It's somewhat random, but I try to keep 17+ hours awake, 8 hours sleep rythm.
The problem is that I stare at the monitor all the day, and I get pinkeye, and that makes me sleepy. Or I just stay up too long because I find something too interesting and don't want to go to sleep.
I don't use alarm clock, so I awake whenever, but it's usually after about 8 hours.