I can fall asleep almost anywhere. I routinely fall asleep in the break room at work. Once, I was helping a friend fix his car, and I fell asleep on his garage floor when he went inside to get water.
But in a hot metal tube tearing through the sky, with my neck all kinked? Get out of here, man.
My last flight had a massive snorer. It was pretty funny at first. At hour 8 it was insanely frustrating. He was woken up four times by various people. Went straight back to snoring.