As every parent knows, infants are seldom on the same sleep-wake cycle as the rest of the family. In human infants, the sleep-wake cycle is initially nearly random. At about 6 weeks of age, the circadian rhythms begin; by about 16 weeks, the baby is entrained. Do infants, in fact, have an endogenous circadian sleep-wake rhythm, or do social cues (parents' schedules) directly control their behavior?
Under normal conditions, the often complex social environment in which most humans live presents many zeitgebers, such as our work schedules or daily caffeine consumption. These factors work with or against our entrainment to the natural light-dark cycle.
Early sleep-wake experimenters eliminated all external cues, such as light and sound, to understand the human sleep-wake cycle. To understand the free running sleep-wake cycle, one of these researchers, Jurgen Aschoff, built an isolation facility known Tier Bunker. Starting in 1961, test subjects stayed in the bunker for 3 or 4 weeks and revealed a sleep-wake cycle of about 25 hours . (Recent research suggests that the human cycle may be closer to 24 hours ). They could turn light on and off at will, but no external cues could reach them. Under these controlled conditions, all subjects displayed clear circadian rhythms, and experimental changes in room temperature seemed to have no effect on their cycle .
Since these early studies, many researchers have gone on to study the human sleep-wake cycle. One of the most intriguing, perhaps, was that of Michael Siffre who spent 6 months alone in a cave .