The researchers note that dentists are reporting an increasing number of patients who complain of dry mouth, especially during sleep or upon awakening.
Mouth breathing during sleep can dry up saliva, which is an important defense mechanism for preventing the mouth becoming too acidic. Previous studies suggest that dry-mouth individuals run a higher risk of erosion than individuals with normal salivary secretion rates.
Acidity leads to loss of tooth enamel through erosion (the direct effect of acid without the influence of bacteria) and tooth decay or caries (the effect of bacteria breaking down foods to produce acid).
For the new study, the researchers decided to investigate what happens to acidity levels in the mouth during open- and closed-mouth sleep.
They measured pH and temperature levels in the mouths of 10 healthy volunteers as they alternately slept without and with a nose clip that forced them to breathe through the mouth.
The volunteers were fitted with a device that continuously measured the pH and temperature of the "palatal aspect of the upper central incisors" inside their mouths. They wore it for two sets of 48 hours.
To force them to breathe through their mouths as they slept, the volunteers wore the nose clip for two nights of the four. To balance any potential bias from the wearing sequence, five of them wore the clip the first night and the others wore it the second night at each of the 48-hour sessions.
The results showed that on average, daytime mouth pH was 7.3, and during sleep it was 7.0. The mean mouth pH during sleep with mouth breathing was 6.6, "which was statistically significant compared with the normal sleep condition," note the authors.