Derived from the Greek word meaning "gnashing of teeth," bruxism is the habitual, involuntary grinding or clenching(きしむ、くいしばる)of the teeth. Sometimes associated with stress, bruxism is caused by the activation(活性化)of the reflex portion of the chewing mechanism at a time when higher brain control remains inactive, usually during sleep. Though common and generally harmless, bruxism can cause damage in the form of cracked teeth, indentations(へこみ、刻み目), and wear(摩耗).