https://www.vice.com/en_us/article/v7emd3/15-people-lived-40-days-in-a-sunless-cave-without-clocks-to-study-time

Fifteen people emerged from a cave in France on Saturday, after spending 40 days living in its depths without sunlight, clocks, or contact with the outside world. 

The volunteers were part of an experiment, called “Deep Time,” which was designed to probe the human brain’s conception of time in an environment without chronological bearings, and to study the group’s overall adaptation to life in isolation.