Diurnal Activity Cycles

My wife and I (both writing theses this spring) and two cats share a very small studio apartment. One way we make maximum use of the space is by having different sleep-wake schedules. I do my most effective writing between 9PM and 1AM. Iris gets the most done from 6AM to 10AM. I have the easiest time concentrating at that time, and only partly because everyone else is asleep. FeLion, our older cat, does some of her best suckling on inanimate objects in the middle of the day. Tigrinum, the kitten, expends most of his energy from 2AM to 6AM, charging around the house and leaping from the top of his cat-tree onto his sleeping humans. Come to think of it, it makes a pretty lousy system.

There is a well established pattern of individuals of different ages waking and sleeping at different times. Older adults and young children tend to go to sleep early and wake up early. Teenagers tend to stay up late and go to bed late. People in all the ages in between are pretty variable. These patterns are well known to most people, but I'm not sure we understand their evolutionary basis. Why did humans evolve to have sleep schedules that vary individually and with age as much as they do? My guess is that it had to do with avoiding competition within groups for space and forage. The old people and the little kids left the cave or the hut early in the morning and foraged before everyone else was up. In the evening, after everyone else was asleep the young people would stay up and try to impress potential mating partners. That was probably the easiest time to find some privacy without straying too far from the village. One thing I wonder about, do other primates have similar variation in their diurnal activity cycles? That right there is a good evolutionary biodemography question.