Teeth-grinding in teenagers could be a sign they are being bullied at school, research suggests.
An oral health charity said parents and schools should be aware of the problem, which can also affect adults who are stressed and anxious.
Teeth-grinding can lead to headaches, worn-down teeth and disrupted sleep - and it appears to be on the rise, experts say.
The sound of grinding has been compared to the noise from a circular saw.
A study published in the Journal of Oral Rehabilitation found that 13 to 15-year-olds who experienced verbal bullying at school were nearly four times as likely to suffer from teeth-grinding at night, or sleep bruxism, than other teenagers.
That equated to 65% among the bullied students, compared to 17% among the others.
The research looked at the experiences of more than 300 adolescents in Brazil.
Dr Nigel Carter, from the Oral Health Foundation, said bruxism was also something to look out for in the UK.
"Grinding teeth may not sound like a priority within the wider picture, but it could prove to give a vital insight into a child's state of mind and could be an important sign for us to identify bullying at an earlier stage."
He said sleep bruxism could be particularly damaging - but people were often unaware they were doing it.
"The first people know of it is usually when they wake up their sleeping partner," Dr Carter said.
The grinding together of the upper and lower teeth can lead to serious dental problems, such as sensitive and worn teeth, chipped or cracked teeth, the loss of teeth as well as pain in the face and jaw.
"Teeth-grinding is around 40 times more powerful than chewing...
"I've seen a man who had worn his teeth right down to the gums."
In most cases, grinders end up with flat areas on their teeth and frayed edges as the teeth start to become very thin.
And a tell-tale sign is a constant headache or sore jaw when waking up.
But the good news is that there are potential treatments.
Dentists can fit you with a dental appliance called a guard or splint, made of hard plastic, which specially fits your teeth and stops them grinding together.
The aim is to try and make the teeth sit comfortably together. When using the appliance, chewing is smooth and the teeth are protected.
Lifestyle changes, such as giving up smoking, cutting back on drinking alcohol and managing stress, are also advised to help improve the problem.
Teeth-grinding can also be caused by sleep disorders such as sleep apnoea and heavy snoring.
Dr Carter estimates that sleep bruxism could affect more than six million people in the UK.
If there is more stress in our lives than ever before, then it's no surprise we are taking it out on our gnashers.
During the day, it has been associated with housework or DIY tasks, and driving - the jobs we love to stress about.
BBC Health News