Life pretty much does it for me. I'm awake 19-20 hours a day, and I grab ... well, pretty much, naps at some point in the night before I have to be up and at it again. I'm practically asleep before my head hits the pillow.
Working out really late at night also helps me - for example, in the winter, if I were to ride my trainer between 10 pm and 11 pm, then workout with weights for half an hour after that ... by 1 am or 2 am when I finally get to bed, I'm ready to SLEEP!