“In 1938, two men spent 32 days inside a Kentucky cave in what may be the first such recorded experiment. In 1962, Michel Siffre (aged 23) spent two months in a cave in the Maritime Alps in France without any time cues. He repeated the experiment several times, the last time in 2000 (at age 61) for 73 days. Others have also run similar experiments of total isolation and the results have always been the same: the more time that is spent in isolation and dark, the longer the sleep-wake daily cycle gets. For example, some subjects could stay awake for 20 hours and sleep for 12 and be perfectly fine. This shows how important the natural cycle of light and darkness that God established for humans can be.
Unknown for many years, the longest recorded continuous stay in a cave was not an experiment but rather a life-saving experience. Thirty-eight Ukrainian Jews hid for nearly two years in the Priest’s Grotto (Popowa Yama), a 77-mile-long gypsum cave labyrinth during World War II.” (Emil Silvestru, The Cave Book, Master Books, Green Forest, AR, 2008, p 27.)
Find that book here: http://www.discovercreation.org/shop/youth-adult-books/the-cave-book/