I have fallen asleep in other circumstances like the cinema, watching tv with my wife, etc.

I haven't completely fallen asleep on a speech but I have been close, when I was in college there was some talks/speeches and I had to battle to keep my eyes open, and I couldn't stop yawning, but won the battle and didn't fall.

my girlfriend-now-wife wasn't amused with my victory and said something like I was yawning too much and I should have showed more respect to this special invited speaker from somewhere I don't remember...