Nasal cycle  The nasal cycle is the often unnoticed alternating partial congestion and decongestion of the nasal cavities in humans and other animals. It is a physiological congestion of the nasal conchae, also called the nasal turbinates, due to selective activation of one half of the autonomic nervous system by the hypothalamus. It should not be confused with pathological nasal congestion. The nasal cycle was studied and discussed in the ancient yoga literature of pranayama. In the modern western literature, it was first described by the German physician Richard Kayser in 1895. In 1927, Heetderks described the alternating turgescence of the inferior turbinates in 80% of a normal population. According to Heetderks, the cycle is the result of alternating congestion and decongestion of the nasal conchae or turbinates, predominantly the inferior turbinates, which are by far the largest of the turbinates in each nasal fossa. Turbinates consist of bony projections covered by erectile tissue, much like the tissues of the penis and clitoris. The turbinates in one fossa fill up with blood while the opposite turbinates decongest by shunting blood away. This cycle, which is controlled by the autonomic nervous system, has a mean duration of two and a half hours. He further observed and documented that the turbinates in the dependent nasal fossa fill when the patient is in the lateral decubitus (lying down on your side) 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 in order to prevent bedsores. Others note that the asymmetric airflow may have some benefit to overall olfactory sensitivity. The nasal cycle is an alternation in both time and between left and right sides, 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. It is possible that the nasal cycle may exacerbate the nasal congestion caused by the common cold, as the lack of motility of the cilia in one half of the nose may lead to an uncomfortable sensation of not being able to shift mucus by blowing the nose. It has been shown that the cilia of the congested side suspend their motility until that side decongests. Thus the cycle ensures that one side of the nose is always moist, to facilitate humidification, which is one of the three functions of the nose, the other two being filtration and warming of inspired air prior to its entering the lungs.