Bruxism

From Wikipedia:
Bruxism or gnashing of teeth, is grinding of the teeth, typically accompanied by clenching of the jaw.

Bruxism is caused by the activation of relfex chewing activity; it is not a learned habit. Chewing is a complex neuromuscular activity that is controlled by reflex nerve pathways, with higher control by the brain. During sleep, the reflex part is active while the higher control is inactive, resulting in bruxism.

Bruxism often occurs during sleep and can even occur during short naps. It is one of the most common sleep disorders.

My Comments:
This teeth grinding is worse that snoring. The noise produced by the former is like mice squeaking and very unpleasant to the ears.