The posting of 5/3/2009, Sleep, discussed the theory that before electric lighting, people went to bed early after it got dark but got up again in the middle of the night for an hour or so. This pattern of dividing the night into two sleep periods was so routine, such a given, that authors rarely mentioned it. The evidence is sketchy, but two sleeping periods could be the norm for humans. As I age, I seem to be reverting to this pattern. I often wake up around 1 or 2 a.m., and getting out of bed for as little as 15 minutes makes it much easier to fall back asleep when I return. It seems natural, and explains why you may be receiving emails then.
A recent online article on another subject discussed the practice of primitive tribes gathering around a fire at night. A sociological study of a contemporary, isolated tribe found that during the day their conversation was much like any office chatter in any city. They complained about the work, gossiped about others, and told rude jokes. But at night around the fireside, the conversation was much different. There, the older members took over, leisurely telling stories of the history and traditions of the tribe.
But, the article added an aside: after going to bed, some of the tribe would get up again at about 2 a.m. and continue the stories for a little longer. Ah, I thought, more evidence of two sleep periods, more evidence that I am sleeping as we are meant to.