When Josie Laures came out of her cave on March 12, 1965, she thought it was February 25. A few days before Antoine Senni came out of his cave, on April 5 that same year, he thought it was February 4.
The two cave explorers emerged from their holes in the French Alps, near Nice, 50 years ago. Each of them set the then-world record for time spent alone in a cave—Laures set the female record at 88 days, and Senni the male record at 126 days—as part of an experiment to see what the effects of extreme isolation and loneliness would be on their bodies and minds.

While in their caves, which were separate but just a few hundred yards apart from each other, Laures and Senni kept in touch with researchers at a control point, who tracked their sleeping and eating habits, as well as memory and vital signs. The researchers did not, however, give Laures or Senni any clues about how time was passing up on the surface. By the time they emerged, wearing dark goggles to spare their cave-accustomed eyes from the sunlight reflecting on the alpine snow, they had lost weeks of time, by their own reckoning.

In chronobiology, the study of biological rhythms and how they relate to the environment, the German word zeitgeber means “synchronizer.” Natural light is the best-known, though not the only, zeitgeber that syncs human sleep patterns up with the Earth’s 24-hour day. Absent any cues from sunlight or even from clocks, Laures’s and Senni’s sleep schedules got wacky—sometimes without them realizing it. A Chicago Tribune article titled “Sleepy Caveman Calls it Quits” proclaims Senni “a great sleeper, sometimes nodding off for 30 hours at a time and waking up believing he had simply had a short nap.” (In years since, researchers have found that people often slip into 48-hour sleep cycles when isolated from the environment.)