Rapid decline in body temperature before sleep: fluffing the physiological pillow?

A novel approach to the analysis of body core temperature was employed in an effort to further clarify the temporal relationship between the nightly decline in body temperature and the timing of the onset of nocturnal sleep. Core body temperature and EEG sleep recordings were obtained from 10 healthy elderly subjects while they lived in the laboratory and self-selected bedtimes and wake-up times. A rate-of-change curve was then generated for each temperature data set, showing the relative magnitude by which body core temperature declined (or increased) from minute to minute across the recording period. The time at which the maximum rate of decline (MROD) occurred was determined, and this time was compared with subjects' self-selected bedtimes and with subsequent EEG-defined sleep onsets. Eight of the 10 subjects' body temperature curves showed a maximum rate of decline well before (mean 41 min) the decision was made to retire. There was a significant positive correlation between the amount of wakefulness within the first hour after initial sleep onset and MROD relative to both bedtime and sleep onset (rs = 0.70; p < 0.04). That is, the closer MROD occurred to either bedtime or sleep onset, the less wakefulness there was within the first hour after sleep onset. The findings indicate that the process of sleep initiation is most likely to occur when body temperature is declining at its maximum rate and is most successfully accomplished at this phase of the temperature cycle.