Did you ever notice you don't have equal air flow through each nostril? Most people alternate between breathing through the left and the right nostril. The only time you are vividly aware of the existence of such a cycle is when you have congestion in just one side of the nose, which results in only being able to breathe more easily every few hours and then feel congested in the other period.

The first scientist, who wrote about this phenomenon in 1895 was a German nose specialist, called Richard Kayser.The phenomenon is still a little bit controversial and gives rise to amusement, because the tissue in each nostril resembles the tissue found in the penis and clitoris, viz. erectile tissue. There is even a condition called 'honeymoon rhinitis', which reflects the phenomenon your nose feels congested after prolonged sexual stimulation, since nose tissues can get excited just like the erectile tissues from sex organs of a honeymooning couple.

In most people, the erectile tissue in one nostril will swell, while at the same time, in the other nostril, it will shrink. So while one nostril is passing a lot of air, the other isn't. And then a little bit later it swaps over. This is called a 'nasal cycle'. A complete 'nasal cycle' takes anything from 40 minutes up to 8 hours, but on average lasts for 3-4 hours. Various factors modify nasal cycle, which include allergies, infections, exercise, pregnancy, fear, temperature humidity and emotions.
Other factors that affect a nasal cycle is being upright or lying. If you lay down on one side, then after about 12 minutes, the erectile tissue in the nostril on that side will begin to engorge and swell so the other side will breathe more easily. A 'nasal cycle' gets weaker though as we age.

The swelling and shrinking seems to be controlled by the central nervous system and related to both the sympathetic (SNS aka fight-or-flight response) and parasympathetic (PNS aka rest and digest response) nerves which are present on both sides of the body as well as the nose.
At any given moment you have sympathetic dominance on one side of the body (and breathe mostly through that nostril) and parasympathetic dominance on the other. Then some time later they switch. Basically the nasal cycle is an indicator of the switch that happens between the SNS and PNS from one side to another, which means you alternate between more active and more restful states.
When the sympathetic system is dominant on the right side and parasympathetic system is dominant on the left, you get dominance in the right nostril and right lungs. It doesn't stop here, but the entire body will be in a more active state with increased heart rate, higher blood pressure, respiration and body temperature and much more.
Once the switch is made and the sympathetic system is dominant on the left side, the left nostril and lung gets dominant, but most of the body will be in a less active state with decreased heart rate, lower blood pressure and body temperature and much more.
Fetuses and babies appear to have a sleep rhythm that closely follows the nasal cycle, which indicates this rhythm is more primitive than the regular circadion day-night rhythm. Even cognition is influenced by the nasal cycle. Right nostril dominance is correlated with enhanced verbal performance, or left brain activity, and left nostril dominance correlated with enhanced spatial performance, like the ability to navigate around objects.

Even though one nostril is less dominant, it is still capable of smell. There is a difference in perception though.
In order to smell, odour molecules must dissolve into the mucus of the olfactory epithelium. Some molecules dissovlve slowly and other molecules dissolve much faster. It just so happens that the non-dominant nostril with the low airflow will allow the body to pick up the smell of slow-dissolving molecules. Vice versa, the fast dissolving molecules will be able to 'touch down' on a larger part of the olfactory epithelium when they come in through the dominant nostril with the fast moving air flow. As a result they will stimulate the brain more.
It is thought the difference in perception of odour molecules is what allows the nose to get 'stereo' smell, just like how we can perceive direction with two ears.

If you are curious how to measure your own cycle, read up here. All you need is a small,clean mirror.

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