The Timing of Defecation within the Sleep-Wake Cycle of Humans during Temporal Isolation

Abstract

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 (α); on average, the interval varied proportionally with α. 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.