Circadian rhythms evaluation

RTS Siffre's cave study: Siffre spent several extended periods in a cave to study the effects of his own biological clock. He deprived himself of exposure to natural light & sound but had access to food & water. After two months in the caves, Siffre believed it was mid-August however it was actually mid-September. A decade later he performed a similar study but for 6 months, in both cases, his 'free running' biological rhythm settled down to one that was just beyond the usual 24 hours (around 25 hours) though he continued to sleep & wake up on a regular schedule. This suggests that his 24-hour sleep-wake cycle was increased by the lack of external cues, making him believe one day was longer than it was, and leading to his thinking that fewer days had passed.

Individual differences: difficult to make generalisations. Supporting studies are based on very small samples of participants, it seems that sleep/wake cycles may vary…