I remember reading somewhere that before the Industrial Revolution introduced shift-based work, people would undertake two periods of sleep - go to bed early, wake up in the middle of the night to go out and do some more activity, and then go back to bed until morning. The article (which I can't find) mentioned that people used these hours for social activity.
While it seems vaguely plausible, I am wondering if it's true. After all, wouldn't the night be too dark to go anywhere?
This BBC News article is pretty clear that "two sleeps" were the norm until the 17th century. It quotes not only historians, but literature that mentions "first sleep" and "second sleep" as relatively ordinary things. As well it mentions a study in the 20th century where some subjects adopted a two-sleep pattern when deprived of external time cues.
There is also a blog entry that uses biblical references and the prayer schedules of monks to make the same claim. It seems to rely on the same book as the BBC article, but uses different examples - which may have been pointed out by the book, but are quite easily checkable (eg, the Bible.)
The shift seems to have happened not so much as a result of the demands of the Industrial Revolution but the invention of artificial light allowing people to stay up later in the evening and then just sleep straight through from say 10 to 6.
