“It is not because you open your jaw that you breathe through your mouth, but because you want to breathe through you mouth that you open your jaw.”
The temporalis muscles are, by mass, among the strongest muscles in the body. As postural muscles, they act to lift the jaw and keep the mouth closed. They are inhibited by the actions of speaking, eating, and drawing in air through the mouth. As such, they may be inhibited in times of stress when more air than can be drawn in through the nose may be required. They may be inhibited when the body is retracted in fear responses and the head is pulled down on the neck. Otherwise, their action should be constant, even while sleeping – they should not simply stop working when you are asleep.
The notion suggested in some books on breathing that the jaw may simply fall open during sleep, leading to mouth breathing, should be questioned. Simply opening the mouth does not necessarily lead to mouth breathing. You can test this by breathing through your nose and opening your mouth – you should not begin drawing in air through the mouth. On the other hand, if you decide to breathe in through your mouth, the temporal muscles will release to let the jaw open.
If you find yourself breathing through your mouth at night, you might consider what you are doing that inhibits the action of the temporal muscles. Perhaps just the habit of curling up and retracting while you sleep causes you to open your mouth in a stress response. If you breathe through your mouth when you are awake, is it because your sinuses are blocked, or is it simply part of an habitual stress response, one that involves pulling the head back and down and shortening the neck and spine. Learning to inhibit such retraction during waking hours should eventually lead to the inhibition of such actions during sleep. However, it may take a while. Taping the mouth shut at night, which some recommend, might lead to awakening when the mouth wants to open, and thus learning not to begin the retraction that leads to opening the mouth, which will lead to more time allowing nose breathing – not a bad thing.