Sleeping for four hours, then waking up and doing something, then sleeping for another three or so hours, referred to as first and second sleeps in some historical texts, seems to have been a common practice up until the late 1600s. Somehow the idea that we ought to sleep for eight hours straight came along only relatively recently.
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”. Ekirch found that references to the first and second sleep started to disappear during the late 17th Century. This started among the urban upper classes in northern Europe and over the course of the next 200 years filtered down to the rest of Western society.