Effect of ambient temperature on the 24-hour sleep-wake cycle in normal and capsaicin-treated rats.

The 24-hour sleep-wake cycle of untreated, normal rats and of capsaicin-treated rats was continuously recorded by telemetry. Recordings were made on two baseline days at 22 degrees C, two days at 29 degrees C, and two final days at 22 degrees C. In untreated animals the daily amount of waking was reduced by the elevated ambient temperature and nonREM sleep was enhanced. This effect was mainly due to the frequent interruption of the dark-time waking episodes by sleep. In capsaicin-treated animals, raising the ambient temperature did not significantly enhance sleep. However, in both groups of rats the slow wave sleep (SWS) fraction of nonREM sleep was increased after elevating the temperature to 29 degrees C. REM sleep showed a minor increase which was significant only for the capsaicin-treated group. The results suggest that a moderate increase of ambient temperature has two effects: (1) It causes an enhancement of sleep by a reduction in the duration of waking episodes, an effect that may represent a heat-defense response. The attenuation of this response in capsaicin-treated rats may be a consequence of the impairment of warm-receptors. (2) It favors the occurrence of SWS and REM sleep.