Thermoregulatory changes around the time of sleep onset

A causal relationship between declining nocturnal core temperature, increasing peripheral temperature, and sleep onset has been reported. The exact trigger for these thermoregulatory changes around sleep onset, however, is unknown. Our aim was to examine one possible trigger: prior knowledge of bedtime. Fourteen young, healthy male subjects (mean age ± sem: 21.9 ± 0.6 years), participated in a randomized, single-blind crossover study, where knowledge of bedtime was manipulated. Following a baseline night, subjects completed three experimental nights: (A) aware of bedtime; (B) no knowledge of bedtime; (C) misinformed about bedtime. In all conditions lights were turned off at each subject's habitual bedtime (determined from sleep diaries), to individually standardize the time of lights off. Polysomnography, rectal and peripheral (foot) temperatures were recorded continuously on each night. There was no significant difference in the time of sleep onset among conditions (mean ± s.d.: 11:55 h ± 0.73 min), and in all conditions sleep onset occurred at the same rectal temperature. A significant difference in the temperature gradient between rectal and peripheral temperatures among the conditions was evident from 90 min to 40 min prior to sleep onset (P = 0.05), with subjects achieving a more rapid rate of temperature change in the B and C condition relative to condition A. The present findings suggest that knowledge of bedtime may modulate changes in temperature around the time of sleep onset. It appears that there is an optimal core body temperature at which to initiate sleep, and changes in the rate of peripheral heat loss may assist in achieving this optimal temperature, and hence facilitate sleep onset.