Key study: Siffre (1975)

Aim: to see what happens to the sleep-wake cycle without zeitgebers such as daylight and clocks.
Participant: Michel Siffre, himself, aged 33.
Procedure: in 1972, he lived alone for about six months in Midnight Cave in Texas, with no daylight, clocks or other time cues.
Electrodes recorded his heart, brain and muscle activity. When he woke, ate or went to sleep, he phoned a team on the surface, who never told him the time. They switched his lights on and off to match his sleep and waking. He also did tests of memory, reactions and physical skill.

His sleep-wake cycle drifted away from 24 hours. It varied a lot, from about 18 hours to more than 48 hours: sometimes he stayed awake for very long periods without realising.
He lost track of time.
He also suffered low mood, poor memory and worse physical skill.
Conclusion: without zeitgebers, the body clock free-runs and does not keep to 24 hours. External cues, especially light, are needed to keep the circadian rhythm in step with the 24-hour day.

Strengths: detailed, objective physiological data were collected with electrodes. It removed zeitgebers in a way that cannot easily be done in a lab, so it gives unique insight into the body clock. It has applications, for example to space travel, submarines and shift work.
Weaknesses: a single participant (and the researcher himself), so results cannot be generalised. The artificial light he used may still have acted as a zeitgeber. Isolation and stress could have affected his rhythms, not just the lack of time cues. Ethical concerns: the study harmed his wellbeing, though he chose to take part.