Well, it is a bit anti-social, although the perpetrator's not doing it on purpose.

If it was irritating then I'd put my personal earphones in and listen to music. If that wasn't sufficient and they were in the aisle seat then I'd tap them on the shoulder to let me out to go to the loo, thereby waking them up in the process. If not, then I might well cough a few times to stir them.

I say this as someone who doesn't usually snore, but on a particular flight fell asleep. Towards the end of the flight, after I'd woken, a member of Cabin Crew mentioned I'd snored (not in a bad way - they really did mention it in context of me sleeping well). However, I was mortified. I felt really bad, inflicting that on others in the cabin. I wish someone had tapped me on the shoulder so I could have shifted my body a bit to stop snoring, I really do.