The common habit of grinding the teeth - bruxism - while asleep can have wide-ranging health consequences.
"The teeth are worn down, and the relief patterns on the crowns of the teeth go," says Thomas Wolf of the Free Association of German Dentists based in Bonn.
This exposes the dentine, which contains nerve endings. As a result the teeth become increasingly sensitive.
"The more you grind or bite hard, the closer you get to the nerve," he says.
In extreme cases, the jaw bones are affected, and the teeth can become loose and even fall out or crumble.
The unconscious grinding is as unpleasant for partners as running nails down a blackboard. While some grind, others bite, pressing the teeth hard against each other. Pushing the tongue up against the teeth, and sucking in the cheeks and biting the inside of the mouth are similar unhealthy nocturnal habits.
All of this takes place with enormous force, with the result that the facial muscles, particularly the jaw muscles, stiffen. This process also involves the back of the neck, through the shoulders and down to the back.
"Jaw muscles are among the most powerful in the body," German physiotherapist Dagmar Schlaubitz, who specialises in the phenomenon, explains.
The stiffened muscles can lead to headaches, tinnitus and problems with vision. Those most severely affected ultimately struggle to open their mouths, as the joint of the jaw is permanently damaged.
Wolf says the cause may lie in the teeth or the jaws being out of alignment.The cause may lie in job stress, a relationship under strain or tension. These can be controlled during the day, only to surface at night.
Wolf sometimes has a mouth guard similar to that used in contact sports made for his patients to wear at night.
"Manual therapy is best suited," Schlaubitz says. "The idea is to relieve the tension and ease the muscles." But grinders need to find a way to switch off and let go over the longer term.
Sufferers also need to monitor their waking behaviour. Teeth should not in general make contact, except when eating.There are various therapies. Wolf uses hypnosis. And many patients can be helped by consulting a psychotherapist. Exercise can help.