Once upon a time I was very much a night person, going to bed several hours past midnight and waking up for school at 8-ish. But since I got a 9-5 job again about 2 months ago, I've gone all sensible(!) I'll be in my bed no later than 10:30 pm and lights out before midnight. I then get up between 7:30 and 8. In my school days I'd go crazy on weekends staying up until 3-4 am and sleeping until past noon (once I even slept 18 hours straight, from midnight to six - I was SO confused when I woke up! I have no idea what happened there...). Of course that would screw everything up and I wouldn't be able to fall asleep on Sunday and I'd be tired for school. But now the rule is that I can't sleep past noon on weekends, so I don't screw up my rhytm too much.
However, if I don't have anything to get up for in a while, I'll fall into the 4-5 to early afternoon pattern in no time (I love being awake in the early hours of the morning in the summertime - it's such a magical time of day, what with the birds and the stillness 🙂 )