[Amadou]

I sleep in two separate three-and-a-half to four hour periods. In my experience I feel much better rested when following this sleeping pattern than when trying to sleep in a constant eight hour block.

Looks like there is historical evidence for a "first sleep" and a "second sleep" and that 8-hour stretches are a product of the last century or two.

http://www.nytimes.com/2012/09/23/opinion/sunday/rethinking-...

(hope the paywall lets you in)