Six healthy female volunteers (22-24 years), physically untrained (unfit), sat in baths of warm or cool water for 90 min, between 14.30 h and 17.30 h, on separate occasions. In the former condition (HOT), rectal temperature (Tr) rose by an average of 1.8 degrees C, and in the latter (COOL), a thermoneutral condition, there was a nil Tr change. All-night sleep EEGs were monitored after both occasions and on baseline nights. Following COOL, there was no significant change in any sleep parameter. After HOT there were significant increases in: sleepiness at bed-time, slow wave sleep, and stage 4 sleep. REM sleep was reduced, particularly in the first REM sleep period.