The final things we observe within the initial six-to-ten-second period is actual clenching of the jaw. It's equivalent to the dog's baring his fangs going into battle,although more subtle. Jaw-clenching,called bruxism when it happens during sleep,it probably affecting close to 30 percent of our population today,and it's thought to be one factor behind the greatly increased amount of periodontal or gum disease, adds Dr. Stroebel.