Back in the 60's a man did several experiments where he isolated himself as well as others in caves for months at a time where they had no contact with outside light nor with any way to measure time.
What they found, was that people tended to go in 48 hour sleep cycles rather than 24. Meaning with out any exposure to natural day to night transitions or anyway to tell time the people slept when they felt tired and woke up when they were ready to.
On average people would stay awake for 36 hours and then sleep for 12 to 14 hours. Meaning if not for the sun rising and setting, the human body would rather stay awake much longer than we typically do.
Another finding was the longer they stayed awake the longer they would stay in a dream state during sleep. For every 10 minutes staying awake, they got a minute of deep sleep dreaming.
Yet another finding was the more someone dreamed the faster their reactions were.
Pretty cool read..
http://www.cabinetmagazine.org/issues/30/foer.php