As arian put it...DUH! Whenever I have a student dozing off in class, I tell them straight up, "The reason you're tired all the time is because you're not getting enought sleep!"
Then they say they do. So I ask "What time do you sleep?" and "What time do you wake up?" Usually they wake up around 7 am or so, which is not out of the ordinary. What IS out of the ordinary is that they're staying up (on school nights) until 3 or 4 in the morning. That's like, 4 hours of sleep max. That's not sleeping, that's just taking an extra-long nap. For a "proper" night's sleep you need to be unconscious for at least 6 and a half hours (preferably 7 or 8 would be best).