Always sleepy even if i sleep enough

I'm 18 years old and I go to school. Every night i go to sleep around 10:30 pm and I wake up at 7 am, I don't have any problems waking up or in the first 5/6 hours awake but almost every day after school i need to have an afternoon nap for about 1 hour. The thing is that i feel like i sleep well and enough but without that nap i can't ve productive( for example i can't study without risking to fall asleep.) Does anybody know how i can "fix" this. (Btw sorry if I have made some mistakes but english is not my first language)