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 partner is one of these. I have photos of him sleeping on planes, trains, boats and cars. He can be worried about something, or stressed about tomorrow, and still fall asleep within 5 minutes of laying down. I am so jealous.