I've always felt better and more productive/enthusiastic when I'm awake late into the night. By contrast, I feel pretty terrible in the morning.

That said, I've been waking up at 05:00 and going to sleep at 21:00-22:00 for several years now. A combination of choice/necessity - most of my clients are several timezones ahead, I like to be awake before they get into the office. I also find it helpful to wake up before family/friends are awake; I don't think interacting with people just after I've woken up (and am thus in a bad mood) is conducive to good relationships.

Sometimes circumstance forces me to stay awake longer than usual, and then I'm suddenly reminded again of how much better my brain works late at night. I haven't yet tried going back to staying up late, but sometimes I'm tempted.