Circadian rhythms occur approximately every 24 hours e.g. body temp or sleep-wake cycle.
Michel Siffre (1972) - the cave study
Siffre spent over 6 months living in a cave in Texas with no natural light or anything else to tell him what time of day it was. His biological clock was allowed to 'free-run' and he was wired up so his body functions could be recorded. Siffre's sleep-wake pattern settled down to 25 hours. We can conclude from…