Thermal load alters sleep

Association between lower body temperature and increased tau pathology in cognitively normal older adults

BACKGROUND

Preclinical studies suggest body temperature (Tb) and consequently brain temperature has the potential to bidirectionally interact with tau pathology in Alzheimer's Disease (AD). Tau phosphorylation is substantially increased by a small (<1 °C) decrease in temperature within the human physiological range, and thermoregulatory nuclei are affected by tau pathology early in the AD continuum. In this study we evaluated whether Tb (as a proxy for brain temperature) is cross-sectionally associated with clinically utilized markers of tau pathology in cognitively normal older adults.

METHODS

Tb was continuously measured with ingestible telemetry sensors for 48 h. This period included two nights of nocturnal polysomnography to delineate whether Tb during waking vs sleep is differentially associated with tau pathology. Tau phosphorylation was assessed with plasma and cerebrospinal fluid (CSF) tau phosphorylated at threonine 181 (P-tau), sampled the day following Tb measurement. In addition, neurofibrillary tangle (NFT) burden in early Braak stage regions was imaged with PET-MR using the [18F]MK-6240 radiotracer on average one month later.

RESULTS

Lower Tb was associated with increased NFT burden, as well as increased plasma and CSF P-tau levels (p < 0.05). NFT burden was associated with lower Tb during waking (p < 0.05) but not during sleep intervals. Plasma and CSF P-tau levels were highly correlated with each other (p < 0.05), and both variables were correlated with tau tangle radiotracer uptake (p < 0.05).

CONCLUSIONS

These results, the first available for human, suggest that lower Tb in older adults may be associated with increased tau pathology. Our findings add to the substantial preclinical literature associating lower body and brain temperature with tau hyperphosphorylation.

CLINICAL TRIAL NUMBER

NCT03053908.

The Temperature Dependence of Sleep

Mammals have evolved a range of behavioural and neurological mechanisms that coordinate cycles of thermoregulation and sleep. Whether diurnal or nocturnal, sleep onset and a reduction in core temperature occur together. Non-rapid eye movement (NREM) sleep episodes are also accompanied by core and brain cooling. Thermoregulatory behaviours, like nest building and curling up, accompany this circadian temperature decline in preparation for sleeping. This could be a matter of simply comfort as animals seek warmth to compensate for lower temperatures. However, in both humans and other mammals, direct skin warming can shorten sleep-latency and promote NREM sleep. We discuss the evidence that body cooling and sleep are more fundamentally connected and that thermoregulatory behaviours, prior to sleep, form warm microclimates that accelerate NREM directly through neuronal circuits. Paradoxically, this warmth might also induce vasodilation and body cooling. In this way, warmth seeking and nesting behaviour might enhance the circadian cycle by activating specific circuits that link NREM initiation to body cooling. We suggest that these circuits explain why NREM onset is most likely when core temperature is at its steepest rate of decline and why transitions to NREM are accompanied by a decrease in brain temperature. This connection may have implications for energy homeostasis and the function of sleep.

Neuronal noise as an origin of sleep arousals and its role in sudden infant death syndrome

In addition to regular sleep/wake cycles, humans and animals exhibit brief arousals from sleep. Although much is known about consolidated sleep and wakefulness, the mechanism that triggers arousals remains enigmatic. Here, we argue that arousals are caused by the intrinsic neuronal noise of wake-promoting neurons. We propose a model that simulates the superposition of the noise from a group of neurons, and show that, occasionally, the superposed noise exceeds the excitability threshold and provokes an arousal. Because neuronal noise decreases with increasing temperature, our model predicts arousal frequency to decrease as well. To test this prediction, we perform experiments on the sleep/wake behavior of zebrafish larvae and find that increasing water temperatures lead to fewer and shorter arousals, as predicted by our analytic derivations and model simulations. Our findings indicate a previously unrecognized neurophysiological mechanism that links sleep arousals with temperature regulation, and may explain the origin of the clinically observed higher risk for sudden infant death syndrome with increased ambient temperature.

Basal forebrain thermoregulatory mechanism modulates auto-regulated sleep

Regulation of body temperature and sleep are two physiological mechanisms that are vital for our survival. Interestingly neural structures implicated in both these functions are common. These areas include the medial preoptic area (POA), the lateral POA, the ventrolateral POA, the median preoptic nucleus, and the medial septum, which form part of the basal forebrain (BF). When given a choice, rats prefer to stay at an ambient temperature of 27°C, though the maximum sleep was observed when they were placed at 30°C. Ambient temperature around 27°C should be considered as the thermoneutral temperature for rats in all sleep studies. At this temperature the diurnal oscillations of sleep and body temperature are properly expressed. The warm sensitive neurons of the POA mediate the increase in sleep at 30°C. Promotion of sleep during the rise in ambient temperature from 27 to 30°C, serve a thermoregulatory function. Autonomous thermoregulatory changes in core body temperature and skin temperature could act as an input signal to modulate neuronal activity in sleep-promoting brain areas. The studies presented here show that the neurons of the BF play a key role in regulating sleep. BF thermoregulatory system is a part of the global homeostatic sleep regulatory mechanism, which is auto-regulated.

Sleep under extreme environments: Effects of heat and cold exposure, altitude, hyperbaric pressure and microgravity in space

Human sleep is sensitive to the individual's environment. The present review examines current knowledge of human sleep patterns under different environments: heat exposure, cold exposure, altitude, high pressure and microgravity in space. Heat exposure has two effects. In people living in temperate conditions, moderate heat loads (hot bath, sauna) prior to sleep provoke a delayed reaction across time (diachronic reaction) whereby slow-wave sleep (SWS) augments in the following night (neurogenic adaptive pathway). Melanoids and Caucasians living in the Sahel dry tropical climate experience diachronic increases in SWS throughout seasonal acclimatization. Such increases are greater during the hot season, being further enhanced after daytime exercise. On the contrary, when subjects are acutely exposed to heat, diachronic decreases in total sleep time and SWS occur, being often accompanied by synchronic (concomitant) diminution in REM sleep. Stress hormones increase. Nocturnal cold exposure provokes a synchronic decrease in REM sleep along with an activation of stress hormones (synchronic somatic reaction). SWS remains undisturbed as it still occurs at the beginning of the night before nocturnal body cooling. Altitude and high pressure are deleterious to sleep, especially in non-acclimatized individuals. In their controlled environment, astronauts can sleep well in microgravity. Exercise-induced sleep changes help to understand environmental effects on sleep: well-tolerated environmental strains may improve sleep through a neurogenic adaptive pathway; when this "central" adaptive pathway is overloaded or bypassed, diachronic and synchronic sleep disruptions occur.

The interaction between sleep and thermoregulation in adults and neonates

The interaction between sleep and thermoregulation leads to different thermoregulatory responses depending on the sleep stage and alterations in sleep when in a cool or warm environment. In the human adult, differences in thermoregulatory efficiency during rapid eye movement (REM) sleep and slow wave sleep (SWS) are less pronounced compared to other mammals: although thermoregulatory processes persist in REM sleep, they are less efficient than during SWS. Cold and warm loads disturb the efficiency and structure of sleep. The duration of REM sleep and (to a lesser extent) of SWS decreases. In contrast, pre-sleep warm loads enhance SWS and improve sleep continuity. This procedure may promote and maintain sleep in depressed patients, whose sleep and body temperature rhythms are modified. In contrast to adults, homeothermic processes in neonates are maintained or even enhanced during active sleep (AS) when compared to quiet sleep (QS). Sleeping in a cool environment increases the duration of AS at the expense of QS. As a result, the thermoregulatory function overcomes the need to conserve energy that would otherwise lead to increased QS. An interaction between sleep, respiration, and thermoregulation may be involved in Sudden Infant Death Syndrome: an alteration in the thermal balance may perhaps induce respiration instability, especially during AS.

Exercise and sleep

This paper reviews the literature on the association between exercise and sleep. The epidemiological and experimental evidence for whether or not acute and chronic exercise promote sleep is discussed, as well as moderating factors and agendas for future directions of study. The expectation that exercise will benefit sleep can partly be attributed to traditional hypotheses that sleep serves energy conservation, body restoration or thermoregulatory functions, all of which have guided much of the research in this field. Exercise is a complex activity that can be beneficial to general well-being but may also stress the body. Differences in the exercise protocols studied (e.g. aerobic or anaerobic, intensity, duration) and interactions between individual characteristics (e.g. fitness, age and gender) cloud the current experimental evidence supporting a sleep-enhancing effect of exercise. In addition, the tendency to study changes in small groups of good sleepers may also underestimate the efficacy of exercise for promoting sleep. Athough only moderate effect sizes have been noted, meta-analytical techniques have shown that exercise increased total sleep time and delayed REM sleep onset (10 min), increased slow-wave sleep (SWS) and reduced REM sleep (2-5 min). The sleep-promoting efficacy of exercise in normal and clinical populations has yet to be established empirically.

Exercise-induced increase in core temperature does not disrupt a behavioral measure of sleep

On separate nights 90 to 30 min before typical bedtime, eight physically active men completed three conditions: seated rest, low-intensity and moderate-intensity cycle exercise. Low-and moderate-intensity exercise had no significant effect on sleep onset latency, the number of awakenings, total sleep time or sleep efficiency as measured by the Sleep Assessment Device. Mean core body temperature was higher during sleep after moderate intensity (36.80+/-0.02 degrees C) exercise compared to both the low-intensity exercise (36.67+/-0.02 degrees C) and rest (36.51+/-0.02 degrees C) conditions. It is concluded that a 1-h bout of moderate-intensity exercise performed shortly before bed elevates core body temperature before and during sleep; however, this elevated temperature does not disrupt behavioral measures of sleep obtained in the home environment in physically active male college students who were somewhat sleep deprived.

Effects of training volume on sleep, psychological, and selected physiological profiles of elite female swimmers

Excessive training is reported to cause sleep disturbances and mood changes. We examined sleep and psychological changes in female swimmers across a competitive swimming season, that is, at the start of the season (onset), during peak training period (peak), and after a precompetition reduction in training (taper). For each phase, polysomnographic recordings, body composition, psychological parameters, and swimming performance were obtained. A daily training log and sleep diary were maintained for the entire study period. Sleep onset latency (SOL) time awake after sleep onset, total sleep time (TST), and rapid eye movement (REM) sleep times were similar at all three training levels. Slow wave sleep (SWS) formed a very high percentage of total sleep in the onset (26%) and peak (31%) training periods, but was significantly reduced following precompetition taper (16%), supporting the theory that the need for restorative SWS is reduced with reduced physical demand. The number of movements during sleep was significantly higher at the higher training volumes, suggesting some sleep disruption. In contrast to other studies, mood deteriorated with a reduction in training volume and/or impending competition.

Circadian rhythms and psychiatric disorders in the elderly

This article reviews changes in circadian rhythms that have been reported to occur in the elderly psychiatric population. Data relating to circadian changes in normal aging are included where relevant. Information was obtained from: (1) a computerized MEDLINE search from 1975 to May 1996; (2) a review of bibliographies of papers obtained through the computerized search; and (3) texts on chronobiology. We could not locate any information relating to circadian rhythms and mania, anxiety, or paranoid disorders in old age. Disruption to the sleep/wake cycle, temperature, melatonin, and motor activity rhythms have been reported in dementia and depression, and disruption to some neuroendocrine and cardiovascular rhythms are reported in dementia. Disruption to circadian rhythmicity has implications for the management of dementia patients: for example, the sleep/wake and behavioral disturbances, and for the long-term management of mood disorders. A number of circadian markers have not been studied and several patient groups have received no research attention to date.

Effect of unilateral somatosensory stimulation prior to sleep on the sleep EEG in humans

The hypothesis that local activation of brain regions during wakefulness affects the EEG recorded from these regions during sleep was tested by applying vibratory stimuli to one hand prior to sleep. Eight subjects slept in the laboratory for five consecutive nights. During a 6-h period prior to night 3, either the left or the right hand was vibrated intermittently (20 min on-8 min off), while prior to night 5 the same treatment was applied to the contralateral hand. The sleep EEG was recorded from frontal, central, parietal and occipital derivations and subjected to spectral analysis. The interhemispheric asymmetry index (IAI) was calculated for spectral power in nonREM sleep in the frequency range 0.25-25.0 Hz for 0.5-Hz or 1-Hz bins. In the first hour of sleep following right-hand stimulation, the IAI of the central derivation was increased relative to baseline, which corresponds to a shift of power towards the left hemisphere. This effect was most prominent in the delta range, was limited to the first hour of sleep and was restricted to the central derivation situated over the somatosensory cortex. No significant changes were observed following left-hand stimulation. Although the effect was small, it is consistent with the hypothesis that the activation of specific neuronal populations during wakefulness may have repercussions on their electrical activity pattern during subsequent sleep.

Human core temperature minimum can be modified by ambient thermal transients

By using slow linear thermal transients (+/- 0.025 degree C/min) of reduced amplitude (+/- 3 degrees C around thermoneutrality), we were able to advance the minimum of human internal temperature (Ti) during nocturnal sleep. During experimental night the minimum of esophageal (Tes) and rectal (Tre) temperature were respectively advanced by 1.6 h (P < 0.01) and 2.6 h (P < 0.001) in comparison to reference night spent at thermoneutrality. It must be emphasized that the provoked advance of nocturnal Ti minimum was not accompanied by any change in sleep latency, efficiency, SWS and REM sleep percentages. The result shows that appropriate ambient temperature transient changes could be used to modify the course of human Ti one of the major biological rhythms usually considered as resistant to sleep-wake rhythm manipulation.

Sleep patterns in depression and anxiety: theory and pharmacological effects

Sleep is invariably disrupted in patients who have depression and in patients with anxiety disorders. Depression and anxiety frequently coexist and are associated with disturbances in various neurotransmitters. The authors explore the relationship between sleep and the two disorders as well as the effects of antidepressants and anxiolytics on sleep architecture. The effects on sleep of various neurotransmitter systems implicated in depression and anxiety are outlined. Lastly, various theoretical models are proposed to account for the above mentioned phenomena and further directions for research are suggested.

Body temperature is elevated during the rebound of slow-wave sleep following 40-h of sleep deprivation on a constant routine

EEG, EMG, EOG and core body temperature were recorded during baseline sleep and during recovery sleep from a 40-h constant routine in 9 male subjects. Slow-wave sleep and slow-wave activity (SWA, EEG power density 0.75-4.5 Hz) were enhanced in the first two nonREM sleep episodes of recovery sleep. Core body temperature was not significantly different in the last 30 minutes before lights out but was significantly higher during recovery sleep in the interval between lights out and sleep onset and during the first nonREM sleep episode. The data demonstrate that an enhancement of SWA/SWS is not necessarily accompanied by lower values of core body temperature, and therefore challenge the notion that SWS is the primary factor responsible for the steep decline of body temperature that occurs at the onset of the nightly sleep episode.

Sleep and waking have a major effect on the 24-hr rhythm of cortical temperature in the rat

The relationship between the time course of cortical temperature (TCRT) and sleep-wake alternation was investigated by correlation analyses and a computer simulation. The data for these analyses were collected in 10 rats in a 4-day experiment (LD 12:12), during which vigilance states and TCRT were determined for consecutive 8-sec epochs. On day 1 baseline recordings were obtained; on day 2 the animals were sleep-deprived; and days 3 and 4 served as recovery days. The correlation analyses revealed that the alternation of sleep and waking accounted for 84% of the variance of TCRT when analyzed for hourly intervals. The residual variance displayed a 24-hr periodicity with an amplitude of 0.15 degrees C. Similar results were obtained in a separate data set of a 2-day experiment, which consisted of a baseline day (LD 12:12) and a day with constant darkness. The periodicity of the residual variance of TCRT can therefore be considered to represent the circadian temperature rhythm not masked by the vigilance states. In the computer simulation, the time course of TCRT was simulated on the basis of the sequence of the vigilance states with an 8-sec time resolution. It was assumed that TCRT increases during waking and rapid-eye-movement (REM) sleep according to an exponential saturating function, and decreases exponentially during non-REM sleep. The simulations could account for 88-93% of the variance of TCRT. We conclude that in the rat, the major part of the variation of TCRT is accounted for by vigilance states, whereas a minor part can be attributed to a direct effect of the circadian pacemaker.

Human slow wave sleep: a review and appraisal of recent findings, with implications for sleep functions, and psychiatric illness

Recent findings concerning human slow wave sleep (hSWS-stages 3 + 4; delta EEG activity) are critically reviewed. Areas covered include the significance of the first hSWS cycle; hSWS in extended sleep; relationship between hSWS, prior wakefulness and sleep loss; hSWS influence on sleep length; problems with hSWS deprivation; influence of the circadian rhythm; individual differences in hSWS, especially, age, gender and constitutional variables such as physical fitness and body composition. Transient increases in hSWS can be produced by increasing both the quality and quantity of prior wakefulness, with an underlying mechanism perhaps relating to the waking level of brain metabolism. Whilst there may also be thermoregulatory influences on hSWS, hypotheses that energy conservation and brain cooling are major roles for hSWS are debatable. hSWS seems to offer some form of cerebral recovery, with the prefrontal cortex being particularly implicated. The hSWS characteristics of certain forms of major psychiatric disorders may well endorse this prefrontal link.

Selective SWS suppression does not affect the time course of core body temperature in men

In eight healthy middle-aged men, sleep and core body temperature were recorded under baseline conditions, during all-night SWS suppression by acoustic stimulation, and during undisturbed recovery sleep. SWS suppression resulted in a marked reduction of sleep stages 3 and 4 but did not affect the time course of core body temperature. These data suggest that sleep stages 3 and 4 of nonREM sleep (i.e. SWS) do not play a major role in the regulation of core body temperature in humans.

Cortical temperature and EEG slow-wave activity in the rat: analysis of vigilance state related changes

Vigilance states, cortical temperature (TCRT), and electroencephalograph (EEG) slow-wave-activity (SWA, mean power density in the 0.75-4.0 Hz range) of ten rats were recorded continuously during a baseline day, and two recovery days (Recovery 1 and 2) after 24 h of sleep deprivation (SD). The short term changes of TCRT were analysed within episodes of nonrapid eye movement sleep (NREMS), REM sleep (REMS) and waking (W), and at transitions between vigilance states. SWA was analysed within NREMS episodes and at W to NREMS (WN) transitions. TCRT increased during episodes of W and REMS, and decreased during NREMS episodes. These changes were a function of episode duration, and, for W and NREMS, of TCRT at episode onset. In Recovery 1 the increase in TCRT at NREMS to REMS (NR) and NREMS to W (NW) transitions tended to be attenuated. SWA within NREMS episodes was enhanced after SD. Over all experimental days, the increase of SWA and the decrease of TCRT in NREMS episodes were not correlated. It is concluded that during recovery from SD the changes in TCRT at state transitions were little affected. The lack of a relationship between changes in TCRT and SWA indicates that separate mechanisms underlie the regulation of brain temperature and sleep intensity.