It seems like I am always tired. Why?

I take a multi vitamin and E vitamin every day. Breakfast usually consists of a gl of OJ, a banana , and a granola bar or cereal. Lunch is usually some fruit, a salad, and a sandwich or equivelant. Dinner usually includes a salad, a starch, meat, and another vegetable. I ride my bike 4 miles a day 5 days a week, I sleep 7-8 hours a night. I drink at least 4 bottles of water a day. I just cant figure out why I am tired all the time.