Relationships between changes in the slope of the body temperature (BT) and the initiation of sleep were examined in 44 subjects ranging from 19 to 82 years of age. Following an adaptation night, subjects remained in the laboratory for a baseline night and 72 hours in temporal and social isolation, with strictly limited behavioral options ("disentrainment") during which continuous electroencephalograph (EEG) and BT were recorded. Polysomnographic sleep variables (e.g. sleep onset, percentage of each sleep stage) were determined for nighttime sleep periods at baseline and during the disentrainment period. Periods of the BT curve surrounding these sleep bouts were examined for minute to minute changes, and the time at which the maximum rate of decline (MROD) in temperature occurred was compared with the time of sleep onset (SO) and sleep quality parameters. On the baseline night, the MROD occurred, on average, 60 minutes prior to SO. During disentrainment, the MROD occurred, on average, 44 minutes prior to SO. The proximity of MROD to SO did not affect subsequent sleep quality on the baseline night, but during disentrainment, there were significant correlations between the interval from MROD to sleep onset and the amount of slow-wave sleep (SWS) obtained during the sleep bout. There were no significant age differences on variables related to MROD on baseline night, but the timing of both MROD and SO were significantly advanced in older, relative to younger, subjects during the disentrainment period. It is suggested that a rapid decline in core body temperature increases the likelihood of sleep initiation and may facilitate an entry into the deeper stages of sleep.