How can I stop grinding my teeth in my sleep?

I have this terrible condition when I am in deep sleep my teeth chatter very badly, like you see in cartoons. It is extremely rapid and painful and scary but because I am in deep sleep I can't get up or make myself stop, like I am in a coma but ONLY alert to what is happening in my mouth and cannot move or control it. It is very frightening and painful and so hard that I feel my teeth will break. What can I do???