What is the cause of teeth gnashing while you are sleeping?

Bruxism - stress: Grinding or "gnashing" of the teeth during sleep is called bruxism, and it can be due to many things, including problems with your bite or your teeth, but by far the most common reason for bruxism is stress. Everyone has stress in their lives, and different people deal with stress in different ways. Some people grind their teeth at night.

Bruxism: Bruxism is a general term for clenching, gnashing, grinding generally caused by stress. Bite discrepencies can also exacerbate this. A nightguard can help prevent damage from bruxism, but will not stop it.