It's pretty easy to discover if you mouth breathe more than you think you do. During the day, just tape your mouth shut using medical tape for at least 10 minutes. Or just seal your lips. If you feel "different" at all, you're mouth breathing way more than you realize. That feeling is your body adapting in real time to new conditions like higher co2. I only share this with you because I realized it for myself. Mouth breathing (aka shallow breathing) impairs circulation and does lots of other crumby stuff. If you play sports or surf, shallow mouth breathing throughout the day negatively impacts your potential.