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.
Only time I could ever sleep on a plane was after a business trip were I worked 18 hours days for almost 2 weeks. I was so tired I fell asleep right after boarding and buckling in. Not even takeoff and landing woke me up. Flight attendant shook me awake. LOL