Nostril breathing increases nitric oxide

[Vince]

Nitric oxide (NO) is made by your nose and sinus mucous membranes, so when you breathe through your nose, you carry a small amount of this gas into your lungs.NO is a potent bronchodilator and vasodilator, so it helps lower your blood pressure and significantly increases your lungs' oxygen-absorbing capacity. NO also kills bacteria, viruses and other germs, so nose breathing helps keeps you from getting sick.
When you breathe through your mouth, NONE of these functions can take place. Mouth breathing is analogous to expecting your body to make use of food by bypassing your stomach – it would be missing some critical steps in the digestive process, and the end result would not be good.
http://fitness.mercola.com/sites/fitness/archive/2013/12/27/proper-exercise-breathing.aspx

 

[Vince Carter]

and Im a terrible mouth breather...

 

[IronSmith]

and Im a terrible mouth breather...

I used to be as well. Took me years to train away that behavior, and correct sleep patterns that go hand in hand with it.

There are exercises and techniques you can use to make headway with it, one of the simplest of them is ensuring your upper and lower jaw meet in the middle, or try extending your lower jaw out in front of the upper for short periods of time.

That relaxed feeling of the lower jaw tucked in behind the upper is the worst culprit of air not passing through the nose properly with the way everything on the face and sinus works.

 

[KSPhD]

This sounded like hokum, but there appears to be some support for this in the scientific literature.
http://onlinelibrary.wiley.com/doi/10.1002/ar.20782/full