text: i clench my teeth when i sleep. is there a reason why? i wake up and my jaw hurts. only when im tired or stressed. anyone know why people do this and how it can be cured? my jae is practically locked when i wake up. i have the same condition, it is called temporo-mandibular joint (tmj) syndrome.  the most common underlying causes of tmj syndrome are stress and a poor bite, together with clenching and grinding of the teeth (bruxism), especially at night.    you might want to consider wearing a special mouth guard at night, which your dentist can make for you.    also, avoid high-stress foods: refined sugar, all white-flour products, all junk foods, candy, colas, potato chips, pies and fast foods.  do not consume high alcoholic beverages as these are common contributing factor in teeth grinding.  tmj syndrome has also been caused by bad posture, habits such as cradling the telephone between the shoulder and jaw, repeated or hard blows to the jaw and chin, or whiplash.  poor dental work and orthodontia may also aggravate the problem, as can habits such as gum-chewing, thumb-sucking and chewing excessively on one side of the mouth.