The alarm clock rings and you slowly lift your body from the bed. Your eyes are not fully open yet and there's a pain in your jaw that is so rigid you can feel the pressure right up to your temples.

"They are indications of teeth grinding at night - something you must tell your dentist about," says Professor Stephan Doering, director of the department of psychosomatic medicine in dentistry at the University Clinic of Munster, western Germany.

The medical term used to describe the unconscious act of teeth grinding and clenching of the jaws is bruxism. Only a small proportion of the people who grind their teeth at night are aware it's happening. "It's usually the partner in bed who complains first," says Doering.

During daylight hours, it's common for people with bruxism to clench their teeth when under stress: "While driving a car, taking an exam or when concentrating on a task."

There are several causes of bruxism. In many cases psychosomatic factors play a role, as well as poor stress-management abilities.

"It's the human version of snarling in animals," says Ingrid Peroz, who lectures in jaw-bone issues at Berlin's Charite hospital. "Unfortunately, people are not allowed to scream when they're in a stressful situation or are feeling aggressive."