It was a person free-running in a cave. Here's one article about it [0] to get you started, and here's another [1] of multiple 9-day studies showing the fairly

It was a person free-running in a cave. Here's one article about it [0] to get you started, and here's another [1] of multiple 9-day studies showing the fairly immediate shift to 24.5 - 25 hour cycles (we're actually Martians?). [0] https://howandwhys.com/michel-siffre-time-experiment-body-sleep-cycle/ https://howandwhys.com/michel-siffre-time-experiment-body-sl... [1] https://www.researchgate.net/publication/339110740_Living_Without_Temporal_Cues_A_Case_Study https://www.researchgate.net/publication/339110740_Living_Wi...