In the absence of artificial light cues, people tend to sleep closer to nine hours per night.

“they took that group of people off they went to the Sierras here in America this sort of beautiful mountain range with no electricity whatsoever no access to artificial light and firstly what was dramatic was that these individuals started going to bed around nine o'clock in the evening this wasn't necessarily just because they didn't have anything to do they actually rated themselves as feeling sleepy er earlier why because they were getting the signal of darkness at the natural time secondly they went from being you know ardent about the fact that they were only six and a half hour sleepers and that's all that they needed to then sleeping a little bit more than nine hours”