Although we don't usually notice it, during the nasal cycle one nostril becomes congested and thus contributes less to airflow, while the other becomes decongested.

No one is sure why the nasal cycle occurs, Benninger said.

Most people aren't aware of the nasal cycle, Benninger said.

If a person lies on their right side, for example, gravity will cause that lower nostril — the right nostril — to become more congested?

If the cycle has designated the right nostril to be naturally more congested at the time, there's no appreciable effect?

But if the nasal cycle has made the left nostril more congested and the right nostril is congested because of side sleeping, breathing may be difficult, and the person may wake up.

Usually, people notice the cycle only if they have consistent blockage of one side of their nose, Benninger noted.

Although your nose may feel stuffy when you have a cold, that’s not due to the nasal cycle.

In general, both nostrils are congested when you’re sick, so you’re going to have trouble breathing through your nose no matter where you are in the cycle, Benninger said.

There are ways to decongest both nostrils at once, which can temporarily lead someone to breathe more equally out of them until the nasal cycle resumes.