In 1962, French geologist Michel Siffre climbed into a glacier cave in the Alps with no clock and no sunlight and stayed for two months, and when his team called him up on 14 September he insisted it was still 20 August, some of the earliest evidence that human circadian rhythm drifts free of any external cue
ScienceBlog.com,
On 16 July 1962, a 23-year-old French geologist named Michel Siffre lowered himself 130 metres into a glacier cave called…