How many hours would a day last for a person who can’t tell the time of day by external signs?

French speleologist Michel Siffre carried out several experiments on the matter, including one from 1964 to 1972. He placed himself in a specially equipped bunker with no light, constant temperature and humidity, and absolute silence. He spent several months like this.

The results showed that his inner clock altered: he needed 36 hours awake and 12 hours of sleep. Later, he conducted similar experiments with other volunteers in a cave — with the same result.