A week ago, I used up a stretch of vacation time to indulge in a week of personal productivity (and video gaming). During this stretch of time (from one Saturday to the next Sunday) I managed to sleep one less times than there were nights.
That is to say, I did not sleep when it was dark - I slept when I felt tired enough to do so. This happened to occur less often than nighttime. To see a similar sleep schedule displayed in a handy timeline, check out this XKCD comic.
Over Christmas weekend (4 days), I tried the same thing - I fell asleep very late the evening of the first day, very early the morning of third day, and again the afternoon of the fourth day.
This is the end of that experiment - over 4 days, I have slept 3 times. As I go to work right now, I have already been awake for 8 hours. I will probably feel like sleeping not long after I leave work.
The odd sleep schedule has some benefits: it's not hard to fall asleep (because my bed time is defined by when I feel sleepy), and depending on my sleep patterns, there is the possibility of actually spending less time sleeping every week.
The abnormal waking hours definitely felt odd at times, though I feel that I started to get used to them a bit. I'm considering trying a similar schedule over the course of a work week some time. If I do, I will report back with my findings.