Go night owls. Mornings have crushed me ever since I was a kid. I got the nickname coma because, with morning classes in college, no one could find me. I'd always be asleep afterwards!
Then I had a job where you had to get in 8a every day. It literally broke my sleep schedule so I'd only get 4 hours per night. No bueno for a bipolar. The mania it triggered was fun, but not in a "hey let's do this for the rest of my life" sort of way.
Before I turned into bizarro-man, I actually went to a sleep doctor about the issue. She said I had "delayed sleep phase disorder". The Wikipedia entry for it is spot on for night owls. But there's the implication that it's... well... a disorder. The suggestion of light therapy to become day-shifted was also no bueno for a bipolar.
I think being night-shifted is as natural as being left-handed. I also think people who try to change that don't have as much information or understanding about it as we do.