Effects of a nocturnal environment perceived as warm on subsequent daytime sleep in humans

We studied the influence of a nocturnal environment perceived as warm on the subsequent daytime sleep of healthy human subjects (20-25 years old). From 00:00 to 8:00, they were kept awake and exposed to either a thermoneutral and comfortable (CN) or a warm and uncomfortable (EW) environment, as assessed by the predicted mean vote/percentage of persons dissatisfied questionnaire (PMV/PPD). The subjects then slept from 8:00 to 14:00 in a thermoneutral environment. Sleep was scored according to the Rechtschaffen and Kales criteria. Rectal temperature was recorded from 22:00 to the end of the sleep period. Compared to CN, a significant but moderate hyperthermia (0.3-0.4 degrees C) occurred rapidly in EW, and was maintained throughout the night. This modest difference disappeared during subsequent sleep spent at thermal comfort. Exposure to a warm uncomfortable environment before bedtime significantly increased the duration (+37%) and percentage of rapid eye movement sleep (REMS). This hypnic response could be due to interactions occurring between thermoregulatory, circadian, and sleep mechanisms. It could also be ascribed to synergic actions of the neurophysiological (among others, involving the hypothalamo-pituitary-adrenal axis) and psychological (involving memory processing) processes developed when the organism faces a moderate stress.