The effect of afternoon body heating on body temperature and slow wave sleep.

Recent evidence suggests that body temperature at sleep onset affects the subsequent level of slow wave sleep. According to one hypothesis, the actual temperature is the critical factor determining the relationship. An alternative proposal is that it is the rate of fall of body temperature following sleep onset. These hypotheses were tested by measuring rectal temperature and sleep, following late afternoon passive heating in a warm bath and during a control condition. Passive heating increased rectal temperature, which then returned rapidly toward the control level. However, immediately before lights out rectal temperature was still higher in the passive heating condition, a difference that continued throughout the night. Following passive heating the amount of slow wave sleep was higher in the early part of the night. These results support the hypothesis that body temperature at sleep onset and the amount of slow wave sleep are positively related.