I feel sleepy even after decent sleep. (15F)

I'm preparing for a competitive exam. I sleep usually at 12:30 and wake up at 6:30, additional nap for 1.5 hour in afternoon. Still from waking up to 6p.m, (during classes and after coming back too) I feel sleepy and tired. How can I fix this?

(I don't consume caffeine or like that which may mess up sleeping habits)