After nightfall, most people have fallen into a sound sleep. However, some people (especially children), will grind their teeth while sleeping. Since there is no food in the mouth when grinding teeth at night, and the saliva secretion also correspondingly decreases, the teeth cannot be lubricated. This forms a "dry grinding" like pushing an empty millstone. If you often "dry grind", the wear on your teeth can be quite severe. Over time, the worn teeth will become sore or painful to varying degrees, and sometimes it can even cause temporomandibular joint dysfunction.