Not that I've ever managed or ever will manage to get more than about 20 minutes of contiguous sleep on a plane (I'm tall, and have a hard time falling asleep in ideal conditions, which planes are not) but having a hoodie with a very large hood can be helpful. Ideally, one made of a thin material, so you don't cook yourself a la barbacoa. You then pull the hood over your head and face, and pull the drawstrings. People leave me alone when I do. For this reason, I call hoodies "portable privacy".