Work on relaxing more periodically throughout the day, since tension at night, even when asleep, is the result of psychological factors. Remove the psychological causes, and you will help treat the physical component. This requires persistent and consistent meditation to get at the root of the problem.
You might want to consider getting treated by a doctor or seeing a specialist, since if this happens while you sleep and you still can't control it, you might want to consider homeopathic, allopathic, or holistic treatments. Some investigation into clinics in your area might be fruitful.