What's wrong with these guys? Don't they know how to keep their heads from plummeting to your shoulder? If you're really that tired, you should be buying one of those weird head pillows to make sure you stay in your lane. Then comes the question of how to wake them up without touching their bloated faces. Usually a quick shoulder jerk will do the trick.
Or maybe set your headphones around your neck anad blast the angriest Eminem track you can find. If nothing else that person will have some crazy violent dream in which his his plane crashes into the side of a mountain and he's forced to eat his own arm. That should wake him.