Delayed sleep phase disorder is a recognized disorder that can be difficult or impossible to control:
http://en.wikipedia.org/wiki/Delayed_sleep_phase_disorderThat isn't an excuse to stop trying—there are things you can do that often help: a combination of correctly timed melatonin and light therapy can shift your circadian rhythm back towards normal. Nonetheless, it's an actual medical condition.
To everyone else: if you think you have this problem, do something about it. There's a healthy chance you can fix it.