Airline meals are designed to help you sleep. The ones they serve after take-off are usually very carb heavy - rice, potato, pasta. If it’s a long flight, then the second meal is usually protein rich - eggs, etc.
If you’re not getting a meal on-board then take something carb heavy like a bagel.
Anything easy to digest would help to fall asleep. Usually you’d know best what works for you - anything ‘new’ might cause problems. More importantly though, staying asleep might be easier if you have not drunk and eaten too much, because you’ll probably not get back to sleep after a sprint to the lavatory.
Glycinate is supposed to be the easiest on the bowels but I would advise a test before you depart. I have no issues with it, but we all vary. I sleep much better with glycinate. You might ask in r/supplements for other opinions.
Carbs are your best friend: pasta, rice, or a good ol’ peanut butter sandwich knocks me out like a toddler after recess. Bananas help too, but let’s be real, a heavy meal and a window seat do most of the work.
I avoid greasy food. I’ll also look at the menu before sitting down. Some countries have decent things like sushi, stir-fry, salad bars. Another approach is to pay for the lounge or check your credit card. It’s not the greatest food but it’s more comfortable waiting and all you can eat with some selection.