Six young men were exposed to a thermoneutral environment of air temperature (Ta) 20 degrees C for 5 days and nights followed by an acclimation period of 5 days and nights at Ta 35 degrees C and 2 recovery days and nights at Ta 20 degrees C. Electrophysiological measures of sleep, esophageal temperature, and mean skin temperature were continuously monitored. The total nocturnal body weight loss was measured by a sensitive platform scale. Compared with the 5 nights of the baseline period at 20 degrees C, sleep patterns showed disturbances at 35 degrees C. Total sleep time was significantly reduced, while the amount of wakefulness increased. The subjects exhibited fragmented sleep patterns. The mean duration of REM episodes was shorter at 35 degrees C than at 20 degrees C of Ta, while the REM cycle length shortened. In the acclimation period, there was no change in sleep pattern from night to night, despite adaptative adjustments of the thermoregulatory response. The protective mechanisms of deep body temperature occurring with heat adaptation did not interact with sleep processes. Upon return to baseline condition, a recovery effect was observed on a number of sleep parameters which were not significantly affected by the preceding exposure to prolonged heat. This would suggest that during exposure to dry heat, the demand for sleep could overcome that of other regulatory functions that are temperature-dependent. Therefore, a complete analysis of the effect of heat on sleep parameters can be assessed only if heat exposure is compared with both baseline and recovery periods.
The changes in the central control of sweating were investigated in five sleeping subjects under neutral and warm conditions [operative temperature (To) = 30, 33, and 34 degrees C; dew-point temperature = 10 degrees C]. Esophageal (Tes) and mean skin (Tsk) temperatures, chest sweat rate (msw,1), and concomitant electroencephalographic data were recorded. Throughout the night, msw,1 was measured under a local thermal clamp of 38 degrees C. Results showed that the thermal environment exerted a strong influence on both the levels and the time patterns of body temperatures. Moreover, local sweating rate correlated positively with Tes, and this relationship varied according to sleep stages. For a given Tes level, there was a sleep stage-related gradation in msw,1 that was higher in slow-wave sleep (SWS) than in stage 1-2 and the lowest in rapid-eye-movement (REM) sleep. This is explained by a change in the excitability or the sensitivity of the thermoregulatory system. The msw,1 differences between stage 1-2 and SWS are accounted for by a decrease in the Tes threshold (Tset) for sweating while the slope of the msw,1-Tes relation remains unchanged. The lower msw,1 in REM sleep is explained by a lesser slope for the msw,1-Tes relation without any Tset change from stage 1-2.
Five young unacclimatised subjects were exposed for 4 h at 34 degrees C (10 degrees C dew-point temperature and 0.6 m X s-1 air velocity), while exercising on a bicycle ergometer: 25 min work--5 min rest cycles for 2 hours followed by 20 min work--10 min rest cycles for two further hours. 5 experimental sessions were carried out: one without rehydration (NO FLUID) resulting in 3.1% mean loss of body weight (delta Mb), and four sessions with 20 degrees C fluid ingestion of spring water (WATER), hypotonic (HYPO), isotonic (ISO) and hypertonic (HYPER) solutions to study the effects of fluid osmolarity on rehydration. Mean final rehydration (+/- SE) after fluid intake was 82.2% (+/- 1.2). Heart rate was higher in NO FLUID while no difference among conditions was found in either delta Mb or hourly sweat rates. Sweating sensitivity was lowest in the dehydration condition, and highest in the WATER one. Modifications in plasma volume and osmolarity demonstrated that NO FLUID induced hyperosmotic hypovolemia, ISO rehydration rapidly led to plasma isoosmotic hypervolemia, while WATER led to slightly hypoosmotic normovolemia. It is concluded that adequate rehydration through ingestion of isotonic electrolyte-sucrose solution, although in quantities much smaller than evaporative heat loss, rapidly restored and expanded plasma volume. While osmolarity influenced sweating sensitivity, the plasma volume changes (delta PV) within the range -6% less than or equal to delta PV + 4% had little effect on temperature adjustments in our conditions.
Body temperature regulation was studied in 6 male subjects during an acclimation procedure involving uninterrupted heat exposure for 5 successive days and nights in a hot dry environment (ambient temperature = 35 degrees C, dew-point temperature = 7 degrees C; air velocity = 0.2 m.s-1). Data were obtained at rest and during exercise (relative mechanical workload = 35% VO2max). At rest, hourly measurements were made of oesophageal and 4 local skin temperatures, to allow the calculation of mean skin temperature, and of body motility and heart rate. During the working periods these measurements were made at 5 min intervals. Hourly whole-body weight loss was measured at rest on a sensitive platform scale while in the working condition just before starting and immediately after completing the bicycle exercise. The results show that, in both exercise and at rest, the successive heat exposures increased the sweat gland output during the first 3 days. Afterwards, sweat rate decreased without any corresponding change in body temperature. For the fixed workload, the sweat rate decline was associated with a decrease in circulatory strain. Adjustments in both sweating and circulatory mechanisms occur in the first 3 days of continuous heat exposure. The overall sweat rate decline could involve a redistribution of the regional sweating rates which enhances the sweat gland activities of skin areas with maximal evaporative efficiencies.
Effects of dehydration (3% of initial body weight) on temperature regulation were investigated in 5 men during intermittent exercise of 4 h duration at a dry air temperature of 34°C. Relative mechanical work load was 50% of the subject's steady state heart rate, which was 170 beats • min -1. During rehydration from the 70th min to the end of the exercise, the subjects drank, every 10 min in equal portions, an amount of water (20°C) totaling up to 80% of the body weight loss recorded during dehydration runs. Continuous measurements were made of rectal (Tre) and mean skin (TSk) temperatures and of whole body weight loss. Chest sweating rate (m,) was measured from a capsule located under a local thermal clamp (36°C). Blood samples were obtained during rest periods and after the 1st and the 4th hour of exercise. Compared to dehydration runs, water intake did not always cause an increase of msW while body temperatures always decreased. Dehydration resulted in a decrease in plasma volume and in increases of plasma osmolality, [Nat] and [K+]. Water intake induced a thermoregulatory response whose intensity largely differs from one body area to another. The change in the slope of the relation of msW to TTe features a decrease in the sensitivity of the thermoregulatory system with dehydration. The whole body water loss is significantly correlated with the change in plasma volume and body temperatures (Tre, Tsk)• This suggests that the reduced sweating response observed during dehydration can be related to plasma hypovolemia. Experimental evidence supports the finding that continuous exercise in the heat leading to 2-3% dehydration of body weight results in a sweat rate decline and a body temperature rise, when compared to that observed during the hydrated state
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