Why am i so tired all the time? (10 points)?

I'm constantly tired and i get around 8-9 hours sleep a night, i also try and eat healthy im not over weight and do enough exercise. I find it hard to concentrate or even stay awake for long periods of time. I have been to the doctors because i get a cold just about every month, she tested my blood and the results came back normal... anyone know what could be wrong?