Grinding or clenching the teeth during sleep is called sleep bruxism.
It can range from occasional mild clenching to repeated episodes strong enough to damage teeth or leave the jaw sore.
The movement is involuntary
People usually are not dreaming about chewing something and deciding to grind.
Sleep bruxism appears to involve brief changes in nervous-system activity during sleep.
Jaw muscles activate strongly even though the person is not consciously controlling them.
Stress can make it worse
Stress and anxiety are commonly associated with increased clenching or grinding in some people.
But stress is not the only possible contributor.
Sleep patterns, medications, alcohol, caffeine, smoking, and other factors can also play roles.
Why does the jaw hurt in the morning?
Clenching muscles for long periods is work.
The jaw muscles can become tired and sore.
Some people also develop headaches or tenderness around the face.
Can it damage teeth?
Yes.
Repeated grinding can wear tooth surfaces, damage dental work, or contribute to cracks.
That is one reason dentists sometimes recommend a night guard.
The guard does not necessarily stop the nervous system from producing the grinding behavior, but it can help protect the teeth.
Is this related to other sleep movements?
Only broadly.
A hypnic jerk is a sudden muscle contraction during the transition into sleep.
Sleep bruxism involves repeated jaw-muscle activity during sleep itself.
Different behavior.
Different timing.
Does everyone who grinds know they do it?
No.
A partner may hear the sound first.
A dentist may notice wear.
Some people discover it only after waking repeatedly with jaw pain.
The jaw can become surprisingly busy during a night when the rest of you is supposedly resting.
And that’s the short explanation.
Related explanations: Why do we twitch when falling asleep?, Why do we wake during the night?, and Why do we feel tired after sleeping?