Siffre’s Cave Study

Siffre spent several extended periods underground to study the effects on his own biological rhythems
He was deprived of exposure to natural light and sound, but had access to adequate food and drink
He resurfaced in mid-September 1962 after two months in the caves believing it was mid-August
His free-running biological rhythm settled down to the one that was just beyond the usual 24 hours (approx. 25 hours)
He continued to fall asleep and wake up on a regular schedule

This study showed the power of endogenous pacemakers.

Aschoff and Wever (1976)

Participants spent 4 weeks in a WW2 bunker deprived of natural light
All but one of the participants displayed a circadian rhythm between 24 and 25 hours
The study and the Siffre study both show that the sleep-wake cycle may be slightly longer than 24 hours but has been modified due toe exogenous zeitgebers