Because your nostrils are good buddies that take turns doing the work. No seriously. Even when you're not sick, nostrils split the workload, with the a majority of the air going in and out one nostril, and a much smaller amount passing through the other. "Every few hours, your autonomic nervous system, which takes care of your heart rate, digestion and other things you don't consciously control, switches things up and your other nostril does all the heavy lifting for a little while," explains Mental Floss . "...[B]ut when you're sick and really congested, the extra mucous often makes the nostril that's on break feel much more backed up."