HowMuch Sleep is Enough

It depends on genetics.  A good article can be found at
http://www.entrepreneur.com/article/232784

Excerpt: Unfortunately, for most of us, sleep is the one non-negotiable. To operate optimally, 90 percent of the human population needs somewhere between seven to nine hours a night, says Ying-Hui Fu, a human geneticist at the University of California-San Francisco. She estimates that a small segment of the population -- about 3 to 5 percent – needs less than 6.5 hours, while an even smaller sliver (Fu places it at less than one percent) can function normally on less than five hours.

As one of the miniscule percentage 'tis nice.

 

If you think you might be a short sleeper the consistent wake time usually to the minute seems to be the characteristic to be aware of even on the first day off after a long week. (Or two.)  One short sleeper I talked to trained himself to "Feet on the floor at 4:44" every day.