Naps as integral parts of the wake time within the human sleep-wake cycle

Thirteen subjects lived singly in an isolation unit without temporal cues for an average time span of 32 days. They signaled the times when they woke up, took a meal, and retired, noting in their diaries what kind of sleep (nap or main sleep) they were going to have. In five subjects the free-running circadian rhythms remained internally synchronized. The other eight subjects became internally desynchronized, with different periods in the sleep-wake cycle and in the rhythm of body temperature, or developed a "circa-bi-dian" rhythmicity (i.e., a state of the circadian system in which the temperature rhythm regains synchrony with the sleep-wake cycle in a 2:1 ratio). The duration of naps was positively correlated with the duration of wake time (after subtraction of the nap), as well as with the duration of wakefulness preceding the nap. In contrast, the duration of main sleep was negatively correlated with the preceding wake time. It is concluded that naps are integral parts of the wake time, and that they follow the rules known from intermeal intervals and the perception of long time intervals such as 1 hr.