It is caused by stress and/or anxiety and is expressed through teeth grinding and jaw clenching. 

Most people who grind their teeth and clench their jaw are not aware they’re doing it ,doing it during sleep or while concentrating or under stress.
Symptoms include facial pain, headaches, earache, broken teeth or fillings, severe wearing to the teeth and pain and stiffness in the jaw joint and surrounding muscles which can lead to temporomandibular disorder.