I maintain that falling asleep in the cinema has nothing to do with the quality of the movie itself.
A darkened cinema, a comfortable seat...sometimes people don't actually realise how tired they are, and their lack of sleep (and who here can honestly say they always get the full 8 hours every night?) will often catch up with them during that moment.
And let's be honest...do people only fall asleep during movies they don't like?
It has nothing to do with the movie not holding your interest...I have been gripped by some movies and yet still struggled to keep my eyes open.
I have a good friend who is an absolute movie freak...he had a big screen tv, laserdisc player and an early form of surround system at home even back in the 80's, and went to the cinema practically every day.
But...at regular intervals, for about 10 - 15 mins at a time he regularly fell asleep during movies at the cinema.
I sat and watched Aliens in his house...and he was fast asleep after 10 mins and slept for the whole movie.
Alcohol of course plays a huge part...another friend of mine used to invite me to watch movies at his house. He would open a bottle of whisky, have a few slugs and within about 30 mins he was out for the night. I often used to go home and just leave him comatose in the chair.
I seriously think this whole 'bad movies putting you to sleep' thing is a myth...on a similar note it's right up their with the ridiculous concept of yawning because you are bored.