why do I always feel sleepy whenever I start studying?

[xaweerf501]

I sleep 7-8 hours, I go out in the morning to walk or jog, I excise almost daily, drink 2 L of water everyday and still, I feel so sleepy when im trying to study. ( I avoid studying on my bed and strictly only study on tables). I can't focus on studying because my eyelids feel too heavy. How do I fix this? whats wrong with me?