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.
The one time I slept in a plane for more than an hour was when I wrapped my head in a blanket and had an empty seat next to me to lean on.