Frenchman Michel Siffre has spent months at a time living in caves deep underground without the usual external markers of night and day. He found that without these markers, his sleep cycles tended to

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Michel Siffre's experiments in underground living without external time cues found that his sleep-wake cycle tended to extend, with one notable period settling into a 48-hour cycle. This demonstrated the flexibility and adaptability of human circadian rhythms in the absence of natural light and dark cues.

Frenchman Michel Siffre Has Spent Months at a Time Living