health status, loss of physical function, and primary sleep disorders. Psychosocial influences are likewise described, and their relevance to sleep complaints is discussed. These aging-related changes are subsequently incorporated into a conceptual model that describes sleep complaints as a consequence of multiple and interdependent predisposing, precipitating, and perpetuating factors, akin to a geriatric syndrome. The discussion concludes by applying the conceptual model to the sleep-related care of an older person with insomnia and daytime drowsiness and suggesting that the diagnostic assessment consider, in addition to primary sleep disorders, multiple domains, including medical, physical, cognitive, psychological, and social matters, with the intent of developing an overall therapeutic plan and establishing long-term follow-up.

The energetic savings of sleep versus temperature in the Desert Iguana (Dipsosaurus dorsalis) at three ecologically relevant temperatures

One of the proposed ecological functions of sleep is to conserve energy. The majority of studies that support this theory have been done on endothermic animals whose body temperatures drop during sleep due to the reduced neurological control of thermoregulation. In the present study, we examined typical temperatures to which the Desert Iguana, Dipsosaurus dorsalis, is exposed to in the field and found that mean high temperatures ranged from 24-58 degrees C throughout the active portion of the year. We also examined the ecological savings that sleep could provide for this ectothermic iguana using a closed system respirometer. We found that laboratory-acclimated iguanas are able to save significantly more (27.6%) energy by sleeping than by being awake and that field iguanas also had significant savings of energy (69.1%) while asleep. However, iguanas could save more energy by remaining awake at cooler temperatures than by sleeping at warmer temperatures. In addition, we found no correlation for time of night with metabolic rate. Our study supports the hypothesis that one potential function of sleep is to conserve energy.

Short sleep duration is associated with reduced leptin levels and increased adiposity: Results from the Quebec family study

OBJECTIVE

To explore cross-sectional associations between short sleep duration and variations in body fat indices and leptin levels during adulthood in a sample of men and women involved in the Québec Family Study.

RESEARCH METHODS AND PROCEDURES

Anthropometric measurements, plasma lipid-lipoprotein profile, plasma leptin concentrations, and total sleep duration were determined in a sample of 323 men and 417 women ages 21 to 64 years.

RESULTS

When compared with adults reporting 7 to 8 hours of sleep per day, the adjusted odds ratio for overweight/obesity was 1.38 (95% confidence interval, 0.89 to 2.10) for those with 9 to 10 hours of sleep and 1.69 (95% confidence interval, 1.15 to 2.39) for those with 5 to 6 hours of sleep, after adjustment for age, sex, and physical activity level. In each sex, we observed lower adiposity indices in the 7- to 8-hour sleeping group than in the 5- to 6-hour sleeping group. However, all of these significant differences disappeared after statistical adjustment for plasma leptin levels. Finally, the well-documented regression of plasma leptin levels over body fat mass was used to predict leptin levels of short-duration sleepers (5 and 6 hours of sleep), which were then compared with their measured values. As expected, the measured leptin values were significantly lower than predicted values.

DISCUSSION

There may be optimal sleeping hours at which body weight regulation is facilitated. Indeed, short sleep duration predicts an increased risk of being overweight/obese in adults and is related to a reduced circulating leptin level relative to what is predicted by fat mass. Because sleep duration is a potentially modifiable risk factor, these findings might have important clinical implications for the prevention and treatment of obesity.

A phylogenetic analysis of the correlates of sleep in birds

Quantitative comparative studies of sleep have focused exclusively on mammals. Such studies have repeatedly found strong relationships between the time spent in various sleep states and constitutive variables related to morphology, physiology, and life history. These studies influenced the development of several prominent hypotheses for the functions of sleep, but the applicability of these patterns and hypotheses to non-mammalian taxa is unclear. Here, we present the first quantitative analysis of sleep in a non-mammalian taxon (birds), focusing on the daily amount of time spent in slow-wave sleep (SWS) and rapid-eye movement (REM) sleep as determined by electrophysiological methods. We examined the relationships between constitutive and sleep variables in 23 avian species following earlier studies in mammals, but also considered an index of exposure to predators while asleep and controlled for shared evolutionary history among taxa. Overall, our results were very different from those obtained for mammals. Most remarkably, the relationships between both SWS time and REM sleep time and all constitutive variables were very weak and markedly non-significant, even though we had adequate power to detect correlations typical of the mammalian data. Only an index of exposure to predation during sleep was significantly related to sleep time, which is the only result common to both birds and mammals. Our results suggest that further insight into the function(s) of sleep across the animal kingdom may require an expansion of sleep research beyond the current mammalian paradigm.

A Phylogenetic Analysis of Sleep Architecture in Mammals: The Integration of Anatomy, Physiology, and Ecology

Among mammalian species, the time spent in the two main "architectural" states of sleep--slow-wave sleep (SWS) and rapid-eye-movement (REM) sleep--varies greatly. Previous comparative studies of sleep architecture found that larger mammals, those with bigger brains, and those with higher absolute basal metabolic rates (BMR) tended to engage in less SWS and REM sleep. Species experiencing a greater risk of predation also exhibited less SWS and REM sleep. In all cases, however, these studies lacked a formal phylogenetic and theoretical framework and used mainly correlational analyses. Using independent contrasts and an updated data set, we extended existing approaches with path analysis to examine the integrated influence of anatomy, physiology, and ecology on sleep architecture. Path model structure was determined by nonmutually exclusive hypotheses for the function of sleep. We found that species with higher relative BMRs engage in less SWS, whereas species with larger relative brain masses engage in more REM sleep. REM sleep was the only sleep variable strongly influenced by predation risk; mammals sleeping in riskier environments engage in less REM sleep. Overall, we found support for some hypotheses for the function of sleep, such as facilitating memory consolidation or learning, but not others, such as energy conservation.

Control of cerebral blood flow during sleep and the effects of hypoxia

During wakefulness, cerebral blood flow (CBF) is closely coupled to regional cerebral metabolism; however CBF is also strongly modulated by breathing, increasing in response to both hypercapnia and hypoxia. During stage III/IV non-rapid eye (NREM) sleep, cerebral metabolism and CBF decrease whilst the partial pressure of arterial CO2 increases due to a reduction in alveolar ventilation. The reduction in CBF during NREM sleep therefore occurs despite a relative state of hypercapnia. We have used transcranial Doppler ultrasound to determine middle cerebral artery velocity, as an index of CBF, and have determined that NREM sleep is associated with a reduction in the cerebrovascular response to hypercapnia. This reduction in reactivity would, at least in part, allow the observed reductions in CBF in this state. Similarly, we have observed that the CBF response to hypoxia is absent during stage III/IV NREM sleep. Nocturnal hypoxia and hypercapnia are major pathogenic factor associated with cardio-respiratory diseases. These marked changes in cerebrovascular control that occur during sleep suggest that the cerebral circulation may be particularly vulnerable to cardio-respiratory insults during this period.

Toward an integrative theory of sleep and dreaming

Non-rapid-eye-movement sleep (NREMS) is triggered by the accumulation of adenosine, as a result of the perceptual overload of the brain cortex. NREMS starts in the most burdened regions of the cortex first and then eventually, after the released adenosine has reached the ventrolateral pre-optic nucleus area of the hypothalamus, triggers the "general NREMS pattern". This is accompanied by the usual familiar changes in the thalamocortical system. When NREMS reaches the slow-wave sleep (SWS) phase, with its predominant delta activity, brain metabolism drops significantly with the brain temperature, and this is recognized by the alarm system in the pre-optic anterior hypothalamus and/or the other thermostat circuit in the brainstem as a life-threatening situation. This alarm system triggers a reaction similar to abortive or partial awakening called rapid-eye-movement sleep (REMS), which is aimed at restoring the optimal body-core temperature. As soon as this restoration is accomplished by the activation of the brainstem-to-cortex ascending pathways, NREMS may continue, as may the interchange of the two sleep phases during the entire sleep period. During both NREMS and REMS, the same essential pattern occurs in the cortex: the loops "used" during the previous waking period, now deprived of external input, replay their waking activity at a lower frequency, one which enables them to restore the membrane's potential (possibly by means of LTD). During REMS, however, the cholinergic flood originating in the LTD/PPT nuclei of the pons tegmentum, increases in the basal forebrain and, provoking theta activity in the medial septum is extended to the hippocampus, causing the circuits that are active at that particular moment in the cortex, to store the information they carry as memory. This is the explanation of both the memory improvement known to be related to REMS and of dreams. Both phenomena are clearly side effects of REMS.

Short sleep duration is associated with reduced leptin, elevated ghrelin, and increased body mass index

BACKGROUND

Sleep duration may be an important regulator of body weight and metabolism. An association between short habitual sleep time and increased body mass index (BMI) has been reported in large population samples. The potential role of metabolic hormones in this association is unknown.

METHODS AND FINDINGS

Study participants were 1,024 volunteers from the Wisconsin Sleep Cohort Study, a population-based longitudinal study of sleep disorders. Participants underwent nocturnal polysomnography and reported on their sleep habits through questionnaires and sleep diaries. Following polysomnography, morning, fasted blood samples were evaluated for serum leptin and ghrelin (two key opposing hormones in appetite regulation), adiponectin, insulin, glucose, and lipid profile. Relationships among these measures, BMI, and sleep duration (habitual and immediately prior to blood sampling) were examined using multiple variable regressions with control for confounding factors. A U-shaped curvilinear association between sleep duration and BMI was observed. In persons sleeping less than 8 h (74.4% of the sample), increased BMI was proportional to decreased sleep. Short sleep was associated with low leptin (p for slope = 0.01), with a predicted 15.5% lower leptin for habitual sleep of 5 h versus 8 h, and high ghrelin (p for slope = 0.008), with a predicted 14.9% higher ghrelin for nocturnal (polysomnographic) sleep of 5 h versus 8 h, independent of BMI.

CONCLUSION

Participants with short sleep had reduced leptin and elevated ghrelin. These differences in leptin and ghrelin are likely to increase appetite, possibly explaining the increased BMI observed with short sleep duration. In Western societies, where chronic sleep restriction is common and food is widely available, changes in appetite regulatory hormones with sleep curtailment may contribute to obesity.

Memory consolidation in sleep; dream or reality

We discuss several lines of evidence refuting the hypothesis that procedural or declarative memories are processed/consolidated in sleep. One of the strongest arguments against a role for sleep in declarative memory involves the demonstration that the marked suppression or elimination of REM sleep in subjects on antidepressant drugs or with brainstem lesions produces no detrimental effects on cognition. Procedural memory, like declarative memory, undergoes a slow, time-dependent period of consolidation. A process has recently been described wherein performance on some procedural tasks improves with the mere passage of time and has been termed "enhancement." Some studies, but not others, have reported that the consolidation/enhancement of perceptual and motor skills is dependent on sleep. We suggest that consolidation or enhancement, initiated in waking with task acquisition, could in some instances extend to sleep, but sleep would serve no unique role in these processes. In sum, there is no compelling evidence to support a relationship between sleep and memory consolidation.

Chronic Nightmares After Severe Burns: Risk Factors and Implications for Treatment

Although patients frequently experience sleep disturbances and nightmares in the first weeks after a severe burn, information is scarce on the course and prevention of this problem. Prolonged experience of nightmares in adults is one of the symptoms of posttraumatic stress disorder. The aim of this work was to determine risk factors for developing chronic nightmares after severe burns. Personality traits and coping strategies were assessed. As part of a follow-up study of patients treated at the Burn Center at Uppsala University Hospital, Uppsala, Sweden, between 1980 and 1995, the questionnaires of 166 patients (34 females, 132 males, average age 50 years, average burn size 25% TBSA, full-thickness burn size 10% TBSA, average time since burn 11.4 years) were analyzed. The effects of individual personality traits and coping strategies on the frequency of nightmares were evaluated by regression analysis. Nightmares were reported by 43% of the patients, by females more frequently than males. The frequency of nightmares was shown to be associated with the size of the full-thickness burn. The use of Avoidance or Revaluation/Adjustment scales as coping strategies and the presence of Somatic Trait Anxiety as a personality trait were associated with a higher frequency of nightmares after correction for gender. In contrast, persons seeking Emotional Support as a coping strategy reported significantly fewer nightmares. Certain personality traits and coping strategies apparently increase the risk of having nightmares after a severe burn. Helping persons at risk develop different coping strategies may be a possible means of prevention or treatment.

Effect of controlled-release melatonin on sleep quality, mood, and quality of life in subjects with seasonal or weather-associated changes in mood and behaviour

This study aimed to explore the effects of melatonin on sleep, waking up and well being in subjects with varying degrees of seasonal or weather-associated changes in mood and behaviour. Fifty-eight healthy adults exhibiting subsyndromal seasonal affective disorder (s-SAD) and/or the negative or positive type of weather-associated syndrome (WAS) were randomised to either 2 mg of sustained-release melatonin or placebo tablets 1-2 h before a desired bedtime for 3 weeks. Outcome measures were changes from baseline in sleep quality, sleepiness after waking, atypical depressive symptoms and health-related quality of life by week three. Early morning salivary melatonin concentrations were measured at baseline and treatment cessation in all subjects. Melatonin administration significantly improved the quality of sleep (P=0.03) and vitality (P=0.02) in the subjects with s-SAD, but attenuated the improvement of atypical symptoms and physical parameters of quality of life compared to placebo in the subjects with WAS, positive type.

Activation of brown adipose tissue thermogenesis increases slow wave sleep in rat

Considering the thermoregulatory role of slow wave sleep (SWS), we wondered whether the sole increase of brown adipose tissue (BAT) thermogenesis could enhance this sleep state. We tested this hypothesis by administering to rats an agonist (BRL 37,344) of the beta-3 adrenoceptor subtype that is massively localized in BAT cell membrane and that is known to activate BAT thermogenesis. Sleep was electrographically characterized. The temperature of interscapular BAT (Tibat) and cortex (Tco) were also assessed. Tibat significantly increased 2-3 h after BRL injection (but not Tco), concomitantly with SWS (+56-57%). At the maximum of Tibat, a significant positive correlation was found between their changes and those of SWS. We demonstrated for the first time that sleep (and especially SWS) can be affected by the specific activation of BAT.

Sleep and methods of assessment

Sleep is a complex behavior; adequate sleep is essential for healthy functioning and even for survival. Poor sleep quality and insufficient sleep have been linked to increased risk for various illnesses, as well as with an increased prevalence of excessive daytime sleepiness and reduced quality of life. Daytime sleepiness can lead to dangerous outcomes associated with drowsy driving and has become an important public health issue. Routine health examinations that include questions about sleep habits, daytime sleepiness, and problems with sleep at night can help to educate people about the importance of good sleep habits. They provide a way to identify sleep disorders so that appropriate therapies can be instituted or proper referrals to a sleep specialist can be given.

The role of sleep in learning and memory

Sleep has been implicated in the plastic cerebral changes that underlie learning and memory. Indications that sleep participates in the consolidation of fresh memory traces come from a wide range of experimental observations. At the network level, reactivations during sleep of neuronal assemblies recently challenged by new environmental circumstances have been reported in different experimental designs. These neuronal assemblies are proposed to be involved in the processing of memory traces during sleep. However, despite this rapidly growing body of experimental data, evidence for the influence of sleep discharge patterns on memory traces remains fragmentary. The underlying role of sleep in learning and memory has yet to be precisely characterized.

Quality of sleep and its daily relationship to pain intensity in hospitalized adult burn patients

Sleep disturbances are frequently reported in victims following burn injuries. This prospective study was designed to assess sleep quality and to examine its daily relationship to pain intensity within the first week of hospitalization. Twenty-eight non-ventilated patients were interviewed during 5 consecutive mornings (number of observations=140) to collect information about perceived quality of sleep (visual analogue scale, number of hours, number of awakenings, presence of nightmares). Pain intensity was assessed at rest (nighttime, morning, during the day) and following therapeutic procedures using a 0-10 numeric scale. Seventy-five percent of patients reported sleep disturbances at some point during the study although, in most patients, sleep quality was not consistently poor. Pooled cross-section regression analyses showed significant temporal relationships between quality of sleep and pain intensity such that a night of poor sleep was followed by a significantly more painful day. Pain during the day was not found to be a significant predictor of poor sleep on the following night. These results support previous findings that perceived quality of sleep following burn injury is poor. Moreover, they show a daily relationship between quality of sleep and acute burn pain in which poor sleep is linked to higher pain intensity during the day.

Behavioral, neurophysiological and evolutionary perspectives on unihemispheric sleep

Several animals mitigate the fundamental conflict between sleep and wakefulness by engaging in unihemispheric sleep, a unique state during which one cerebral hemisphere sleeps while the other remains awake. Among mammals, unihemispheric sleep is restricted to aquatic species (Cetaceans, eared seals and manatees). In contrast to mammals, unihemispheric sleep is widespread in birds, and may even occur in reptiles. Unihemispheric sleep allows surfacing to breathe in aquatic mammals and predator detection in birds. Despite the apparent utility in being able to sleep unihemispherically, very few mammals sleep in this manner. This is particularly interesting since the reptilian ancestors to mammals may have slept unihemispherically. The relative absence of unihemispheric sleep in mammals suggests that a trade off exists between unihemispheric sleep and other adaptive brain functions occurring during sleep or wakefulness. Presumably, the benefits of sleeping unihemispherically only outweigh the costs under extreme circumstances such as sleeping at sea. Ultimately, a greater understanding of the reasons for little unihemispheric sleep in mammals promises to provide insight into the functions of sleep, in general.

Corticosterone and nocturnal torpor in the rufous hummingbird (Selasphorus rufus)

Three experiments were designed to investigate whether corticosterone (CORT), known to have a role in restoration of energy homeostasis, regulates nocturnal torpor, an energy conservation state used by some small mammals and birds to offset environmental challenges to energy balance. In two experiments, one during autumn migration and one during early spring molt, captive rufous hummingbirds (Selasphorus rufus) were fed control and dilute (85% strength) nectar on alternate days. In migratory birds, torpor occurred more frequently over all, and nectar dilution resulted in increased torpor duration and increased concentration of CORT in evening but not midday cloacal fluid (CF) samples. In molting birds, torpor occurred infrequently on both control and food dilution days, but, although there was a significant increase in evening CF CORT on food dilution days, torpor duration did not increase significantly in response and there was no correlation between torpor duration and CF CORT at either time of day. Daily CF CORT patterns showed an increase from midday to evening during migration, but the reverse pattern during the molt. In a third experiment, CORT administered in the nectar elevated the use of torpor and depressed food intake. The results of these three experiments support the hypothesis that CORT is involved in the regulation of torpor, but suggest that some feature of the CORT signal other than concentration per se may be required to fully explain seasonal changes in the relations among energy challenge, CORT, and nocturnal torpor in hummingbirds.

Adaptations and pathologies linked to dynamic stabilization of neural circuitry

Brain circuits for infrequently employed memories are reinforced largely during sleep by self-induced, electrical slow-waves, a process referred to as "dynamic stabilization" (DS). The essence of waking brain function in the absence of volitional activity is sensory input processing, an enormous amount of which is visual. These two functions: circuit reinforcement by DS and sensory information processing come into conflict when both occur at a high level, a conflict that may have been the selective pressure for sleep's origin. As brain waves are absent at the low temperatures of deep torpor, essential circuitry of hibernating small mammals would lose its competence if the animals did not warm up periodically to temperatures allowing sleep and circuit reinforcement. Blind, cave-dwelling vertebrates require no sleep because their sensory processing does not interfere with DS. Nor does such interference arise in continuously-swimming fishes, whose need to process visual information is reduced greatly by life in visually relatively featureless, pelagic habitats, and by schooling. Dreams are believed to have their origin in DS of memory circuits. They are thought to have illusory content when the circuits are partially degraded (incompetent), with synaptic efficacies weakened through infrequent use. Partially degraded circuits arise normally in the course of synaptic efficacy decay, or pathologically through abnormal regimens of DS. Organic delirium may result from breakdown of normal regimens of DS of circuitry during sleep, leaving many circuits incompetent. Activation of incompetent circuits during wakefulness apparently produces delirium and hallucinations. Some epileptic seizures may be induced by abnormal regimens of DS of motor circuitry. Regimens of remedial DS during seizures induced by electroconvulsive therapy (ECT) apparently produce temporary remission of delirium by restoring functional or 'dedicated' synaptic efficacies in incompetent circuitry. Sparing of sensory circuitry in fatal familial insomnia seemingly owes to supernormal circuit use in the virtual absence of sleep. ECT shocks and cardioverter defibrillation may have analogous remedial influences.

Vigilance states and body temperature during the circadian cycle in fed and fasted pigeons (Columba livia)

Fasting induces nocturnal hypothermia in pigeons. Slow-wave sleep (SWS) and paradoxical sleep (PS) are associated with reduced heat production in pigeons. The possibility that fasting-induced nocturnal hypothermia is related to increased SWS and PS was examined by comparing body temperature (Tb) and vigilance states when pigeons were fed and fasted. The results showed that when Tb is decreasing near the beginning of the dark phase, the percentage of total recording time (%TRT) spent in SWS and PS was elevated in fasting due to increased frequency of episodes and increased duration of PS episodes. When Tb was low during the middle segment of the dark phase, SWS was elevated in fasting due to increased episode duration. However, fasting did not alter PS, which increased in %TRT across the segment due to increased episode frequency. When Tb was rising during the final hours of dark, SWS remained elevated in fasting and %TRT in SWS and PS was relatively high. SWS and PS may promote the fasting pigeon's entry into, and maintenance of, nocturnal hypothermia.

Memory, sleep and the evolution of mechanisms of synaptic efficacy maintenance

The origin of both sleep and memory appears to be closely associated with the evolution of mechanisms of enhancement and maintenance of synaptic efficacy. The development of activity-dependent synaptic plasticity apparently was the first evolutionary adaptation of nervous systems beyond a capacity to respond to environmental stimuli by mere reflexive actions. After the origin of activity-dependent synaptic plasticity, whereby single activations of synapses led to short-term efficacy enhancement, lengthy maintenance of enhancements probably was achieved by repetitive activations ("dynamic stabilization"). One source of selective pressure for the evolutionary origin of neurons and neural circuits with oscillatory firing capacities may have been a need for repetitive spontaneous activations to maintain synaptic efficacy in circuits that were in infrequent use. This process is referred to as "non-utilitarian" dynamic stabilization. Dynamic stabilization of synapses in "simple" invertebrates occurs primarily through frequent use. In complex, locomoting forms, it probably occurs through both frequent use and non-utilitarian activations during restful waking. With the evolution of increasing repertories and complexities of behavioral and sensory capabilities--with vision usually being the vastly pre-eminent sense brain complexity increased markedly. Accompanying the greater complexity, needs for storage and maintenance of hereditary and experiential information (memories) increased greatly. It is suggested that these increases led to conflicts between sensory input processing during restful waking and concomitant non-utilitarian dynamic stabilization of infrequently used memory circuits. The selective pressure for the origin of primitive sleep may have been a resulting need to achieve greater depression of central processing of sensory inputs largely complex visual information than occurs during restful waking. The electrical activities of the brain during sleep (aside from those that subserve autonomic activities) may function largely to maintain sleep and to dynamically stabilize infrequently used circuitry encoding memories. Sleep may not have been the only evolutionary adaptation to conflicts between dynamic stabilization and sensory input processing. In some ectothermic vertebrates, sleep may have been postponed or rendered unnecessary by a more readily effected means of resolution of the conflicts, namely, extensive retinal processing of visual information during restful waking. By this means, processing of visual information in central regions of the brain may have been maintained at a sufficiently low level to allow adequate concomitant dynamic stabilization. As endothermy evolved, the skeletal muscle hypotonia of primitive sleep may have become insufficient to prevent sleep-disrupting skeletal muscle contractions during non-utilitarian dynamic stabilization of motor circuitry at the accompanying higher body temperatures and metabolic rates. Selection against such disruption during dynamic stabilization of motor circuitry may have led to the inhibition of skeletal muscle tone during a portion of primitive sleep, the portion designated as rapid-eye-movement sleep. Many marine mammals that are active almost continuously engage only in unihemispheric non-rapid-eye-movement sleep. They apparently do not require rapid-eye-movement sleep and accompanying non-utilitarian dynamic stabilization of motor circuitry, because this circuitry is in virtually continuous use. Studies of hibernation by arctic ground squirrels suggest that each hour of sleep may stabilize brain synapses for as long as 4 h. Phasic irregularities in heart and respiratory rates during rapid-eye-movement sleep may be a consequence of superposition of dynamic stabilization of motor circuitry on the rhythmic autonomic control mechanisms. Some information encoded in circuitry being dynamically stabilized during sleep achieves unconscious awareness in authentic and var

Functional neuroanatomy of human slow wave sleep

The distribution of regional cerebral blood flow (rCBF) was estimated during sleep and wakefulness by using H215O positron emission tomography (PET) and statistical parametric mapping. A group analysis on 11 good sleepers (8 with steady slow wave sleep, SWS) showed a significant negative correlation between the occurrence of SWS and rCBF in dorsal pons and mesencephalon, thalami, basal ganglia, basal forebrain/hypothalamus, orbitofrontal cortex, anterior cingulate cortex, precuneus, and, on the right side, in a region that follows the medial aspect of the temporal lobe. Given the known decrease in global cerebral blood flow levels during SWS, these negative correlations suggest that rCBF is decreased significantly more in these cerebral areas than in the rest of the brain. The marked rCBF decreases in the pons, mesencephalon, thalamic nuclei, and basal forebrain reflect their close implication in the generation of SWS rhythms. The influence of these rhythms on the telencephalon usually are thought to be global and homogeneous. In contrast, our results show that rCBF is decreased more in some cortical areas (especially in orbitofrontal cortex) than in the rest of the cortex. We hypothesize that cellular processes taking place during SWS might be modulated differently in these regions. Given the functions of the ventromedial frontal areas, we surmise that SWS might be particularly critical for the adaptation of behavior to environmental pressures. This hypothesis is supported indirectly by results of sleep deprivation experiments.

Origin and evolution of sleep: roles of vision and endothermy

The origin of both sleep and memory appears to be closely associated with the evolution of mechanisms of enhancement and maintenance of synaptic efficacy. After the origin of activity-dependent synaptic plasticity, whereby single activations of synapses led to short-term efficacy enhancements, lengthy maintenance of the enhancements probably was achieved by repetitive activations ("dynamic stabilization"). These are thought to have occurred either in the course of frequent functional use, or to have been induced spontaneously within the brain to maintain synaptic efficacies in circuits that were in infrequent use. The latter repetitive activations are referred to as 'non-utilitarian' dynamic stabilization. With the evolution of increasing repertories and complexities of behavioral and sensory capabilities-with vision usually being the vastly preeminent sense-brain complexity increased markedly. Accompanying the greater complexity, needs for storage and maintenance of hereditary and experimental information (memories) also increased greatly. It is suggested that these increases led to conflicts between sensory input processing during restful waking and concomitant 'non-utilitarian' dynamic stabilization of infrequently used memory circuits. The selective pressure for the origin of primitive sleep may have been a need to achieve greater depression of central processing of sensory inputs-largely complex visual information-than occurs during restful waking. The electrical activities of the brain during sleep (aside from those that subserve autonomic activities) may function largely to maintain sleep and to dynamically stabilize infrequently used circuitry encoding memories. Sleep may not have been the only evolutionary adaptation to conflicts between dynamic stabilization and sensory input processing. In some ectothermic vertebrates, sleep may have been postponed or rendered unnecessary by a more readily effected means of resolution of the conflicts, namely, extensive retinal processing of visual information during restful waking. By this means, processing of visual information in central regions of the brain may have been maintained at a sufficiently low level to allow adequate concomitant dynamic stabilization. As endothermy evolved, the skeletal muscle hypotonia of primitive sleep may have become insufficient to prevent sleep-disrupting skeletal muscle contractions during 'non-utilitarian' dynamic stabilization of motor circuitry at the accompanying higher body temperatures and metabolic rates. Selection against such disruption during dynamic stabilization of motor circuitry may have led to the inhibition of skeletal muscle tone during a portion of primitive sleep, the portion designated as "rapid-eye-movement sleep." Many marine mammals that are active almost continuously engage only in unihemispheric non-rapid-eye-movement sleep. They apparently do not require rapid-eye-movement sleep and accompanying 'non-utilitarian' dynamic stabilization of motor circuitry because this circuitry is in virtually continuous use. Studies of hibernation by arctic ground squirrels suggest that each hour of sleep stabilizes brain synapses for as long as four hours.

The function of fetal/neonatal rapid eye movement sleep

The hypothesis is put forward that rapid eye movement (REM) sleep in early life serves as (1) an indicator for the degree of brain maturation and (2) the promoter of further brain development. This hypothesis, although not exclusive, differs (a) from the theory of Roffwarg et al. that REM sleep substitutes for 'wakefulness' during the period (early life) in which wakefulness is limited, (b) from the theory of Crick and Mitchson, i.e., the 'unlearning' hypothesis of REM sleep, (c) from the theory of Jouvet, i.e., that REM sleep is a time for genetic read-out and (d) from the theory of Freud, i.e., that dreams fulfil our wishes (in other words, activation of neuronal systems that were disproportionally activated during wakefulness).