I don't know what's wrong with me??

This lately, I don't know what's wrong with me because seem like I am oversleep. I don't know how I can oversleep because before this, I used to sleep early and wake up early, sleep in the afternoon for only 15 minutes. But, this lately, damn~ I sleep about 1pm but guess what? I wake up about 7.30am. The worst part is I still feel sleepy although I have sleep for about 6 hours 30 minutes. WTH!! Then, I will sleep again in the afternoon which I'm planning to sleep about 15 minutes only but, I oversleep about one to two hours.. Now, I sleep about eight hours one day. I think it's too much because I think I only supposed to have six hours only. Not more than that. I don't know how I'm going to change this habit. I have to change it or I will have less time to study.. Guys, help me how to overcome this problem please??