Interesting article on sleep by James Clear:
On February 13, 1972, Michel Siffre climbed into a cave in southwest Texas. It would be six months before he saw daylight again.
Siffre was a French scientist and a pioneer in chronobiology, which is the study of biological rhythms. The most well-known of these biological rhythms is the circadian rhythm, which controls the human sleep-wake cycle, and Siffre was on a mission to learn how, exactly, it worked.
Full James Clear Article: The Science of Sleep: How to Sleep Better and Have More Energy.