Bruxism (from the Greek βρυγμός (brugmós), gnashing of teeth) is grining of the teeth, typically accompanied by clenching of the jaw. It is an oral parafunctional activity that occurs to some extent in most humans. Bruxism is caused by the activation of reflex chewing activity; it is not a learned habit. Chewing is a complex neuromuscular activity that is controlled by reflex nerve pathways, with higher control by the brain. During sleep, the reflex part is active while the higher control is inactive, resulting in bruxism. In most people, bruxism is mild enough not to be a health problem; however, some people suffer from significant bruxism that can become symptomatic.