At first Ruth Heatherington thought her migraines, the cleaving headaches that had stalked her as a young woman, were coming back. Yet this time there was something more ominous than the ache — a burning in her face.

"I woke up with it every morning," said Heatherington, 54, of Long Beach, Calif. "One side of my face would be sore all day long."

It was the kind of pain that turns the simplest routines into maddening chores. A mother of five, with 10 grandchildren and a full schedule of family activities, Heatherington suddenly was concerned about her health.

The mystery was alarming. Was it something to do with blood pressure? Some kind of progressive disease? It was time to see a doctor.

But then her husband, Dale, made a curious discovery. He woke up one night and noticed an unsettling sound, like footsteps on gravel or a low growling or tearing noise. Tracing it to its source, he turned to his wife, now stirring, and said, "What are you doing?"

She made an appointment with a dentist, Morris Fink of Los Angeles, who made the official diagnosis: bruxism, or teeth grinding.

"Most people have no idea they’re doing it," Fink said, "and don’t like to admit it even if they do know."

Heatherington’s muscles were sore, her jaw ached, and after grinding through the night for months or perhaps longer, her molars were beginning to flatten. One was fractured, and had begun to tilt forward under the pressure.

"Like it was trying to escape," she said.

According to Jack Broussard, director of the University of Southern California’s Oral Health Center, stress by itself doesn’t cause teeth grinding, nor do sleeping problems, although grinding is usually worse at night.

Rather, bruxism is a central nervous system habit that almost everyone develops at some point during childhood.

For reasons no one understands, the repetitive side-to-side or back-and-forth motion becomes more pronounced in 15 percent to 20 percent of people, beginning as early as puberty or as late as retirement age. These people clench and grind most often — and with greater force — at night, during the dream-filled and most active phase of slumber, REM sleep.

Over months and years the grinding wears away crowns, flattens molars and can create small cracks in teeth, which is what alerts dentists to the problem. If untreated, teeth can fracture, requiring replacement or restoration, dentists say.

"Some people grind side to side with big, drastic movements that are very noticeable," Broussard said, "but others clench their jaws and go through small micro-motions that are not so obvious, so they might be getting headaches or facial pain and have no idea why."

Doctors have tried to treat bruxism with talk therapy, as if it were an anxiety problem. They have also tried techniques designed to improve and deepen sleep, from cutting down on caffeine and alcohol to increasing physical exercise, which can help.

Some doctors have suggested that people who are moody or anxious by nature are especially prone to the problem. But grinders are no more likely to be depressed than anyone else, it appears, and often they’re good sleepers.

That’s why dentists tend to favor intraoral appliances — plastic mouth guards, often called night guards — which are considered the standard treatment for bruxism. Most insurance plans will cover at least part of the $300-to-$800 cost of the devices.

After identifying the problem, Fink first fixed Heatherington’s broken molar, fitting a crown on the tooth. He then took an imprint of her upper teeth, and within weeks Heatherington had a mouth guard that snapped into place.

"The guards aren’t a cure; people usually continue to grind their teeth," Fink said, "but we can protect the teeth, and stop the damage."

Mouth guards don’t sit well at first for many people. It can take a week or more to get used to sleeping with a piece of plastic in your mouth, and some never feel comfortable. Some severe grinders wear out the hard plastic in a year or so, and need to be fitted all over again, while many others return every few years to be refitted.

"All I was worried about was that the thing wouldn’t stay in place, that it would keep falling out," Heatherington said.

No such trouble: In her case, the guard prevented more tooth damage, and seems to have reduced clenching and grinding.

"My face was sore for a little while, but then the pain was gone, and I could sleep well again," she said. "I got so used to the night guard that sometimes I’d forget to take it out when I woke up."

To stop the grinding itself, some dentists recommend simple exercise. In one, patients touch the tip of their tongue to the roof of their mouth, just behind the teeth, then open slowly, and close, without touching the teeth; this is repeated a half-dozen times.

Broussard said the exercise keeps the jaw joint properly aligned and trains the muscles to move as they should, while keeping people aware of the clenching and grinding, at least during the day.

Another technique puts the jaw in a natural resting position: Lick the lips, swallow and relax the jaw. In this relaxed position the lips and teeth are slightly parted.

After a couple of months, Heatherington began sleeping without the guard on some nights, to see what happened. The facial pain and headaches did not return, and she’s convinced that she’s grinding far less than before, if at all. Now she uses the mouth guard only two or three times a week.

"I just wear it when I’m feeling especially stressed," she said. "But just having it gives me a sense of security; psychologically, I feel I’m cured."