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.
Melatonin helps. It’s a natural chemical your brain releases to help make you drowsy at bedtime and you can get it over the counter to help sleep. It also resets your biological clock which makes it helpful for jet lag.
That with noise cancelling headphones or earplugs, a sleep mask to cover the eyes, and laying as flat as possible (even paying for business class) all help.