A few contenders, but I'd probably go with Traffic. My friend and I were excited to go see that after all the rave reviews, and it just about put us into a coma. The guy behind us nodded off a few times, and his wife had to keep waking him up when he'd start to snore. That was more entertaining than anything in the film.
I've now given this film 2 chances and have concluded it could be used as an antidote to caffeine.
Syriana. The one that popped into my mind when I saw the thread title. Other movies that made me go to sleep were The Dark Knight Rises, Avatar and Fellowship of the Ring, though in defense of Fellowship I was sitting at the front row because the theater was packed and I had to look up all the time which made me fatigued after two hours so I missed the last part which I later revisited on blu-ray.
I'm pretty sure I answered this question on the board recently, it was Jane Campion's 'The Piano' for me. Hated it. My wife would say Ridley Scott's '1492 - Conquest of Paradise'. She fell asleep. I love that movie