Though you may not pay attention to it unless you're sick, you're always breathing more heavily from one nostril than the other.

During the day, the sides switch and the other nostril goes into 'work mode', but why?

This process is automated by the aptly named autonomic nervous system, which is the same system that controls many things your body does all by itself such as digestion and heart rate.

For your nose, this system controls your 'nasal cycle', so that each nostril operates effectively.

The nasal cycle happens, according to the US National Library of Medicine, several times during the day, and is only brought to your attention if your nose is clogged up more than usual.

In order to open one side of your nose and close the other, your body inflates tissue with blood in the same way that a man gets an erection, except, you know, in your nose.

"Increased blood flow causes congestion in one nostril for about 3 to 6 hours before switching to the other side. There is also increased congestion when one is lying down, which can be especially noticeable when the head is turned to one side," Jennifer Shu reports for CNN.

It's believed that this cycle helps round out your sense of smell. As Matt Soniak reports for MentalFloss, some smells are better picked up by fast moving air through your nose, while others take more time and are detected better with slow-moving air.

If one side of your nose is wide open and the other is slightly closed, you get all of the smells.

The process also gives each side of your nose a break, since a constant stream of heavily flowing air can dry it out and kill off the small hairs that protect you from foreign contaminants.

When you get sick, the whole process can become unbearable, because the one nostril that is effectively 'turned off' feels way, way more clogged than the other, Soniak says.

Basically, the clogged-up feeling is just amplified by the cold.

So the next time you feel like you're only breathing from one side of your nose, remember that it's a natural, automatic system working to keep you smelling properly, and to make sure your nose doesn't get dried out by a constant onslaught of dirty air.

A version of this article was first published in February 2016.