It really all starts from infancy. As humans we are nose breathers. Yes, we can breathe through our mouths, but that is actually only for when we have a blockage in the nose, or we are in need of a higher volume of air exchange in the lungs than our noses alone are capable of channeling (i.e. allergies, deviated septum, intense exercise). When we are infants and breastfeeding, our noses should be clear. It is also important to be breast fed more often than bottle fed (best is to be exclusively breast fed). The suckling action of the infant stimulates and trains the proper peristaltic motion of the tongue for swallowing. Because the human nipple can deform and shape to the infant's mouth and tongue, the pressures exerted by the tongue are balanced throughout the mouth, and especially on the upper palate. With the nipple from a bottle, the rubber is too firm to deform properly and this can cause an improper arch as well as a vacuum effect in the mouth, which causes the palatal arch to be too narrow and too high; which in turn can cause tooth crowding and distortion. This causes the nasal passages to not develop wide enough to allow proper air flow through the nose. When this happens, the tongue cannot rest in its proper position (fully relaxed on the roof of the mouth). The tongue is almost like a retainer in that it consistently applies pressure to the teeth to keep them in line. This in turn can lead to a slightly occluded (blocked) airway when the tongue rests low and back in the mouth. This then causes the head to move forward to create more space in the airway. The common thought is that forward head posture is caused by modern technology. It is my belief that the technology just exacerbates a condition that already exists. This is why we do not see forward head posture in all people who use computers and cell phones for extended periods of time.
If allergies are an issue, or there is any nasal blockage then we will breathe through our mouths. There are many problems with this. First, the nose has innate, built-in protective measures (nasal hairs, mucus, increases the humidity of the air, increased nitric oxide production which kills pathogens). Second, the air flowing through the nose creates pressures in the cranial vault which allows for proper cranial development. Without the airflow, the palatal arch can raise up, the septum can deviate and drop, and the nasal passages can narrow (this is similar to the phenomenon of orca whales in captivity having drooped fins. They need the pressure of the water to evenly push on their fins while diving deep and swimming in long straight lines. In captivity they cannot dive deep and they swim in circles. The shape of a body structure is very much dictated by the environment. This will be discussed in a later post). Breathing through the mouth also creates an improper environment in the mouth where it becomes too dry and the pH balance gets disturbed, leading to things such as halitosis (bad breath), cavities, inflamed gums, as well as inflamed tonsils because they now have to do the protective job that the nose structures were supposed to be performing (think tonsilitis and strep throat).
When the tongue can sit properly on the roof of the mouth and proper swallowing mechanics are developed during infancy, then the arch develops properly, the cranial vault (structure of the skull that encompasses the brain) expands properly, and the airways stay open. The reverse is when the nasal passages are blocked due to inflammation, allergies, a deviated septum, or a downstream effect of improper arch development and we are obliged to breathe through our mouths. In the short term this is ok. That is why the body has built-in redundancy. However, if any of these conditions become chronic, then it can be detrimental to our health. This is not just true of young, developing humans, but even after we have reached adulthood. Our structures can change through consistent use (our bodies respond to forces, with the guidance of our genetics. Again, this will be in the post about function and structure). This means that even if you are currently a mouth breather, you have a narrow arch, allergies, deviated septum, etc., you can still make changes that will benefit you; learning proper tongue posturing, doing self releases on the muscles around your mouth, head, and throat, working on clearing any allergies, consulting a specialist about your deviated septum, and even taping your mouth shut at night while you sleep.
Bottom line, where you rest your tongue is more important than most people think. And breathing is arguably the most important thing that we do everyday. So pay attention to what your tongue is doing and how you are breathing over the next couple days. If you rest your tongue on the roof of your mouth and breathe through your nose; cool. If not, consider working on your tongue posture and alleviating whatever is blocking your nasal passages.