Sleeping with your mouth open can make it as acidic as if you had just drunk a glass of orange juice, according to dental researchers who say this may promote decay and weakening of the teeth

The mouth's acidity level is typically neutral, with a pH of about 7.

Otago University post-graduate student Joanne Choi and colleagues experimented with 10 healthy volunteers by sometimes attaching a nose clip that made them breathe through their mouths.

The participants also had a small device clipped to their teeth to measure acidity and temperature and transmit the data to an external recorder.


Acidity increased slowly in everyone's mouths during sleep, but increased more over a longer period when participants were forced to mouth-breathe.

The average pH level during sleep with forced mouth-breathing was a mildly acidic 6.6, but the levels reached 3.6, which is about the level found after drinking a fizzy drink or orange juice.

This is significantly more acidic than the critical threshold of pH 5.5, at which tooth enamel starts to erode.

"This study is the first to continuously monitor intra-oral pH changes in healthy individuals over several days," says Ms Choi. "Our findings support the idea that mouth-breathing may indeed be a causal factor for dental diseases such as enamel erosion and caries."

When a tooth's hard enamel surface starts to erode -- or demineralise -- it begins to yellow as the softer material beneath, the dentine, is exposed and the tooth is left more vulnerable to wearing down. Caries are tooth decay or cavities caused by acids produced by harmful bacteria processing sugars in the mouth.

Ms Choi says the study helps to explain the observation of dentists that people who tend to sleep with their mouths open because of conditions such as sleep apnoea have higher rates of tooth decay.

Other than regular tooth brushing, she doesn't have any specific advice from the study, although she hopes to do further research to investigate what might help to address mouth-breathing acidity.

The study was published in the Journal of Oral Rehabilitation.