Same here. If I start early (particularly due to a meeting), it's difficult to do any serious mental work for the rest of the day because I consistently forget what I'm working on. I can force myself to work anyway, but then I'm more prone to make mistakes. It's much better for me to ease into work.

I learned a few years ago that defying my internal clock even has physical effects such as poor digestion and terrible stomach pain. The popular advice to rise early is harmful in my case.