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.
There’s definitely a lot to be said about the exhaustion from long flights. I used to travel often between east Asia and the States, so I had to get used to sleeping on planes out of necessity. I’d always try to get a long layover in either Seoul or Beijing because their airports allowed for sleep. Seoul had a layover area where there were reclining lounges that were really comfortable to sleep on, and Beijing has a hotel in the airport where you can get a single room with a bed. It also has a shower, which is awesome when you’re getting off of a 10+ hour flight.