I've just been like that since I was in high school. I'm not really sure why. I've always stayed up until at least midnight for as long as I can remember, but started going to bed around 3ish once I got to high school. For a few weeks, I started going to bed as soon as I got home from school (about 3pm) and would sleep until around 3am, then I would wake up and kind of dick around for about three or four hours and then take another nap about an hour before I left for school. Plus, when I lived in Miami, I would regularly get out of work at about 3:30am, so that didn't do much to help me.
The last time I had a (more) regular sleep schedule was when I was working in NYC. I would wake up at 5 to catch the 5:30 train into the city and transfer to the subway which took me to my job in the Upper West Side, where I would arrive at about 7 am. I would get out at around 10:30 or 11pm (since the restaurant was brand new, it had plenty of hours to give). I would do the same train-hopping process in reverse, which would bring me home usually by around 1am. Then I would get the only sleep I could before doing it all again the next day.
Yea, that's the most regular my sleep has been for almost my entire adult life.