Why am I so tired since exams/school term had finished?

School ended for the term/semester 4 days ago and I have a 3 week holiday before the third term starts. Since Saturday I've been sleeping 8-10 hours at night and doing relaxing activities during the day like watching movies, playing Wii or going shopping. I wake up tired and by the late afternoon I feel like I could go to sleep again even though I've only been awake for a few hours. Why am I so tired now that I'm actually not busy and how can I fix this?