Bruxism is clenching or grinding your teeth. Most people are not even aware that they are doing this. Some people grind their teeth only during sleep. This is called "nocturnal bruxism" or "sleep-related bruxism." It can have a variety of causes, nothing more than a habit. It also can be a result of the body's reaction when the teeth do not line up or come together properly. It also can be a symptom of certain rare diseases of the nerves and muscles in the face. People with severe bruxism can break dental fillings or damage their teeth. Rubbing the teeth together can cause the outer layers of enamel to wear away, exposing dentin. This can result in tooth sensitivity.