Effects of humid heat exposure in later sleep segments on sleep stages and body temperature in humans

This study sought to investigate the effects of humid heat exposure in later sleep segments on sleep stages and body temperature in humans. The subjects were eight healthy males, from whom informed consent had been obtained. The experiments were carried out under three different sets of conditions: a control climate [air temperature (Ta)=26°C, relative humidity (RH)=50%] (C); a humid heat climate (Ta=32°C, RH=80%) (H); and a humid heat exposure in later sleep segments (C for the first 3 h 45 min, followed by a 30-min transition to H, which was then maintained for the last 3 h 45 min) (C–H). Electroencephalogram, EOG, and mental electromyogram, rectal temperature (Tre), and skin temperature (Tsk) were continuously measured. The total amount of wakefulness was significantly increased in H compared to C–H or C. Compared to C, wakefulness in C–H and H was significantly increased during later sleep segments. Tre and mean Tsk were significantly higher in H than in C–H or C. In C–H, Tsk and Tre increased to levels equal to those observed in H after Ta and RH increase. Whole body sweat loss was significantly lower in C–H and C than in H. These results suggest that humid heat exposure in the later sleep segment reduces thermal load as compared to full-night humid heat exposure. In daily life, the use of air conditioning in the initial sleep hours can protect sleep and thermoregulation.     

Effects of airflow on body temperatures and sleep stages in a warm humid climate

Airflow is an effective way to increase heat loss—an ongoing process during sleep and wakefulness in daily life. However, it is unclear whether airflow stimulates cutaneous sensation and disturbs sleep or reduces the heat load and facilitates sleep. In this study, 17 male subjects wearing short pyjamas slept on a bed with a cotton blanket under two of the following conditions: (1) air temperature (Ta) 26°C, relative humidity (RH) 50%, and air velocity (V) 0.2 m s⁻¹; (2) Ta 32°C, RH 80%, V 1.7 m s⁻¹; (3) Ta 32°C; RH 80%, V 0.2 m s⁻¹ (hereafter referred to as 26/50, 32/80 with airflow, and 32/80 with still air, respectively). Electroencephalograms, electrooculograms, and mental electromyograms were obtained for all subjects. Rectal (Tre) and skin (Ts) temperatures were recorded continuously during the sleep session, and body-mass was measured before and after the sleep session. No significant differences were observed in the duration of sleep stages between subjects under the 26/50 and 32/80 with airflow conditions; however, the total duration of wakefulness decreased significantly in subjects under the 32/80 with airflow condition compared to that in subjects under the 32/80 with still air condition (P < 0.05). Tre, Tsk, Ts, and body-mass loss under the 32/80 with airflow condition were significantly higher compared to those under the 26/50 condition, and significantly lower than those under the 32/80 with still air condition (P < 0.05). An alleviated heat load due to increased airflow was considered to exist between the 32/80 with still air and the 26/50 conditions. Airflow reduces the duration of wakefulness by decreasing Tre, Tsk, Ts, and body-mass loss in a warm humid condition.     

Ambient temperature-dependent thermoregulatory role of REM sleep

Changes in ambient temperature produce complex effects on sleep–wakefulness. In order to find out the mechanisms involved in temperature-sensitive changes in sleep in rats, their thermal preference, body temperature and sleep were studied before and after the destruction of both peripheral and central warm receptors, by systemic administration of 375 mg/kg capsaicin. Though the pre-treated rats preferred to stay mostly at the ambient temperature of 27 °C, post-treated rats strayed freely into chambers having ambient temperature of 30 °C and 33 °C. Sleep and body temperature of these rats were studied for six hours each, when they were kept at an ambient temperature of 18–36 °C. Total sleep time, especially REM sleep, was maximum at 30 °C in pre-treated rats, but this REM sleep peak at 30 °C disappeared after capsaicin administration. Body temperature increased sharply in post-treated rats, at ambient temperatures above 30 °C. Apart from the ability to defend body temperature at high ambient temperature, avoidance of warm ambient temperature and increase in REM sleep are the behavioral measures which are lost in post-treated rats. Results of this study suggest that the ambient temperature-related increase in REM sleep at 30 °C could be part of the thermoregulatory measures.     

Effects of head cooling on human sleep stages and body temperature

The purpose of this study was to confirm the effect of head cooling on human sleep stages and body temperature. Nine healthy male volunteers with a mean age of 25 +/- 3.77 years served as subjects. The experiments were carried out under three different sets of conditions: 26 degrees C, relative humidity (RH) 50% (26/50); 32 degrees C, RH 80% (32/80); and 32 degrees C RH 80% with the use of a cooling pillow (32/80 HC). The subjects slept from 2300 hours to 0700 hours with a cotton blanket, wearing short-sleeved pyjamas and shorts on a bed, which was covered with a sheet. Electroencephalograms, electro-ouclogram, and mental electromyelograms were recorded through the night. Rectal temperature (Tre) and skin temperature (Tsk) were measured continuously. Whole-body sweat and the tympanic temperature (Tty) were measured before and after sleep. Wakefulness significantly increased at 32/80 than at 26/50; however, no significant difference was observed between 32/80 HC and 26/50. Tre and mean Tsk were higher both at 32/80 and 32/80 HC than at 26/50. The whole-body sweat loss was significantly greater and Tty in the morning was higher at 32/80 than 32/80 HC and 26/50. These results suggest that head cooling during sleep may help to decrease the whole-body sweat rate during sleep under humid heat conditions.     

Thermoregulation, metabolism, and stages of sleep in cold-exposed men

Four naked men, selected for their ability to sleep in the cold, were exposed to an ambient temperature (Ta) of 21 degrees C for five consecutive nights. Electrophysiological stages of sleep, O2 consumption (VO2), and skin (Tsk), rectal (Tre), and tympanic (Tty) temperatures were recorded. Compared with five nights at a thermoneutral Ta of 29 degrees C, cold induced increased wakefulness and decreased stage 2 sleep, without significantly affecting other stages. Tre and Tty declined during each condition. The decrease in Tre was greater at 21 degrees C than at 29 degrees C, whereas Tty did not differ significantly between conditions. Increases in Tty following REM sleep onset at 21 degrees C were negatively correlated with absolute Tty. VO2 and forehead Tsk also increased during REM sleep at both TaS, whereas Tsk of the limb extremities declined at 21 degrees C. Unsuppressed REM sleep in association with peripheral vasoconstriction and increased Tty and VO2 in cold-exposed humans, do not signify an inhibition of thermoregulation during this sleep stage as has been observed in other mammals.     

EEG patterns and body temperatures in man during sleep in arctic winter nights

Two Caucasian males, aged 19 and 22, slept at night in sleeping bags (9.0 clo) in an unheated tent at ambient temperatures between –25 and –35°C in the Arctic. Electroencephalographic (EEG) sleep studies were conducted for two baseline nights (19–21°C), 10 cold exposure nights and 2 recovery nights (19–21°C). Rectal and skin temperatures, and heart rates were also recorded. The subjects suffered disturbances in sleep patterns involving an insomnia composed of an increased wakefulness, a decrease in slow wave sleep and a deprivation in rapid eye movement (REM) sleep. Dissimilarities appeared between the subjects which may be related to differences in thermoregulatory responses.     

Physiologic responses of dairy calves to environmental heat stress

The effects of thermal stress were identified in terms of a calf's systemic response. The following physiological parameters were monitored during successive exposure of eight Holstein calves to five temperature levels ranging from 15.5°C to 37.7°C at 60% RH: stroke volume, heart rate, arterial systolic and diastolic pressures, plasma cortisol and thyroxine levels, and internal and skin temperatures. Results indicated that 3 to 4- week-old male calves respond to acute heat stress only above 32.2°C at 60% RH and do not demonstrate a marked attempt to acclimate until at least four to five hours of exposure at 37.7°C.     

Responses of cattle to the combined exposure to diurnal temperature rhythm (−5 to 25°C) and to simulated high-altitude (4,000 m)

Eight 1/2-year old calves were exposed in a climatized altitude chamber to the following four conditions: 400 and 4,000 m at constant T_a (17°C), 400 and 4,000 m at alternating T_a (–5° to 25°C). Each exposure lasted for 24 h and for the rhythmic conditions included a cold night and warm midday hours, supplemented by infrared heaters. During exposure, hourly measurements were made of heart rate, respiratory rate, rectal and three skin temperatures. Every 3-h blood samples were collected for the determination of 10 blood variables. The following main results were obtained: (a) Altitude alone caused increases in respiratory rate, heart rate, erythrocyte number, haemoglobin, specific gravity of blood and plasma, LDH and all four body temperatures. (b) In the rhythmic exposures, high correlation coefficients were found between ambient temperature on the one hand and skin temperatures (0.88 to 0.94), rectal temperature (–0.43) and respiratory rate (0.49) on the other hand. A change in ambient temperature by 1°C lead, on average, to a change in ear temperature by 1.2°C. (c) in response to falling ambient temperature during the night, rectal temperature and heart rate increased. This was interpreted as indicating a compensatory elevation in meta bolic heat production. At the same time, there was haemoconcentration as shown by elevations in erythrocyte number, haematocrit and haemoglobin. This haemoconcentration might have reflected splenic discharge, possibly supplemented by some loss of water from the plasma. (d) The warm environmental conditions around midday produced mild heat responses in terms of elevated values for respiratory rate, heart rate and body temperatures. (e) It is concluded that the rhythmic temperature with alternating stress of cold and mild heat, especially in combination with high altitude, was a strain on the animals and that they were forced to expend extra energy for combatting altitude- and temperature stress, energy which no longer would be available for productive processes.Presented at the Eight International Congress of Biometeorology, 9–14 September 1979, Shefayim, Israel.     

Comparison of two cool vests on heat-strain reduction while wearing a firefighting ensemble

This study evaluated the effectiveness of a six-pack versus a four-pack cool vest in reducing heat strain in men dressed in firefighting ensemble, while resting and exercising in a warm/humid environment [34.4°C (day bulb), 28.9°C (wet bulb)]. Male volunteers (n = 12) were monitored for rectal temperature (T _re), mean skin temperature (T _sk), heart rate, and energy expenditure during three test trials: control (no cool vest), four-pack vest, and six-pack vest. The cool vests were worn under the firefighting ensemble and over Navy dungarees. The protocol consisted of two cycles of 30 min seated rest and 30 min walking on a motorized treadmill (1.12 m · s^–1, 0% grade). Tolerance time for the control trial (93 min) was significantly less than both vest trials (120 min). Throughout heat exposure, energy expenditure varied during rest and exercise, but no differences existed among all trials (P > 0.05). During the first 60 min of heat exposure, physiological responses were similar for the four-pack and six-pack vests. However, during the second 60 min of heat exposure the six-pack vest had a greater impact on reducing heat strain than the four-pack vest. PeakT _e andT _sk at the end of heat exposure for 6-pack vest [mean (SD) 38.0(0.3)°C and 36.8(0.7)°C] were significantly lower compared to four-pack [38.6 (0.4)°C and 38.1(0.5)°C] and controls [38.9(0.5)°C and 38.4(0.5)°C]. Our findings suggest that the six-pack vest is more effective than the four-pack vest at reducing heat strain and improves performance of personnel wearing a firefighting ensemble.     

Shivering modulation in humans: Effects of rapid changes in environmental temperature

During cold exposure, increase in heat production is produced via the activation of shivering thermogenesis and nonshivering thermogenesis, the former being the main contributor to compensatory heat production in non-acclimatized humans. In rats, it has been demonstrated that shivering thermogenesis is modulated solely by skin thermoreceptors but this modulation has yet to be investigated in humans. The aim of this study was to determine if cold-induced shivering in humans can be modulated by cutaneous thermoreceptors in conditions where increases in heat loss can be adequately compensated by increases in thermogenic rate. Using a liquid-conditioned suit, six non-acclimatized men were exposed to cold (6 °C) for four 30 min periods, each of them separated by 15 min of heat exposure (33 °C). Core temperature remained stable throughout exposures whereas skin temperatures significantly decreased by 12% in average during the sequential cold/heat exposures compared to baseline (p<0.0001). Shivering intensity and metabolic rate increased significantly during 6 °C exposures (3.3±0.7% MVC, 0.40±0.0 L O₂/min, respectively) and were significantly reduced during 33 °C exposure (0.5±0.1% MVC, 0.25±0.0 L O₂/min; p<0.005 for both). Most importantly, shivering could be quickly and strongly inhibited during 33 °C exposure although skin temperature often remained below baseline values. In conclusion, under compensatory conditions, cutaneous thermoreceptors appear to be a major modulator of the shivering response in humans and seem to react rapidly to changes in the microclimate right next to the skin and to skin temperature.     

Dynamic Mapping of the Temperature Fields of the Brain

The thermal fields and biopotentials of the brain were studied in 11 healthy subjects in the states of quiet wakefulness and sleep (stages I–IV). To this end, a new method of dynamic radiomapping was applied in parallel with the traditional method of EEG recording. The method of dynamic radiomapping is based on measuring the brain thermal radiation in the decimeter (40 cm) wave range. It allows the integral brain temperature to be recorded from deep inside and up to 2.5 cm from the surface with the help of 12 antennas applied to the skin. The temperature of the cerebral cortex of the human subject in the state of quiet wakefulness varied stochastically in the range of deviations of ±0.3°C in all areas. Changes in the brain functional state, i.e., the transition from wakefulness to sleep, were accompanied by either an increase in the variation range to ±0.5°C or the appearance of stationary foci of heating (by 0.9–1.3°C) or cooling (by –0.7°C) of individual locations and amplitudes.     

Circadian and seasonal variation of the body temperature of sheep in a tropical environment

Nychthemeral and annual rhythms of the rectal temperature were determined for Corriedale sheep in a tropical climate. The minimum rectal temperature averaged 39.55° C at 0500 hours in summer, and 38.87° C at 0600 hours in winter. The maximum was 40.03° C in summer (1700 hours) and 39.33° C in winter (1830 hours). Annual cycle of the rectal temperature showed a minimum in July and maximum in December.     

Circadian Rhythms of Core Body Temperature and Urinary Noradrenaline Secretion under the Influence of Skin Pressure due to Foundation Garments Worn during Wakefulness

The present experiment investigated the effects of skin pressure by foundation garments (brassiere plus girdle) worn during wakefulness on the circadian rhythms of core temperature and endocrine secretion. Eight healthy females (18–23 yrs) maintaining regular sleep-wake cycles for a week prior to participation in the experiment served as participants. The experiments were performed from June to August, 1999, using a bioclimatic chamber controlled at 26.5 ± 0.2°C and 62 ± 3% RH. Ambient light intensity was controlled at 500 lx from 07:30 h to 17:30 h, 100 lx from 17:30 h to 19:30 h, 20 lx from 19:30 h to 23:30 h and there was total darkness from 23:30 h to 07:30 h. The experiment lasted for 58?h over 3 nights. The participant rose at 07:30?h in the morning of the first day and retired at 23:30 h, adhering to a set schedule for 24 h but without wearing foundation garments. From 07:30 h to 23:30 h of the second day the participant wore foundation garments but did not wear foundation garments during the sleep. Rectal and leg skin temperatures were continuously measured throughout the experiment. Urine was collected every 4 h for the analysis of catecholamines. Skin pressure applied by the foundation garments was in the range 11–17 gf/cm² at the regions of abdomen, hip, chest and back. The main results were as follows: Rectal temperature was significantly higher (p < 0.01) during wakefulness, but significantly lower (p < 0.01) during sleep with than without foundation garments. Furthermore, the amplitude of rectal temperature was larger with than without foundation garments (p < 0.033). Urinary noradrenaline was mostly lower with foundation garments throughout the day and night. The results suggest that skin pressure by foundation garments worn during wakefulness could influence the level of core body temperature and noradrenaline secretion not only during wakefulness, but also during sleep.     

Assessment of spinal temperature sensitivity in conscious goats by feedback signals

Summary In two conscious goats with chronically implanted spinal thermodes, fifty-six experiments were carried out at two environmental conditions of + 5 °C DB and 30 °C DB. The temperature of the spinal cord was altered by perfusing the thermodes with water whose temperature, as measured at the inlet of the thermodes, varied between 30 °C and 43 °C. Heat production, respiratory evaporative heat loss, rectal and oesophageal temperatures were measured. At the lower air temperature, spinal cord cooling resulted in an elevation of rectal temperature, while spinal cord heating caused a fall in rectal temperature. At the higher air temperature, spinal cord cooling did not result in an increase of rectal temperature. As in the lower air temperature, spinal cord heating caused a fall hi rectal temperature. The experiments suggest that the generation of spinal warm signals is independent of air temperature between +5 °C and 30 °C, while spinal cold signals are not generated in the absence of skin cold signals.     

Relationship between skin blood flow and sweating rate, and age related regional differences

To examine the mechanisms and regional differences in the age-related decrement of skin blood flow, 11 young (age 20-25 years) and 10 older (age 64-76 years) men were exposed to a mild heat stress by immersing their feet and lower legs in water at 42 degrees C for 60 min, while they were sitting in near thermoneutral conditions [25 degrees C and 45% relative humidity (rh)]. During the equilibrium period (25 degrees C and 45% rh) before the heat test, no group differences were observed in rectal (Tre) and mean skin (Tsk) temperatures or mean arterial pressure (MAP). During passive heating, Tsk was significantly lower in the older men 20 min after commencing exposure (P<0.001), although there were similar increases in Tre in both groups. Exposure time and age did not affect MAP. The local sweating rate (m(sw)) and the percentage change in skin blood flow by laser Doppler flowmetry (%LDF) relative to baseline values on the chest, back, forearm and thigh were significantly lower in the older men (P<0.001), especially on the thigh. After starting the heat exposure, three temporal phases were observed in the relationship between %LDF and m(sw) at most sites in each subject. In phase A, %LDF increased but with no increase in m(sw). In phase B, m(sw) increased but with no secondary increase in %LDF. Finally, in phase C, there were proportional increases in %LDF and m(sw). The increase in %LDF in phase A was significantly lower on the forearm and thigh (P<0.05) for the older men, but not on the chest and back. In phase C, the slopes of the regression lines between %LDF and m(sw) were lower for the older men on the back (P<0.03), forearm (P = 0.08) and thigh (P<0.03), but not on the chest. These results would suggest that the age-related decrement in skin blood flow in response to passive heating may be due in part to a smaller release of vasoconstrictor tone and to less active vasodilatation once sweating begins. Regional differences exist in the impaired vasoconstriction and active vasodilatation systems.     

Effects of hyperthermia and hypothermia on spontaneous contractions of the adult rabbit testicular capsule

Spontaneous contractions of the isolated testicular capsule of the adult rabbit have been found to be markedly sensitive to heat and cold stress. Testicular capsular contractions may provide a propulsive pumping action for transporting nonmotile sperm out of the testis and into the epididymis where they can then attain motility. An optimal temperature for the amplitude of spontaneous contractions of the rabbit testicular capsule occurred at 32 – 34°C. An increase in thein vitro organ bath temperature from 37 to 40°C caused a marked decrease in the amplitude of spontaneous contractions. A complete and irreversible cessation of spontaneous contractions occurred at 48°C for at least 30 min after cooling to 37°C. A decrease in temperature from 37 to 26°C resulted in a marked decrease in frequency and amplitude progressing to a complete but reversible cessation of spontaneous contractions at 16°C. Marked changes in the frequency and amplitude of spontaneous contractions of the isolated testicular capsule began to be observed when the tissue was exposed to organ bath temperatures of 3°C above and below the normal intra-testicular temperature. These data suggest that exposure of men to fever or excessively hot baths as well as swimming in excessively cold water or extreme cold weather exposure may have inhibitory effects on testicular capsular spontaneous contractions which may interfere with sperm transport.     

The effects of two types of clothing on seasonal cold tolerance

This study investigated the effects of two types of clothing, leaving legs covered or uncovered, on seasonal cold tolerance in women. Experiments were carried out to compare cold tolerance at an ambient temperature (T _a) of 10° C in December between two groups of subjects, who wore either knee-length skirts (skirt group) or full-length trousers (trouser group) for 3 months from September to November. The main results are summarized as follows: rectal temperatures continued to fall for 40 min in the trouser group when the subjects were covered by a blanket, while it became stable in 30 min in the skirt group; rectal temperatures showed greater increases in the skirt group when the blanket was removed after 40 min exposure to T _a of 10° C; metabolic heat production was kept significantly lower in the skirt group when uncovered or covered by a blanket at T _a of 10° C; metabolic heat production was negatively correlated with mean skin temperature and was always higher in the trouser group when measured at the same mean skin temperature; in the uncovered condition diastolic blood pressure increased significantly in the trouser group but not in the skirt group. These results would suggest that the subjects who wore skirts for 3 months from September to November had improved their ability to tolerate the cold.     

The influence of prior wakefulness on REM sleep

Data from studies of naps and of shifted sleep were used to determine the relationship between two measures of rapid eye movement (REM) sleep (percentage of REM in the first 2 hr of sleep and REM latency) and prior wakefulness. For each sample, we calculated the difference between the observed value and that predicted by a cosine function that estimated the circadian rhythm of REM sleep propensity. The difference values were found to correlate reliably with hours and log hours of prior wakefulness. We conclude that while REM sleep is regulated in part by an endogenous circadian oscillator, it is also influenced by the duration of prior wakefulness.     

Résistance des abeilles (Apis mellifica L. var.ligustica) a des températures élevées

Summary The following is a study of resistance of worker honey bees (Apis mellifica L. var.ligustica) to high temperatures and of the effect of nutrition upon it.Survival of honey bees of spring generation was studied at 45, 50, 55 and 60°C during 15, 30, 45 and 60 minutes exposures. The survival rate was established at the end of their exposure, and 24 hours later. Lethal effects of heat were evident immediately after a 30 minute exposure to 50° C. These effects could be noticed in the survivers 24 hours following exposure.Effect of nutrition on heat resistance at 32° C and at 50° C was studied on one group of bees supplied with a 30 % honey solution; on another- with water only, and on a third group which served as a control (no food). Their survival at 32° C after 12 hours exposure was 100%, 81% and 48%, respectively. However, an exposure duration of 45 minutes at 50°C resulted in a survival of 22% of control bees as compared to a total survival of those fed on honey solution.     

Exercise in a cold environment after sleep deprivation

Seven subjects exercised to thermal comfort in a cold environment (O degrees C, 2.5 m X s-1) after normal sleep (control) and following a 50-h period of sleep deprivation. Resting core temperature (rectal) taken before the subject entered the cold environment was significantly lower (-0.5 degrees C, P less than 0.05) following the 50-h period of wakefulness. However, rectal temperature was not different after 15 min of exercise during the two exposures, suggesting that the subjects stored heat more rapidly during the first 15 min of exercise after sleep deprivation. No significant differences in self-chosen exercise intensity, significant differences in self-chosen exercise intensity, heart rate, metabolic rate, or exercise time were evident between the control and sleep deprived exposures. Fifty hours of sleep deprivation failed to alter the core temperature response during exercise in severe cold stress, and subjects chose identical work rates to minimize fatigue and cold sensation. The results suggest that the 50-h sleep deprivation period was not a true physiological stress during exercise in a cold environment. (Supported by Contract #DAMD 17-81-C1023.)