Need help with my sleep!?

During the one month holiday, i sleep around 3 to 4 am everyday and wake up at like 1pm and i got pretty used to that. Now that schook reopened, im still sleeping around the same time and i need to wake up at 6am! Everyday after i come back from school, i would be exhausted and just take a 2 or 3 hour nap. i would then feel refreshed and cant sleep until 3am. its the same process everyday!! i usually sleep in class and i need that to stop. help:D