When you get a cold and have a stuffy nose, you may notice that only one nostril is stuffed up at a time, and which nostril that is changes throughout the day. That’s because we only breathe through one nostril at a time, even when we’re healthy. (The nostril that’s stuffed is just the one that happens to be resting at the moment.)
This is called the nasal cycle, and you can prove it to yourself by putting a hand under your nose. You’ll probably feel your breath more on one side than the other; if you feel it in both, you’ve caught the cycle during its transition. Wait a little while and feel again. I’m breathing through my left nostril right now.