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.
Waking up crying every 2 hours? I guarantee that whoever came up with that saying to mean sleeping well never had kids.
I struggled to sleep on planes even before I had an energetic little kid to keep under control. I don’t like sleeping while sitting up so I only ever manage to sleep lightly for a couple of hours at most.