It's called sleep bruxism, and it refers to the grinding
or clenching of teeth
. There's a waking version, too -- an unconscious clenching of the teeth
, most often owing to stress -- but the origins are different and the effects are seldom anywhere near as bad as during sleep, when certain of the body's protective mechanisms are turned off. Left untreated, it can cause damage to the teeth and surrounding tissue, headaches and jaw pain.