Melatonin isn't a sleep drug. It's something your brain makes naturally at the end of the day when it gets dark etc. As I understand it, it's part of your bio clock and is not much more than your brain telling your body it's time to go to sleep. But if you're on a plane and you're nervous about flying, you won't fall asleep any more than in any other situation where you're nervous and not lying in your bed. You don't necessarily need something stronger, you need something different.