All living organisms are subject to Biological rhythms; they are governed by internal biological 'clocks' called endogenous pacemakers.
The sleep/wake cycle is an example of Circadian Rhythms as it is governed by the endogenous pacemaker, the suprachiasmatic Nucleus (SCN).
Exogenous zeitgebers such as light can reset the SCN.
Siffre was a self styled caveman who studied the effects of several periods underground on his own biological rhythms. He deprived himself of exposure to natural light and sound but he had adequate supply of food and water. He reported back to his team when he felt tired or hungry to record. Resurfacing in mid September, Siffre believed it to be mid August.
Siffres sleep/wake cycle changed to 25 hours
Other research was carried out by Ashcoff and Wever (1976) where a group of participants were convinced to spend 4 weeks in a WW2 bunker deprived of natural light. Findings were that all but one participant displayed a circadian rhythm of 24-25 hours suggesting that one sleep/wake cycle may be slightly longer than 24 hours but 24 hours is biologically entrained.
