I was just reading Heather Armstrong's post about flying with kids and I remembered that my mom once told me that when I was little, like two or four, and we'd fly 20 hours from London to Taiwan, I wouldn't sleep. Not at all. I'd pace up and down the airplane aisle all night.
Also when I was wee, after eating a big meal, I'd sit there in my high-chair and sing.
To this day I still can't sleep on planes and I still sing to myself after a big meal. It must be genetic.