Sleepy time thoughts

Why is it that through out the day some could say I’m a space cadet due to me not paying attention a lot during the day or being an airhead cause I might not have any thoughts on anything but tell me why when it’s time to go to bed your head always thinks of stuff that stops you from sleeping?… It can’t just be a me thing I know that for sure but I feel like there could be no possible explanation… I’m writing this to stop thinking so much in my head and with this focus on a few things in a line you know.