Feeling tired/sleeping alway even after sleeping for hours?

some of my friends said that i sleep too much . i am still a student and somehow everytime i sleep i dream . on exam days are worse . example math test on thur i dream of math equation or solution on fri right after the test. i sleep ALOT but i always feel that my brain is not working . why?
Update: i tried sleeping only a certain amount of time but it didnt work i just simply went back to sleep . i did try work out and even drink coffee to help me stay awake but then i felt back to bed. somehow whenever i wake up my feet is kinda weak and during exam i even miss out a words NOT causing the whole question wrong
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