Have kids. (preferably more than one) Then let me know if you can see how it could happen.
When you're trying to juggle a spouse, a full time job, multiple children and making time to play a few games with your friends when the rest of the family is asleep...it becomes pretty easy to see how it could happen. It doesn't happen to me because I'm a night owl, and gaming when tired ends up affecting my mornings more than my evenings, but it doesn't surprise me at all that I've had more than one friend fall asleep while gaming in the past.
Hmm, I can see that providing quite an incentive to fall asleep. I suppose I should clarify a bit - I don't understand when people my age, university fellas, fall asleep while gaming. I can understand that maybe, just maybe the coursework would be enough to cause an extreme amount of stress to fall asleep... but I'm a full time computer science student full up on units (they wouldn't let me take any more classes) as well as a part time job, leaving me with barely even the weekends to play (because I also work weekends, even Sundays, which means good money). I guess people are just built differently.
As for kids, though. Christ, I had to do a babysitting job once. Babysitting all my little kid cousins. The horror.