There is historical evidence for 'segmented sleep' up until the 17th century - see this BBC article:
http://www.bbc.co.uk/news/magazine-16964783
"........these references describe a first sleep which began about two hours after dusk, followed by waking period of one or two hours and then a second sleep. "It's not just the number of references - it is the way they refer to it, as if it was common knowledge," Ekirch says. During this waking period people were quite active. They often got up, went to the toilet or smoked tobacco and some even visited neighbours. Most people
stayed in bed, read, wrote and often prayed"
It may be that we were never meant to sleep all through the night in one session.....?