Mouth breathing really increases the chance of waking up with sinus pressure and a stuffy nose.
It might seem logical that mouth breathing occurs because the nose is congested, but that is not always the case. The brain of a mouth breather thinks carbon dioxide is being lost too quickly from the nose and stimulates the goblet cells to produce mucous in the nose to slow the breathing.
This creates a cycle of mouth breathing triggering mucous formation, nasal passage blocking, leading to more mouth breathing. So in fact, mouth breathing can cause nasal congestion leading to more mouth breathing.
When children breathe through their mouths during the day chances are that they also breathe through their mouths at night. Mouth breathing at night is directly connected to altered levels of carbon dioxide and oxygen in the blood stream. When less oxygen is able to reach the brain, learning and the ability to focus at school becomes a problem for many children.