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.