I have just read this article on sleep on the BBC website. The article suggests that our current habit of going to sleep for a solid eight hours (give or take an hour) is not natural and that our ancestors in pre-industrial times tended to sleep in two four-hour spells, first sleep, second sleep with a one or two-hour spell awake in between.
When we visited the Musée de la Grande Chatreuse last year, I found that this was the pattern that the Carthusian monks (whose lifestyle hasn’t changed much since the c10th) followed. They wake up during the night to say the office of Matins. Of course unless you are a Carthusian monk you don’t have to pray:
A doctor’s manual from 16th Century France even advised couples that the best time to conceive was not at the end of a long day’s labour but “after the first sleep”, when “they have more enjoyment” and “do it better”.
What I do know, is that when I was at sea working on traditional watches, four hours on eight hours off and sleeping twice per day usually about four hours at a time, I normally felt better rested than I do now sleeping in a solid eight-hour block.