What can lack of sleep and stress in school do to your health?

Hi! I am not an insomniac but I only get 3 hours of sleep a night, including weekends. This is because of school and my workload. My teacher also claims I am high strung and that I need to relax more. I am never relaxed and when I am doing something that is not homework or studying, i feel that I am wasting time and I feel guilty about it. I know that it is normal to relax, but for some reason, I can not convince myself. So, what exactly can stress, no relaxation, and lack of sleep do to a person. I am 15 btw, if that makes a difference.