I've fallen asleep at the computer a couple of times...First time that I can recall, I was watching Eragon with my window open as soon as I came home from school. The weather was PERFECT, and a gentle breeze blew about while the sun shined on me...and I promptly fell asleep because I was just THAT relaxed.

Second time, after a HUGE emotional/mental breakdown because of an incident between my parents that really upset me and made me almost literally go crazy, I started to fall asleep at the computer after I had settled down and stopped crying.

I'm sure it's happened more than that, but those two times stick out the most in my mind.