"So how much sleep do you need? When researchers put test subjects in environments without clocks or windows and ask them to sleep any time they feel tired, 95 percent sleep between seven and eight hours out of every 24. Another 2.5 percent sleep more than eight hours."

The broken link there is meant to point to (http://www.sleepfoundation.org/article/how-sleep-works/how-m...) and I can't spot where it mentions this study.

In any case, you put me in a room with no clock or windows (and presumably no serious stimuli as it's likely to influence the results) and I can bet you I'll sleep twice as much - out of pure boredom.