Most people experience a subtle, often unnoticed, shift in airflow between their nostrils throughout the day. This phenomenon, known as nasal cycling, is a normal physiological process. Your nose constantly adjusts its internal environment without conscious awareness.
What is Nasal Cycling?
Nasal cycling involves the alternating congestion and decongestion of the nasal passages, resulting in one nostril being more open for airflow than the other. Air is predominantly drawn through one side of the nose before gradually shifting to the other. In approximately 70% to 80% of healthy adults, this cycle naturally alternates airflow, with a typical duration ranging from two-and-a-half to six hours.
You can observe this phenomenon by placing a finger under each nostril to feel the difference in airflow. Alternatively, holding a small mirror beneath your nose and exhaling will reveal more condensation from the currently dominant nostril. This pattern is distinct from congestion caused by illness or allergies.
The Biology of Alternating Nostrils
The underlying mechanism of nasal cycling is regulated by the autonomic nervous system, which controls involuntary bodily functions. The sympathetic and parasympathetic branches of this system play opposing roles in regulating blood flow to structures within the nasal passages called turbinates. These turbinates are bony projections covered by erectile tissue, similar to other tissues in the body.
When the sympathetic nervous system dominates one side, it constricts blood vessels, causing the turbinates in that nostril to shrink and decongest, allowing for greater airflow. Simultaneously, the parasympathetic nervous system increases blood flow to the turbinates in the opposite nostril, causing them to swell and become more congested. This reciprocal swelling and shrinking, orchestrated by a central regulator like the hypothalamus, creates the alternating dominance observed during nasal cycling.
Why Do We Have a Dominant Nostril?
While the exact evolutionary purposes of nasal cycling are still being researched, several compelling hypotheses exist.
Preventing Dryness and Infection
One proposed function is to prevent the nasal passages from drying out and to protect against infection. By regularly alternating airflow, each nostril gets a period of rest, allowing its mucous membranes and cilia to remain moist and recover from continuous exposure to inhaled air. This helps maintain the nose’s ability to filter, warm, and humidify incoming air.
Optimizing Sense of Smell
Another theory suggests that nasal cycling optimizes our sense of smell. Different odor molecules are absorbed and detected more effectively at varying airflow speeds. The faster airflow in the decongested nostril may be better for detecting some scents, while the slower airflow in the more congested nostril allows for better detection of others. This difference in airflow through each nostril provides a wider range of olfactory detection. Additionally, the cycle may influence brain hemisphere activity and optimize airflow for different breathing demands.