Sleeping in shifts

From a page with notes, links and thoughts about the history of human sleep and what might be natural, Sandra's words:

I like the sentinal theory. I’ve often thought that teenagers’ propensity to stay up late might have been very useful in “the old days” (caves, camps or castles) because they could keep watch while they talked to each other. And their sleeping in the daytime while others are awake is seen as sloth in modern days by too many people, but I think as long as they get sleep, it shouldn’t matter so much what time it is.

What about sleep? sleep in history and culture
SandraDodd.com/sleep/outside
photo by Sandra Dodd (and it's a link)