French geologist Michel Siffre’s 63-day experiment in a dark cave revealed the human body has an internal clock that operates independently of external time cues. By stripping away markers of day and night, Siffre uncovered a personal timer that slowly drifted beyond the standard 24-hour day. His results have had far-reaching implications for chronobiology, influencing research on jet lag, space travel, and shift workers’ fatigue.
Source: https://www.earth.com/news/geologist-who-accidentally-discovered-humans-have-an-internal-clock-by-spending-63-days-underground