French cave explorer Michell Siffre is a man with firsthand knowledge of the mechanics of sleep. At 23, he lived for two months in a glacial cave, 120 meters beneath the surface. Down there, in the perpetual dark, in the absence of environmental time cues, he confirmed a then-emerging theory that the human biological clock is slightly longer than 24 hours.

Ever since, he has devoted himself to the study of sleep, conducting similar experiments with researchers from around the world. In November 1999, 37 years after his first stay in the cave, he returned for another extended period to celebrate the last Christmas of the century and the new millennium.

Unfortunately, he missed the crucial moment because his internal clock ticked even more slowly compared to when he was a young man.