6.4 Interaction Between the Sleep-Wake Cycle and the Rhythm of Rectal Temperature

The human circadian system has been considered as controlled by two — or even more — oscillators which are represented by the sleep-wake cycle and the rhythm of rectal temperature (Wever 1975). Both rhythms are influenced by environmental time cues but also affect each other mutually. The interaction between the two rhythms is especially obvious in the absence of external stimuli, i.e., under constant conditions, where it usually results in mutual synchronization between the two rhythms. However, in about 30% of subjects examined under constant conditions, the two rhythms show different average periods in the steady state; nevertheless, they continue to interact mutually. It is only in this state of internal desynchronization that the direction of the interaction between the two rhythms can be evaluated.