I did an experiment when I was in my early twenties: I went to bed when tired and got up when I woke up. My time awake shifted an hour or two every day, soon moving my sleep time to fall squarely during day time. I functioned just fine with this schedule but the bad thing about it was it shifted me out of my social life - hard to keep that going when you are asleep when your friends are awake and vice versa.