The nasal cycle is the unnoticeable alternating partial congestion and decongestion of the nasal cavities in humans. It is a physiological congestion of the nasal concha due to selective activation of one half of the autonomic nervous system by the hypothalamus. It should not be confused with pathological nasal congestion. Although various aspects of the nasal cycle have been studied and discussed in the ancient Yogic Literature, in the modern western literature, it was first described by the German physician Richard Kayser in 1895.
In 1927 Heetderks spoke about the alternating turgescence of the inferior turbinates in 80% of a normal population. The turbinates in one fossa filled up while the opposite turbinates decongested. This cycle, which is controlled by the autonomic nervous system as described above, had a mean duration of two and a half hours. He further observed and documented that the turbinates in the dependent nasal fossa filled when the patient was in the lateral decubitus (lying down) position. Some postulate that this alternating positional obstruction has the purpose of causing a person to turn from one side to the other while sleeping. The nasal cycle is an alternating one, with the total resistance in the nose remaining constant. In patients with a fixed septal deviation and intermittent nasal obstruction, the interplay of the nasal cycle becomes evident; the sensation of obstruction frequently mirrors the congestion phase.
The nasal cycle's value becomes evident when one considers that the function of the nose is to warm, humidify, and filter nasally inspired air. These humidifying and filtering functions are dependent on the presence of moist respiratory epithelium, which contains both mucus-producing goblet cells and cilia, which are microscopic hairs that are covered with a layer of "mucus blanket" to which inspired particles stick and are swept (by the cilia) posteriorly into the throat to be swallowed. The presence of two nasal fossae, or chambers, that function in an alternating pattern prevents excessive drying, crusting, and infection, which are the likely results of a static passage that is open to constant airflow, especially in desert regions. (These consequences are observed in patients with atrophic rhinitis, in which the nasal mucosa and turbinates atrophy and cease to function, and in patients who have had overly aggressive turbinectomies.) "Empty nose syndrome" refers to noses which have been subjected to excessive surgical procedures that result in dryness, crusting, ozena (a constant foul odor), infection, and sometimes bleeding.
Investigators have demonstrated that, in a normally functioning nose, while one nasal fossa is congested, its ciliary beating slows in frequency or even ceases, in order for moisture to accumulate in preparation for resuming the nasal functions when that side decongests as the other side congests. Physiologically speaking, a human has two noses (making necessary two nostrils) that work in concert to provide constant humidification, filtering, and warming. The turbinates also slow the flow of air through the nose by causing turbulence, allowing more time for these functions to take place.
The nasal septum is the midline partition that divides the two nasal passages, or fossae. An absolutely straight nasal septum has seldom, if ever, been observed. Significant deviation of the nasal septum can cause abnormal patency of one side of the nose with corresponding narrowing of the opposite nasal fossa. Since the subjective perception of nasal airflow is dependent on some resistance to airflow, the patient with a deviated septum may paradoxically sense that the abnormally open side is more congested than the narrowed side. However, endoscopic exam in other cases sometimes shows that the septum is positioned in a cantilevered fashion, causing posterior narrowing of what is the abnormally "open" side anteriorly.
An interesting observation is that, by an unknown mechanism, the mucosa of an abnormally patent nasal fossa thickens, and the mucosa of an abnormally narrowed nasal fossa thins, in the body's apparent attempt to restore symmetry. The turbinate mucosa of an abnormally patent nasal fossa typically thickens, sometimes leading to the diagnosis of "turbinate hypertrophy", but its thickening is simply the body's mechanism for reducing abnormal patency and is not necessarily an indication for surgical reduction or amputation. If the passage is surgically reduced by surgically addressing the septal deformity, the turbinate size will adjust downward physiologically.
The dynamic portions of the nose (including the nostrils, vestibules, and lumen) can be narrowed due to the Venturi effect. Identification of collapsible nasal structures is important and should be addressed during preoperative review. Surgery can then be planned to widen the angle between the upper lateral cartilages and the septum with spreader grafts or to stiffen the nasal side wall with batten or umbrella graft placement.
The nasal cycle should not be confused with pathological nasal congestion: individuals with normal nasal breathing usually do not realize their breathing is asymmetric unless there is underlying nasal obstruction. In pathological conditions, however, the nasal cycle may influence the symptoms.
Research on the effects 
In 1994, breathing through alternate nostrils showed effects on brain hemisphere symmetry on EEG topography. D.S. Shannahoff-Khalsa published in 2007 on the effect of this cycle and manipulation through forced nostril breathing on one side on the endogenous ultradian rhythms of the autonomic and central nervous system.
- Josephson, J. S. (2006). Sinus Relief Now: The Ground-Breaking 5-Step Program for Sinus, Allergy, And Asthma Sufferers. Penguin Group. p. 15. ISBN 978-0-39953-298-6.
- Huizing, E. H.; de Groot, J. A. M. (2003). Functional Reconstructive Nasal Surgery. Thieme. p. 52. ISBN 978-1-58890-081-4.
- Swara Yoga, Swami Mukti Bodhananda, Bihar School of Yoga, December 1999
- Richard Kayser: Die exakte Messung der Luftdurchgängigkeit der Nase. Arch. Laryng. Rhinol. (Berl.) 8, 101 (1895)
- Cummings: Otolaryngology: Head & Neck Surgery, 4th ed
- Stancák A Jr, Kuna M (1994 Oct). "EEG changes during forced alternate nostril breathing". Int J Psychophysiol (Department of Physiology and Clinical Physiology, Charles University in Prague) 18 (1): 75–9. doi:10.1016/0167-8760(84)90017-5. PMID 7876041. Unknown parameter
- D.S. Shannahoff-Khalsa: Selective Unilateral Autonomic Activation: Implications for Psychiatry. CNS Spectr. 2007, 12(8):625-634