This publication presents a new analysis of experiments that were carried out in human subjects in isolation from time cues, under supervision of Jurgen Aschoff and Rutger Wever at the Max Planck Institute for Behavioural Physiology (Erling-Andechs, Germany, 1964-1974). Mean rectal temperatures (t(b)) were compared between subjects who showed internal desynchronization (ID) and internal synchronization (IS) of the endogenous rhythms of sleep-wakefulness and of body temperature. The results showed that t(b) was reduced in long ID (circadian sleep-wake cycle length [(SW)] > 27 h) and increased in short ID ((SW) <22 h) relative to IS. In subjects with both ID and IS sections in the complete record, these differences were also found when comparing only the IS sections: Low t(b) during IS predicts the later occurrence of long ID, and high t(b) predicts the incidence of short ID. While this association is associated with sex differences in t(b), it also occurs within each sex. To the extent that the variation in t(b) reflects the variation in heat production (metabolic rate), the results are consistent with the proposition that the spontaneous frequency of the human sleep-wake oscillator is associated with the metabolic rate, as suggested on the basis of the proportionality of meal frequency and sleep-wake frequency. The finding thus has implications for our views on spontaneous sleep timing.