Data were collected from 14 human subjects who lived singly in an isolation unit without temporal cues. The subjects used buttons to signal the times when they woke up, took a meal, defecated, and retired. Under these conditions, the "free-running" circadian rhythms (e.g., the sleep-wake cycles and the rhythm of body temperature) remained internally synchronized in 7 subjects (mean circadian period = 24.47 hr); in the remaining 7 subjects the sleep-wake cycle lengthened beyond 28 hr, desynchronizing from the rhythm of body temperature (internal desynchronization; mean sleep-wake cycle = 33.45 hr). In all subjects, the interval from wake-up to defecation increased with the duration of wake time (alpha); on average, the interval varied proportionally with alpha. Furthermore, the interval from the last main meal (dinner) to defecation the following day was proportional to the sleep-wake cycle--either that which included dinner but preceded the defecation, or that which followed the dinner but included the defecation. It is concluded that a lengthening of the sleep-wake cycle (and of the wake time) results in a slowing down of the processes of digestion and evacuation of the bowels, in parallel with an apparent reduction of total energy expenditure.