Experiments have been performed upon human volunteers in an Isolation Unit to investigate the effects of different routines of sleep and wakefulness upon circadian rhythms in deep body temperature and some urinary constituents. Irregular sleeping routines, whether as a single randomly-timed 8-hour sleep or as two randomly-arranged 4-hour sleep periods, were associated with free-running rhythms with periods greater than 24 hours, even though mealtimes continued to be taken as customary times of day. If one of the 4-hour sleep periods - the anchor sleep - was taken at the same time each day (0000-0400; 0400-0800; 0800-1200 or 1200-1600), but the other sleep continued to be taken at irregular times, then, after a few days, the circadian rhythms became stabilized with periods indistinguishable from 24 hours. There was a shift in phase of the stabilized rhythms when compared with the phase during control conditions, the size of which indicated that the time of sleep, rather than mealtimes, was synchronizing the rhythms. Some of the implications of these findings for people working shifts or other irregular schedules are discussed.