Only one of your nostrils is fully open at any one time. The nasal cycle means that one of your nostrils is constricted for various physiological reasons, and this swaps around every two and a half hours.
My wife warned me not to write about this, because once you’re aware of it you’ll find it very difficult to put it out of your mind. It’s like being reminded about your breathing – all of a sudden it switches to manual instead of automatic. So, I’m sorry in advance.
Basically, tissues inside the nose swell up and constrict a nostril, limiting airflow – although not blocking it off entirely. Try breathing in through your nose now and see which nostril is more open.
One of the functions of the nose is to humidify air passing through it (via the nasal cilia), and this alternation gives each nostril a chance to rest and not be moist. It’s also hypothesized that this enhances the sense of smell (because some smells require low airflow to detect and others fast airflow) and – because it alternates – encourages people to roll over in their sleep.
I don’t know enough about medicine to evaluate the veracity of these ideas, but I like the idea of my nostrils swapping roles every two hours. Nasal teamwork!