Sleep patterns. Nature or Nurture?

[Gawdzilla]

For reasons not germane to the topic I started going to bed before 8 pm and getting up at 4 am a few years ago. I kept that habit until a few months ago. Then I decided to go back to a more rational schedule. It's taken me four months to train my body to stay up "late" and get up "late", but changing sleep habits can be done, for a lot of people. Professional insomniacs have my sympathy.