Six male subjects slept nude except for shorts on a bed made from nylon webbing at 5 different ambient temperatures (TaS): 21, 24, 29 (thermoneutrality), 34 and 37 degrees C. Standard electrophysiological recordings were obtained and analyzed for sleep stages. Temperature displayed a significant quadratic trends for nearly every sleep variable, such that TaS above or below thermoneutrality had similar effects on sleep patterns. Multiple comparisons showed that 21 degrees C was the most disruptive condition, and that cold TaS were generally more disruptive to sleep than warm TaS. There were marked individual differences in sensitivity of sleep to cold. Decreases in REM sleep in humans produced by heat or cold probably result from a general disruption of sleep processes rather than being specifically related to the status of the thermoregulatory system during REM sleep.