clenching your jaw shut tight when sleeping is a sign of stress. it can cause TMJ - temporo mandibular joint dysfunction. do you also grind your teeth. clenching your jaw when sleeping indicates that your muscles are not relaxing at night. do you wake up feeling tired and not refreshed? there is a device you can get at drug stores that you put over your bottom teeth so that you cannot clench your jaw at night. i suggest you invest in one. i had a problem with jaw clenching for years. i also found that placing a bolster pillow under my knees when sleeping on my back was helpful in helping my body relax too. (i used to tighten my legs when sleeping; the bend in the knees keeps those muscles relaxed.) all the best to you.