I've never gotten that tired but when I worked at Taco Bell people would pass out in the drive through. Quite often. Disturbing I know.
One couple really made me laugh because both of them had fallen asleep. When I yelled out the window at them the man (who was driving) jumped awake and then reached over and hit his wife to wake her up. We were all laughing at that. I told him I don't know why he thought she needed to wake up he was the one driving.