WHY???

Why is that when you want to go to sleep you can't go to sleep, and when you can't go to sleep you want to go to sleep.

For example...right now I want to go to sleep, but can't seem to fall asleep, but at about 12:45, right after lunch at school, when I obviously can't go to sleep (because that would cause serious chaos in my classroom), I want to go to sleep....

Someone please give me the answer to this riddle.

???