In 1962, French geologist Michel Siffre started going underground. He conducted a series of experiments that involved sending human subjects, including himself, into dark caves alone, for months, without any clocks or calendars or contact with the outside world, aside from daily check-ins with his research team above. The subjects lived in total darkness except for a lightbulb that would turn on when they awoke and off when they went to sleep. ⁠ ⁠ _____⁠ ⁠ His goal was to study isolation, but in the process, he wound up showing that humans have a biological clock: an internal mechanism that controls when the body sleeps and wakes, among other functions. In the cave, without exposure to natural light, his subjects’ internal clocks fell out of sync with the 24-hour day/night cycle taking place above, warping their sense of time. Some fell into a 48-hour rhythm, sometimes staying awake for 36 hours and then sleeping for 12. ⁠ ⁠ When researchers told them their experiments were over, some ...