Bruxism (from the Greek βρυγμός (brygmós), "gnashing of teeth") is characterized by the grinding of the teeth and typically includes the clenching of the jaw. It is an oral parafunctional activity that occurs in most humans at some time in their lives. In most people, bruxism is mild enough not to be a health problem. While bruxism may be a diurnal or nocturnal activity, it is bruxism during sleep that causes the majority of health issues; it can even occur during short naps. Bruxism is one of the most common sleep disorders.