Watch this video and be educated!
It has all the basic info on mouth vs nose breathing. In a nutshell, we are MEANT to breath through our noses, not our mouths. I have battled, for as long as I can remember, to breath through my nose at night. It just seems to get stuffy and closed up. I have to sleep with a glass of water by my bed to quench my dry mouth at night.
But after this video suggesting that you can even tape your mouth shut at night with micropore tape to train yourself to nose breathe, I decided to try my darndest to do it naturally, with no tape. I spent a good half an hour or more last night, before bed, blowing my nose, trying to clear it, and just holding my mouth shut to "force" myself to breath through my nose. It wasn't easy but after a while, my nasal passages opened up and I was able to get air through both nostrils. And...I am super chuffed to report that I managed to sleep the whole night with my mouth shut.
How do I know? Because the twice I awoke for a wee, I could feel my mouth shut. I didn't need to drink as much water. And when I awoke this morning, my mouth was still shut and not dried out as usual.