This autonomic response with it's sudden surge of adrenaline is used to prepare the body for either fighting or fleeing danger, a predator, or any life- threatening situation. This is the response mechanism that enables a woman to lift a car off of her child, for example. Remind yourself you are NOT in danger, you are ok, and that the feelings you are having are caused by the adrenaline surge and not real danger. Also try to resist getting up and pacing around until you are more alert and less likely to injure yourself. At least, this has been my own experience.
Some people have episodes less than once per month, without harm to themselves or others.
Some people experience episodes less than once per week, without harm to themselves or others. It would be wise to contact a sleep disorder clinic if you fall into the second or third category, certainly. I have a link to sleep disorders clinics, below. The term rythmic movement disorder is preferred as different body areas may be involved in the movement activity. These movements can also be accompanied by gutteral noises that sound pretty primeavel.