http://en.wikipedia.org/wiki/Delayed_sleep_phase_disorder

I can't say for sure whether things like full spectrum bulbs actually help us sleep, or if that's just psychosomatic, but there definitely exist people who naturally sleep during the day and wake up at night. Left to my own devices, I go to bed at dawn rather reliably, sleep ~8 hours, and wake up feeling incredibly rested. Any other sleep cycle tends to leave me feeling restless and tired, but I've learned to force myself to conform to "normal" society.