[SubuSS]

Did your family setup etc also change around this time?

I have an unsubstantiated belief that age / family etc affects this. As you add up more commitments, you start having to work with / for others meaning a lot of overlap time is required. This also means morning is the only time you can get to do your own stuff and eventually the habit sets in.

Anecdotally you can see this in kids who've committed to sports etc. Early. They tend to become morning folk just out of habit even if they party it up for certain years.

[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.