Sweating during night sleep

Sleep and global warming: How will we sleep when the Earth is hotter?

Societal concern about climate change and global warming has grown worldwide along with the concomitant awareness that health will be impacted deeply. Among living beings, humans have quite large capacities for adaptation to varied temperature conditions. Despite their tropical origin, they live under all Earth climates, such as polar, temperate, altitude, arid, and tropical climates, using a wide range of behavioral and physiological adaptive responses. We address the adaptive abilities of human sleep-wake regulation and its interplay with thermoregulation under different natural climates. Sleep represents one-third of our living time and is also a major determinant of morbidity and mortality; shortening sleep duration increases mortality and multimorbidity. In addition, major advances in sleep neurology have occurred in the last decades. Some have been extensively reviewed, notably comparative sleep physiology among animals, allowing one to hypothesize about the functions of the different sleep states, as well as their relation to cognitive neuroscience or body biorhythms. However, the question of the sleep adaptive capacity of humans to global warming has barely been addressed. We examine "normal" sleep and thermoregulation in young adults residing in temperate conditions. We then review the sleep and thermoregulatory reactions under various climatic conditions, demonstrating the role of sleep changes as potent adaptive responses to living under natural hot climatic conditions. As a result, we show that humans are well-equipped to adapt to severe climates.

The impact of sleepwear fiber type on sleep quality under warm ambient conditions

BACKGROUND

Sleep disturbance in adults with no health concerns is often linked to the thermal environment. This study assesses the impact on sleep quality of sleepwear made from fibers with different thermal insulation and hygral properties. This randomized cross-over study investigated the effects on sleep quality of sleepwear made from cotton, polyester and Merino wool in adults aged 50-70 years, at an ambient temperature of 30 °C and a relative humidity of 50%.

METHODS

Thirty-six healthy participants completed four nights of sleep study with polysomnography. Participants were categorized by body mass index as <25 kg·m^-2 or ≥25 kg·m^-2, age as <65 years or ≥65 years, and by Pittsburgh Sleep Quality Index (PSQI) as poor sleepers (PSQI≥5) or good sleepers (PSQI<5).

RESULTS

Small, but statistically significant sleep benefits were observed for wool over cotton and polyester sleepwear for multiple sleep parameters, while neither cotton nor polyester was responsible for any statistically significant sleep benefit over the 11 sleep parameters examined. The key findings were: 1) A significant sleepwear effect was observed for sleep onset latency (SOL), p=0.04. 2) For older participants, sleeping in wool significantly reduced SOL (12.4 mins) compared with cotton (26.7 mins, p=0.001) or polyester (21.6 mins, p=0.001). 3) A statistically significant effect was found for sleep fragmentation index (p=0.01) in which wool sleepwear (12.1 no·h^{- 1}) was lower than polyester (13.7 no·h^{- 1}) (p=0.005), but not different to cotton (13.3 no·h^{- 1}). 4) Poor sleepers had less wakefulness when sleeping in wool compared to cotton (p=0.047). 5) And Poor sleepers had higher rapid eye movement sleep latency in polyester than in cotton (p=0.037) or in wool (p=0.036).

CONCLUSION

Statistically significant benefits for wool sleepwear were observed on average for all participants and, in particular, for the older and poorer sleepers. There were no significant differences in any sleep variables between sleepwear types for the BMI sub-group.

Oral intake of encapsulated dried ginger root powder hardly affects human thermoregulatory function, but appears to facilitate fat utilization

The present study investigated the impact of a single oral ingestion of ginger on thermoregulatory function and fat oxidation in humans. Morning and afternoon oral intake of 1.0 g dried ginger root powder did not alter rectal temperature, skin blood flow, O2 consumption, CO2 production, and thermal sensation and comfort, or induce sweating at an ambient temperature of 28 °C. Ginger ingestion had no effect on threshold temperatures for skin blood flow or thermal sweating. Serum levels of free fatty acids were significantly elevated at 120 min after ginger ingestion in both the morning and afternoon. Morning ginger intake significantly reduced respiratory exchange ratios and elevated fat oxidation by 13.5 % at 120 min after ingestion. This was not the case in the afternoon. These results suggest that the effect of a single oral ginger administration on the peripheral and central thermoregulatory function is miniscule, but does facilitate fat utilization although the timing of the administration may be relevant.

Effects of thermal environment on sleep and circadian rhythm

The thermal environment is one of the most important factors that can affect human sleep. The stereotypical effects of heat or cold exposure are increased wakefulness and decreased rapid eye movement sleep and slow wave sleep. These effects of the thermal environment on sleep stages are strongly linked to thermoregulation, which affects the mechanism regulating sleep. The effects on sleep stages also differ depending on the use of bedding and/or clothing. In semi-nude subjects, sleep stages are more affected by cold exposure than heat exposure. In real-life situations where bedding and clothing are used, heat exposure increases wakefulness and decreases slow wave sleep and rapid eye movement sleep. Humid heat exposure further increases thermal load during sleep and affects sleep stages and thermoregulation. On the other hand, cold exposure does not affect sleep stages, though the use of beddings and clothing during sleep is critical in supporting thermoregulation and sleep in cold exposure. However, cold exposure affects cardiac autonomic response during sleep without affecting sleep stages and subjective sensations. These results indicate that the impact of cold exposure may be greater than that of heat exposure in real-life situations; thus, further studies are warranted that consider the effect of cold exposure on sleep and other physiological parameters.

Basic Study for Optimal Control of In-Bed Temperature during Sleep

It is important to create a comfortable environment to restful sleep. In this study, we trial-produced an in-bed temperature control system. At first, we statically controlled the temperature in the subject's bed at 32&#176;C by using the system, and examined how this control affected sleep. We were able to confirm that the ratio of slow-wave sleep (SWS) increased in comparison to cases in which the temperature in the bed was not controlled. Next, the temperature in the subject's bed was dynamically controlled at temperature change patterns according to sleep cycles that is as follows; Heating during the REM sleep period and cooling during the SWS sleep period were conducted n the range of 32&#177;2&#176;C, and the case of the opposite phase. The result showed that cooling during the REM period increased the REM sleep share rate. Based on these results, an increase of the REM sleep share rate at around 30&#176;C could be confirmed, indicating a possibility that the REM period thermoneutrality zone shifted to a lower temperature, compared with that of SWS.

Skin sympathetic nerve function during sleep--a study with effector responses

We investigated the effector-organ activities corresponding to skin sympathetic nerve activity (SSNA) during sleep in eight healthy adult volunteers. The following parameters of SSNA were recorded during night sleep: the spontaneous skin vasoconstriction and skin blood flow volume, by laser Doppler flowmetry; sweating, by the ventilated capsule method; and the galvanic skin response (GSR). Fluctuations of sweating and GSR were mainly observed on the dorsal side of the hand during night sleep. The frequency of GSR and sweat rate on the dorsal side of the hand were significantly lower during REM sleep than during NREM sleep. The frequency of spontaneous skin vasoconstriction was higher and blood flow was lower during REM sleep than during NREM sleep. These results indicate that sweating and blood flow in the skin are differentially regulated depending on the sleep stage. Our results also suggest that the sleep-regulating system is closely linked to thermoregulation, which is controlled by the sympathetic nervous system.

Ultradian oscillations in cranial thermoregulation and electroencephalographic slow-wave activity during sleep are abnormal in humans with annual winter depression

The level of core body, and presumably brain temperature during sleep varies with clinical state in patients with seasonal affective disorder (SAD), becoming elevated during winter depression and lowered during clinical remission induced by either light treatment or summer. During sleep, brain temperatures are in part determined by the level of brain cooling activity, which may be reflected by facial skin temperatures. In many animals, the level of brain cooling activity oscillates across the NREM-REM sleep cycle. Facial skin temperatures during sleep in patients with winter depression are abnormally low and uncorrelated with rectal temperatures, although their relationship to EEG-defined sleep stages remains unknown. We therefore measured the sleep EEG, core body and facial skin temperatures in 23 patients with winter depression and 23 healthy controls, and tested the hypothesis that ultradian oscillations in facial skin temperatures exist in humans and are abnormal in patients with winter depression. We found that facial skin temperatures oscillated significantly across the NREM-REM sleep cycle, and were again significantly lower and uncorrelated with rectal temperatures in patients with winter depression. Mean slow-wave activity and NREM episode duration were significantly greater in patients with winter depression, whereas the intraepisodic dynamics of slow-wave activity were normal in patients with winter depression. These results suggest that brain cooling activity oscillates in an ultradian manner during sleep in humans and is reduced during winter depression, and provide additional support for the hypothesis that brain temperatures are elevated during winter depression.

More than a marker: interaction between the circadian regulation of temperature and sleep, age-related changes, and treatment possibilities

The neurobiological mechanisms of both sleep and circadian regulation have been unraveled partly in the last decades. A network of brain structures, rather than a single locus, is involved in arousal state regulation, whereas the suprachiasmatic nucleus (SCN) has been recognized as a key structure for the regulation of circadian rhythms. Although most models of sleep regulation include a circadian component, the actual mechanism by which the circadian timing system promotes--in addition to homeostatic pressure--transitions between sleep and wakefulness remains to be elucidated. Little more can be stated presently than a probable involvement of neuronal projections and neurohumoral factors originating in the SCN. This paper reviews the relation among body temperature, arousal state, and the circadian timing system and proposes that the circadian temperature rhythm provides an additional signaling pathway for the circadian modulation of sleep and wakefulness. A review of the literature shows that increased brain temperature is associated with a type of neuronal activation typical of sleep in some structures (hypothalamus, basal forebrain), but typical of wakefulness in others (midbrain reticular formation, thalamus). Not only local temperature, but also skin temperature are related to the activation type in these structures. Warming of the skin is associated with an activation type typical of sleep in the midbrain reticular formation, hypothalamus, and cerebral cortex (CC). The decreasing part of the circadian rhythm in core temperature is mainly determined by heat loss from the skin of the extremities, which is associated with strongly increased skin temperature. As such, alterations in core and skin temperature over the day could modulate the neuronal activation state or "preparedness for sleep" in arousal-related brain structures. Body temperature may thus provide a third signaling pathway, in addition to synaptic and neurohumoral pathways, for the circadian modulation of sleep. A proposed model for the effects of body temperature on sleep appears to fit the available data better than previous hypotheses on the relation between temperature and sleep. Moreover, when the effects of age-related thermoregulatory alterations are introduced into the model, it provides an adequate description of age-related changes in sleep, including shallow sleep and awakening closer to the nocturnal core temperature minimum. Finally, the model indicates that appropriately timed direct (passive heating) or indirect (bright light, melatonin, physical activity) manipulation of the nocturnal profile of skin and core temperature may be beneficial to disturbed sleep in the elderly. Although such procedures could be viewed by researchers as merely masking a marker for the endogenous rhythm, they may in fact be crucial for sleep improvement in elderly subjects.

Nocturnal sweating and temperature in depression

OBJECTIVE

Patients with depression may have altered thermoregulation, such as high nocturnal core temperatures, decreased daytime sweating and subjective complaints of nocturnal sweating. We sought to compare nocturnal sweating in depressed patients and non-depressed controls, and to assess the impact of REM sleep on sweat rates.

METHOD

Nocturnal sweat rate, nocturnal temperature and REM sleep were measured during the night in 9 controls and 8 depressed subjects; 7 depressed patients were assessed during recovery.

RESULTS

The nocturnal temperature was significantly higher in depressed patients compared to controls, and decreased significantly with recovery. The nocturnal sweat rates of depressed patients did not differ significantly from those of controls, but decreased significantly with recovery. Analyses of sweat rates before, during and after REM sleep indicated a trend for the entire sample to show a decrease in sweat rates during REM.

CONCLUSION

The nocturnal sweating rates in the depressed patients suggest that impaired sweating is not the cause of the high nocturnal temperature commonly found in depressed patients.

A short-term poikilothermic period occurs just after paradoxical sleep onset in humans: characterization changes in sweating effector activity

We examined the changes in sudorific effector activity in five healthy young (21-23 y) subjects just before, during and just after successive paradoxical sleep (PS) phases. Local sweat rates were evaluated minute by minute over the chest (mcs). Previous observations, showing that mcs levels dropped before paradoxical sleep onset was electrophysiologically scored, were confirmed. At the end of this period of mcs depression, which in the present study coincided with paradoxical sleep onset, we show for the first time a short period (3-7 min) (period I) during which sweat production completely disappeared. A second period then followed (period II), at the very beginning of which mcs was re-elicited and thereafter increased in close correlation with paradoxical sleep duration. During period II, the remaining inhibiting influences (maximal during period I) and their releases could be specified by the successive valleys (indicating mcs inhibition) and peaks (indicating release of the mcs inhibition) drawn by the minute by minute mcs changes. These inhibitions became weaker as paradoxical sleep advanced. Given the strategic position of period I (at paradoxical sleep onset) and the total mcs abolition therein observed, it may be assumed that this poikilothermic state is the re-emergence of the 'ancestral' mode of body temperature regulation. From a thermophysiological point of view, period II may be considered as more 'modern' and directly related to the extension of paradoxical sleep in humans. This extension could be underlain by the unique development of our cognitive and/or learning functions.

Dew-point hygrometry system for measurement of evaporative water loss in infants

Evaporation of water from the skin is an important mechanism in thermal homeostasis. Resistance hygrometry, in which the water vapor pressure gradient above the skin surface is calculated, has been the measurement method of choice in the majority of pediatric investigations. However, resistance hygrometry is influenced by changes in ambient conditions such as relative humidity, surface temperature, and convection currents. We have developed a ventilated capsule method that minimized these potential sources of measurement error and that allowed second-by-second, long-term, continuous measurements of evaporative water loss in sleeping infants. Air with a controlled reference humidity (dew-point temperature = 0 degree C) is delivered to a small, lightweight skin capsule and mixed with the vapor on the surface of the skin. The dew point of the resulting mixture is measured by using a chilled mirror dew-point hygrometer. The system indicates leaks, is mobile, and is accurate within 2%, as determined by gravimetric calibration. Examples from a recording of a 13-wk-old full-term infant obtained by using the system give evaporative water loss rates of approximately 0.02 mgH2O.cm-2.min-1 for normothermic baseline conditions and values up to 0.4 mgH2O.cm-2. min-1 when the subject was being warmed. The system is effective for clinical investigations that require dynamic measurements of water loss.

Skin sympathetic nerve activity and effector function during sleep in humans

Multi-unit sympathetic skin nerve activity (SSA) in the peroneal nerve was recorded together with electrical skin resistance, skin blood flow and (in some subjects) finger blood pressure during sleep in 22 sleep-deprived healthy subjects. The average strength of sympathetic activity in different sleep stages was measured during 5-min periods as the area-under-curve of the integrated neurogram. Stage 2 sleep was reached by 15 subjects, stages 3-4 by nine and rapid eye movement (REM) sleep by six subjects. Non-REM sleep was always associated with an increased skin resistance, which was larger in glabrous than in hairy skin (293 +/- 48 vs. 175 +/- 4% of awake control level, n = 10, P < 0.05). Skin blood flow also increased during sleep, with a mean maximal increase of 397 +/- 79% of the awake control level (n = 11, P < 0.05). In spite of these changes of effector function no significant difference in mean SSA was found between the awake control period and periods of non-REM sleep, but during REM sleep SSA increased with 34% (P < 0.05) compared with the immediately preceding stage 2 period. In stage 2 sleep, K-complexes were associated with bursts of SSA followed by transient changes of skin resistance, blood flow and arterial blood pressure. When both skin resistance and blood flow were recorded within the innervation area of the impaled fascicle, single bursts or short periods of increased SSA could be succeeded by increased skin blood flow without concomitant skin resistance change. This indicates the existence of specific sympathetic vasodilator fibres in the skin. Therefore the unchanged strength of multiunit SSA during non-REM sleep in the face of increases of skin resistance and blood flow may be a consequence of an increased sympathetic vasodilator nerve activity combined with decreases of vasoconstrictor and sudomotor traffic.

The effects of two different kinds of quilt on human core temperature during night sleep

Effects of two kinds of quilt with different thermal insulation properties between the upper and lower halves on human core temperature during night sleep were compared at an ambient temperature of 16 degrees C and a relative humidity of 50% in five healthy adult women. One quilt has a thick part (110 mm) in the upper half and a thin part (63 mm) in the lowest half (Quilt A), and the other has a thin part (63 mm) in the upper half and a thick part (110 mm) in the lower half (Quilt B). Subjects, wearing shirts with half-sleeves and breeches, slept on a bed with sleeping mat, being fully covered by either Quilt A or Quilt B from 22:00 to 06:00. The major finding was that rectal temperature fell more quickly in Quilt B after retiring at 22:00, being kept at a lower level during one third of the whole night. We suggest that the reduced level of rectal temperature in Quilt B might be ascribed to lower thermal insulation in the upper half side of the Quilt B and partly to different core-peripheral blood redistribution in the lower extremities between the two kinds of quilt. Rapid fall and lowered level rectal temperature in Quilt B might be of significance for ease in sleep onset and sleep depth.

Sleep-related changes in human muscle and skin sympathetic nerve activities

To characterize the features of sympathetic nerve activity during non-REM sleep, we measured the muscle and skin sympathetic nerve activities (MSNA and SSNA, respectively) using a double recording microneurographic technique. Eight healthy volunteers were monitored by polysomnography (including EEG, EOG, EMG, and ECG) and received acoustic stimuli (880 Hz, 125 ms) during sleep stage 2. The specific discharge properties of MSNA during wakefulness included pulse-synchronicity, short burst duration and a non-responsiveness to arousal stimuli. These were considered to be generated by an inhibitory input from the baroreceptors to the cardiovascular center. In contrast, SSNA lacked pulse-synchronicity, had longer bursts, and was responsive to a variety of stimuli. Burst rates of MSNA and SSNA were reduced during sleep stages 1 and 2 (light sleep) vs. during a wakefulness. Both MSNA and SSNA appeared to be related to spontaneously occurring K-complexes. The baroreflex latency (from the ECG R-wave to the integrated MSNA burst peak) was constant at approximately 1.20 s during sleep, suggesting that pulse-synchronicity was maintained. The MSNA burst evolution time (interval between initiation of the burst and its peak) became longer with a transition to deeper non-REM sleep stages, whereas the SSNA burst evolution times remained constant. K-complexes induced by acoustic stimuli were frequently accompanied by MSNA and/or SSNA. MSNA responded to acoustic stimuli during light sleep, but some bursts lacked a close relation in time to cardiac rhythm.(ABSTRACT TRUNCATED AT 250 WORDS)

Local sweating responses during recovery sleep after sleep deprivation in humans

Changes in the central control of sweating were investigated in five sleep-deprived subjects (kept awake for 40 h) during their recovery sleep under warm ambient conditions [operative temperature (T(o)) was either 35 or 38 degrees C]. Oesophageal (T(oes)) and mean skin (Tsk) temperatures, chest sweat rate (msw,ch), and concomitant electro-encephalographic data were recorded. Throughout the night at 35 or 38 degrees C T(o), msw,ch changes were measured at a constant local chest skin temperature (Tch) of 35.5 degrees C. The results showed that body temperatures (T(oes) and Tsk) of sleep-deprived subjects were influenced by thermal and hypnogogic conditions. The msw,ch levels correlated positively with T(oes) in the subjects studied during sleep stage 1-2 (light sleep: LS), sleep stage 3-4 (slow wave sleep: SWS) and rapid eye movement (REM) sleep. Contrary to what has been reported in normal sleep, firstly, the T(oes) threshold for sweating onset differed between REM sleep and both LS and SWS, and, secondly, the slopes of the msw,ch versus T(oes) relationships were unchanged between REM and non-REM (i.e. LS or SWS) sleep. The changes observed after sleep deprivation were hypothesized to be due to alterations in the functioning of the central nervous system controller.

Thermal exchanges during sleep in anhidrotic ectodermal dysplasia

Anhidrotic ectodermal dysplasia is a congenital syndrome characterized by the absence of sweat glands. A sweating test was performed on such a patient and proved his inability to sweat. Thermal exchanges during night sleep were then measured in this patient and compared with data obtained from a healthy control subject. Ambient conditions were as follows: dry bulb temperature 32.2 degrees C, relative humidity 30%-40%, wind speed 0.7 m.s-1. Polysomnographic recordings showed normal sleep patterns in both subjects, but a "first night effect" in the patient. Rectal (Tre) and mean skin (Tsk) temperatures and loss of mass were monitored continuously throughout the 8-h sleep recording. Loss of mass averaged 34.1 g.h-1 in the patient vs 78.1 g.h-1 in the control subject. No relationship with sleep stages was observed in the patient, in contrast to the control subject who experienced a decrease in evaporation during rapid eye movement sleep. Body temperatures varied little in the patient, but decreased until the 6th h of sleep in the control subject. On two occasions there was a 0.3 degrees C fall in the Tre of the patient during two slow wave sleep (SWS) phases, while Tsk and loss of mass did not change. As thermolytic processes had not varied on these two occasions, it was concluded that the fall in Tre indicated a concomitant decrease in metabolic heat production, in agreement with the assumption that SWS represented a state of energy conservation.

Body temperature and sleep architecture in response to a mild cold stress in women

Six women participated in a seven consecutive night polygraphic sleep study during which both 24-hour rectal, body temperature and wrist activity were continuously sampled and stored at one-minute intervals. The study was designed to investigate the effects of a mild nocturnal cold stress on thermoregulation and sleep. On nights 4 and 5, subjects slept naked, without any bedcovers (mild cold stress) in a warm (26.7-28.3 degrees C) room. The daily mean rectal temperature and the daily nadir (low point) of the circadian temperature rhythm (CTR) showed a significant decrease between the baseline and cold stress conditions. The lowered nadir resulted in a significant amplitude increase in the daily CTR between the baseline and cold stress conditions. There were no significant changes in activity levels across experimental conditions. The ability to maintain a sleep state was significantly impaired during the cold stress. Stage 4 slow wave sleep increased, while Stage 3 decreased in response to the cold stress condition, and there was an associated lengthening of the first NREM period. These data suggest that challenges to the thermoregulatory system can be used as a vehicle to systematically alter sleep architecture in humans.