I was in the exact same situation when I was a kid, I could stay awake in my bed for hours. It was extremely frustrating waking my mother at 3am to tell her that I can’t fall asleep. Nowadays, in my early 20s I can basically fall asleep in less than 3 minutes. I have no clue how this condition disappeared from my life. At least for me physical activity was not a factor in this condition, maybe I’m eating healthier than before (less sugar).