With the threat of UV rays becoming more and more realistic, newer generations are sleeping more and more during the day.
While it seems to the previous generations that we are just being rebellious and insubordinate, I see it as a sign of evolution. Just as in Africa, the skin of the natives has darkened over time to protect them from becoming burned by the radiant heat produced by our sun, we are beginning to avoid that huge fireball in the sky all together.
The ozone layer has been depleting over the decades, and if you think about it, so have the number of morning people. Not because of lack of conditioning, but because of self-preservation. Ask your neighborhood night-owl why they stay up at night, and the answer will probally be somewhat expected:
"I have insomnia." or "I'm just not a day person."
Fact Theory has it that the human body is sensitive to UV rays, so why shouldn't it make sense? It does to me, at least.