Ok... I know you're thinking. "How can I possibly breathe 'wrong'?"
I asked myself the same thing when I first heard this. I thought it was one of those "woo whoo" things that always takes me some time to accept.
But I couldn't get that thought of breathing "wrong" out of my head. My yoga/Pilates instructor always talks about using the "yoga breath" or "Pilates breath," which are different. So... I started researching... was there even a difference between nose breathing and mouth breathing?
Well... wouldn't you know it. THERE IS! And not in the mean way you may be thinking ("mouth-breather").
When you breath thru your mouth, you trigger the fight or flight response in the body. Adrenaline, cortisol, elevated heart-rate, the works. Here's the short reason why:
When you breathe thru your mouth, you are bypassing the body's many filtering methods and taking in WAY more air than you would nasally, signaling that you are in desperate need of oxygen. Your body responds in kind to get out of whatever pickle you need out of right then... like that tiger that you're either trying to catch or evade.
When you breathe nasally, you tell your body you are in rest and digest mode. Plus... your breath... and all those airborne particles... are filtered thru the nose hairs and the sinus mucus. Gross, but an awesome job keeping that stuff out of the lungs.
This is why when you soften with nasal breathing. Try it. Take a breath thru your nose and feel all your muscles gently relax. Your mind calms momentarily. Your jaw unclenches. Your heart rate slows. I won't bore you with all the science, but when air flows over the mucus membrane in the sinus, it creates nitric oxide... the same medicine given to people for acute heart palpitations. COOL, right?
Now try it with a mouth breath. Doesn't work, does it? You feel exactly the same (although your body is escalating a response.).
There's a whole host of other issues with mouth breathing, too... like dehydration, dental decay, chronic fatigue and - I'm not sure if the science has proven that it causes sleep apnea, or if sleep apnea is a symptom of - but they're definitely related.
See a medical professional if this describes you (or if you snore). It could be a sign of a deviated septum, or something else that needs attention. There are treatments, including a CPAP machine that can help you get the rest you need.
In the meantime... start focusing on your breath. Can you give your body the breath it needs?