I PROMISED MYSELF TO STUDY AFTER SCHOOL BUT I'M TOO TIRED

I don't know why, I'm so productive and active at school, and I planned what I'll do after I get home. But once I reached home I just felt sudden exhaustion and drained my energy. So I took a short nap, when I wake up I'm still so tired and lose the energy to do revisions. How can I stay up and still be energised?