In the 1960s, at the peak of the Space Race, scientists were curious how humans would handle traveling in space and living in fallout shelters. Could people cope with extreme isolation in a confined space? Without the Sun, what would our sleep cycles be like? Michel Siffre, a 23-year-old French geologist, decided to answer these Cold War questions by conducting an experiment on himself. For two months in 1962, Siffre lived in total isolation, buried 375 feet inside a subterranean glacier in the French-Italian Maritime Alps, with no clocks or daylight to mark time.

Inside the cave, temperatures were below freezing, with 98 percent humidity. Constantly cold and wet, Siffre suffered from hypothermia, as massive chunks of ice regularly crashed down around his tent. But during his 63 days underground, he only dabbled in madness once. One day, Siffre started singing at the top of his lungs and dancing the twist in his black silk tights. Other than that, he behaved relatively normally.

When Siffre emerged on September 14, he thought it was August 20. His mind had lost track of time, but, oddly enough, his body had not. While in the cave, Siffre telephoned his research assistants every time he woke up, ate, and went to sleep. As it turns out, he’d unintentionally kept regular cycles of sleeping and waking. An average day for Siffre lasted a little more than 24 hours. Humans beings, Siffre discovered, have internal clocks.

The experiment’s success made Siffre eager to conduct more research. Ten years later, he descended into a cave near Del Rio, Texas, for a six-month, NASA-sponsored experiment. Compared to his previous isolation experience, the cave in Texas was warm and luxurious. His biggest source of discomfort were the electrodes attached to his head, which were meant to monitor his heart, brain, and muscle activity. But he got used to them, and the first two months in the cave were easy for Siffre. He ran experiments, listened to records, explored the cavern, and caught up on his Plato.