Jessa Gamble does some research:

[Jürgen Aschoff’s] experiments in a disused Munich bunker in the 1960s were the first to reveal the body’s independent sleep-wake cycle in its naked state. For several weeks, Aschoff’s subjects lived in isolation, collecting their own urine and monitoring their body temperatures. Dim lights were entirely under their control, but no time information from the outside world was allowed, and when Acshoff’s staff arrived with supplies, they even randomized the stubble-length on their faces so as not to give away clues.

Out of that gloom emerged the first proof of the body’s independent clock, cementing Aschoff’s standing as a founder of chronobiology. With no sunrise to provide external calibration, his subjects still tended to sleep for about eight hours. However, their waking period stretched slightly beyond 16 hours, revealing an internal clock that ran 20 minutes slower than the 24-hour day. Their days settled into a pattern of about 24.3 hours. And so with each passing day, the bunker residents went to sleep later and later until they were entirely out of sync with the rhythms of German life bustling above their heads.