During REM sleep, arterial blood flow, neuronal firing rates, metabolism, and temperature increase in many parts of the CNS. Eye muscle tone also increases, and the eyes exhibit bursts of rapid movements. If one of the functions of sleep is to conserve energy, then it is curious that energy is so conspicuously expended in the vicinity of the CNS during REM sleep. The author hypothesizes that homeotherms use REM sleep to produce heat in order to maintain a high, stable temperature in a restricted CNS core during sleep. The fact that several of the active features of REM sleep heat the CNS, and the fact that REM sleep propensity increases when core temperature physiologically decreases, seem consistent with the hypothesis that REM sleep is a regulated mechanism for warming the CNS.