Aim - Investigate the role of light in the sleep-wake cycle
Procedure - spent 2 months, then later 6 months, in an underground cave in Texas. Little light exposure to natural light so had a few external cues of the time of day, but did have access to adequate food and drink. Ha a telephone link and was monitored via computer and video
Findings - Siffre’s sleep-wake cycle was just beyond the usual 24-hour cycle (25 hours) and he continued to sleep and wake up on a regular cycle despite not having external cues
Conclusions - Our ‘natural’ circadian rhythm is 24-25 hours long but is entrained by exogenous zeitgebers like light and time
