Your nostrils work one at a time.

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When we breathe in and out of our nose during the day, one nostril does most of the work at a time, with the duties switching every several hours. This “nasal cycle” is dictated by the same autonomic nervous system that regulates heart rate, digestion, and other unconscious bodily functions and is the reason why—when our nose gets stuffed up—it does so one nostril at a time.