[sjg007]

I think it is biological. Babies sleep all day and feed so the parent must attend. Toddlers learn all day nap and go to bed early. Kids can fit a normal routine as well as teenagers but I think that hormonal changes cause teenagers to stay up later and sleep later. In a hunter-gather society this may have been useful for standing guard and raising alarm. Then you have the grandparent/elders who tend to sleep the least. But I also think it is adaptable so as you say people who do sports tend to do so in the mornings. Also if you have a tough work commute you might have off hours to minimize commute time.