The Bible describes hell as a place of gnashing of teeth, so obviously teeth clenching has been going on for a long time. Teeth clenching is static loading whereas grinding is a rhythmic side-to-side or forward-back movement. Both of these are considered Bruxism which is a response to activation of our sympathetic or “fight-or-flight” nervous system.
Most people have some Bruxism behavior however approximately 10% have symptomatic Bruxism leading to jaw pain, headaches, sleep interruption and tooth damage. The incidence of Bruxism is greatest when young and progressively lessens with advancing age. However the greatest predictive factor for destructive adult Bruxism is childhood Bruxism.
Normally during the day our teeth should be in contact for about 20 minutes; while some teeth clenchers may have their teeth together for an hour or more.