Influence of repeated passive body heating on subsequent night sleep in humans

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Abstract
Experiments were carried out on four healthy male subjects in two separate sessions: (a) A baseline period of two consecutive nights, one spent at thermoneutrality [operative temperature (T o)=30°C, dew-point temperature (T dp)=7°C, air velocity (V a)=0.2 m·s−1] and the other in hot condition (T o=35°C,T dp=7°C,V a=0.2 m·s−1). During the day, the subjects lived in their normal housing and were engaged in their usual activities. (b) An acclimation period of seven consecutive daily heat exposures from 1400 to 1700 hours (T o=44°C,T dp=29°C,V a=0.3 m·s−1). During each night, the subjects slept in thermoneutral or in hot conditions. The sleep measurements were: EEG from two sites, EOG from both eyes, EMG and EKG. Esophageal and ten skin temperatures were recorded continuously during the night. In the nocturnal hot conditions, a sweat collection capsule recorded the sweat gland activity in the different sleep stages. Results showed that passive body heating had no significant effect on the sleep structure of subsequent nights at thermoneutrality. In contrast, during nights atT o=35°C an effect of daily heat exposure was observed on sleep. During the 2nd night of the heat acclimation period, sleep was more restless and less efficient than during the baseline night. The rapid eye movement sleep duration was reduced, while the rate of transient activation phases observed in sleep stage 2 increased significantly. On the 7th night, stage 4 sleep increased (+68%) over values observed during the baseline night. The sweating adaptive mechanisms of heat acclimation persisted only in stage 4 sleep. The results indicated that body temperature rhythmicity was maintained in the heat by an increase in stage 4 sleep which reduced core temperature during the first part of the night.