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 I have a partner to travel with me, I can sleep by holding their hand and leaning on them or by laying on their lap.
I have also slept on semi-strangers lap once.... I'll give some context... We all were apart of an event ... Staff all knew who everyone was, so there was some implicit trust amongst us peers. As a result, I figured I'd ask this one person I was sitting with to hold their hand. The person later asked if I was tired & invited me to lay on their lap, so I could sleep. In any other travel, asking strangers to do such thing could have larger risks obviously.