I posted about his topic on this blog a few months ago I think. It makes a lot of sense from my own experience and what others tell me.
"Extensive research by historian Roger Ekirch shows humans have traditionally slept in two phases. In his book, At Day’s Close: Night in Times Past, he points to more than 500 references from historical texts that speak of dividing the night into two periods of sleep.
It appears it was common for humans to go to bed two hours after dusk, wake in the middle of the night for a couple of hours to smoke, talk, pray or even have sex, then go back to sleep until morning. It wasn’t until the 19th century that people began sleeping in one block."