Meal timing in humans during isolation without time cues.

In an underground isolation unit, 46 subjects lived singly without time cues under conditions of self-selected light-dark (LDs) cycles or constant illumination (LL). They all developed free-running circadian rhythms that either remained internally synchronized (i.e., with equal periods in all functions; n = 24) or became desynchronized by a sudden lengthening (n = 14) or shortening (n = 8) of the sleep-wake cycle. Six further subjects were synchronized to 24 hr by an externally controlled LD cycle. All subjects had to prepare their own meals. Signals were given by the subjects when they woke up, had a meal, and retired. Of the 52 subjects, 43 consistently had three meals per "day," and 9 had two meals, irrespective of the length of the circadian cycle and despite the fact that, due to desynchronization, wakefulness varied from over 30 hr to less than 12 hr. The intervals between meals, between wake-up time and breakfast, and between last meal and bedtime were "stretched" or "compressed" in strong proportionality to the duration of wakefulness.