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.
If it moves, I can sleep. Ship on rough waves? Sleeping. Train? Sleeping. Bus? Sleeping. Cab, especially late at night, a struggle to stay awake. Planes are no different. I often fall asleep during taxi already.