What makes people clench their teeth when they are sleeping?

Usually.......: It is a subconscious release of tension. If you clench, try to relax prior to going to sleep, nothing stressful, do not watch tv , no computer, no newscast. See your dentist, you may benefit from a custom made mouth guard.

Usually stress: Stress is the most common factor. Today we see young adults (20's ) with the tooth wear of a forty year old, due to clenching and grinding at night. An occlusal or nightguard helps protect the teeth. Rarely is this related to a tooth imbalance, though occasionaly it is.

Unknown: The cause of nocturnal clenching is unknown. Any other opinions are speculation. See a TMJ expert for evaluation/treatment of clenching/grinding.