I have had 2 general situations with this.
1. The game is so damned good that I stay up late until I'm basically playing the game while asleep (i.e. walking into walls doing nothing) (e.g. Fallout 3, Fallout New Vegas, Fallout 4, GTA San Andreas)
2. I'm sick, or it's a lazy weekend day, or a rainy day, or something, and I just get tired and fall asleep.
External influence or just a damned good game.
My wife on the other hand, I have walked into a room and see her sleeping, with her 3DS laying on her face because she fell asleep and she just kind of placed it on her face or something. Pretty much exclusively handhelds there.