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?