The scientific study of Yoga particularly is particularly appealing because the Yogis have described the complex interactions between biological anatomy and the metaphysical energetic structures. One of the main observations made by the Yogis, and a foundation for the development of numerous ideas in this essay is that the flow of air through the nostrils relates to the activity of the Ida, Pingala and Sushumma nadi. In particular, Yogis noticed that the breath does not usually flow through both nostrils at the same time. Instead, it cycles through each nostril throughout the day: at some points air is flowing more heavily through the left nostril while at other times, it is flowing through the right nostril. Modern scientists refer to this daily oscillation as the nostril cycle (Mohan M. S., 1991). There is a connection between the nadis and the nostril cycle: when the breath is flowing through the left nostril, Yogis claim that there is activity in the Ida nadi, while if it is flowing through the right nostril, the Pingala nadi is active. The implications of such connection are far reaching and can lead to astonishing insight about the workings of the human autonomic nervous system.
Moreover, the connection between the nostril cycle and the nadis is not limited exclusively to the ANS, but affects the whole physiology and is the foundation for health and longevity. In particular, Yogis explain that the flow in the nostrils should alternate every 1.5 hours in healthy human beings; and, in cases where the breath alternation becomes altered, illness is sure to come (Mukti Bodhananda, 1999, p. 8). Given the connection with the nadis, such a phenomenon is connected with abrupt changes in metabolic activity: when the breath is flowing through one nostril, it indicates that an organ or tissue in the body has excessive heat or cold (Vishnu-devananda, 1988, p. 242). The Hatha Yoga Pradipika, a sacred text on Hatha Yoga, states that “hiccups, asthma, coughing, pain in the head, ears and eyes and other diseases are generated by disturbance of the breath” (Svatmarama, 1971, p. 2:17). This is based on the belief that “incorrect operation of the nostrils creates tension and conflict as well as unnecessary activity” (Johari, 1989, p. 20). Physiologically, if either nadi predominates for an extended period of time, it is a sign that one of the branches of the ANS is overstressed and only half of the brain is being fully used (Mukti Bodhananda, 1999, p. 102). Furthermore, Yogis note that certain activities can affect the regularity of the nostril cycle and therefore certain activities are best performed when a particular nostril is operating (Johari, 1989, p. 23). Finally, Yogis believe that various techniques can be employed to change nostril dominance (see Appendix VI, pg. 81).
Based on the observations of the alternation of the breath, a branch of Yoga emerged which focuses on the alternation of the breath in relation to human energy structures (Mukti Bodhananda, 1999, p. 15). This science is referred to as Swara Yoga, which means the Yoga involving the sound of one’s breath (Mukti Bodhananda, 1999, p. 4). Swara Yoga emphasizes the analysis of the breath and pranic rhythms, while pranayama includes techniques to redirect, store and control prana (Mukti Bodhananda, 1999, p. 4).
The connection between metaphysical anatomy and the breath extends beyond the nostril cycle. According to Yoga, the breath is one of the most direct manifestations of prana, and because of this, breathing exercises are the foundation of pranayama, or the control of prana. This means that by controlling the breath by making it long and steady, all the functions performed by prana can be brought under the Yogis’ control, including the control of the body and mind. Since changes in the breath can be measured scientifically, the Western mind has all the necessary tools to investigate how the ANS can be controlled and the physiological changes that result.
Science and the Nostril Cycle
Research has corroborated that there is a direct connection between breathing and the autonomic nervous system as described by the Yogis. Specifically, breathing is controlled by the Vagus nerve: afferent fibers of this nerve produce the inhalation, while efferent fibers produce the exhalation (Rele, 1931, p. 5). Furthermore, the underlying process that causes the alternation of the nostrils is now well known. Scientists have discovered that congestion in one nostril is produced by vasodilatation due to parasympathetic activity, whereas decongestion is produced by vasoconstriction due to sympathetic nervous system activity (Mohan M. S., 1991). It has been proposed that the hypothalamus regulates the nasal cycle, as well as other rhythms in the body (Shannahoff-Khalsa D. S., 2007, p. 23). Such processes support the connection between the ANS and breathing, and explain why Yoga places such focus on the breath. Finally, supporting the claim that one can manipulate nostril dominance, Mohan confirmed that by adopting a particular lateral posture nostril airflow can be altered (Mohan M. S., 1991). However, many of the other techniques introduced by the Yogis to achieve such change have not been explored scientifically, probably due to the high level of Yoga experience required to achieve change nostril airflow.
Research has found that the alternation of the nostrils is a regular phenomenon among mammals (Gore, 2002). Even though the Yogis claim that the nostril cycle should ideally change every 1.5 hours, a study based on 50 participants reported that the mean duration of the dominance of one nostril was 2.9 hours, with cycles varying from 1 to 8 hours (Hasegawa & Kern, 1978). This discrepancy may arise because the Yogic estimate is based on observations of practitioners of Yoga who are healthy and who regularly focus on the control of the breath, whereas the population studies observed a small sample of people with no Yoga experience. Furthermore, the estimates are based on recordings of only 7 hours.
The discovery of breath alternation by the 19th century German scientist Kayser sparked interest in the nostril cycle in the West (Shannahoff-Khalsa D. S., 2007, p. 4). Since then the main approach to studying the nostril cycle has been studying the effects of Forced Unilateral Nostril Breathing (FUNB). In this technique, participants are instructed to block one nostril and breathe through the other for a period ranging between 1 to 30 minutes while physiological and psychological measures are being taken. The nostril to which participants are assigned is usually independent of the decongested nostril, that is, the nostril with greater airflow. Quantitative data can be collected during FUNB to measure nostril dominance and the respective effects on the brain, and the heart. Long terms studies of the nostril cycle have been very limited. Funk and Clarke (1980) conducted a study for one month, but found only weak patterns in nostril dominance and failed to identify possible factors that could explain the variability.
Measuring Nostril Dominance
In scientific and Yogic literature, various methods have been proposed to measure nostril dominance, with varying degrees of objectivity and accuracy. One can simply block one nostril and breathe normally and then alternate and breathe through the other. There will be one nostril through which breathing will be more comfortable, thus indicating the decongested nostril. Alternatively, one can place the hand vertically in the middle of the face with the fingers facing up touching the midline of the nose. By exhaling vigorously, there will be a clear airflow passing through the palm or the top of the hand. Individuals familiar with nostril dominance can simply breathe normally and note the cooling sensation produced by the air on the upper lip.
For more quantitative measures, one can exhale in a mirror and note which side fogs more than the other. Using more advanced scientific equipment, an exact measure of the percentage of air flowing through each nostril can be recorded. For instance, Gore (2002) used a two channel recorder apparatus in which each nostril is connected to a tube. By measuring the air flow through each tube, a sinusoidal representation of the nostril cycle can be recorded, with the amplitude of the wave indicating the level of congestion. Finally, using a differential pressure transducer, one can note changes in air pressure caused by differences in nostril flow (Mohan, Chandrasekhara, & Yoong, 2001). The last two methods can give a quantitative estimate of air flowing through each nostril. For most purposes, however, it is sufficient to record overall airflow dominance.
The Brain, the Heart and the Nostril Cycle
In exploring possible effects of the nostril cycle on the brain, it is essential to first understand the differences in cognitive processes attributed to each cerebral hemisphere. In very general terms, the left hemisphere is connected with language processing whereas the right hemisphere is more involved in visual processing. In particular, research reveals individual differences in the degree to which one cerebral hemisphere rather than the other is used to process information (Torrance, 1982). This is referred to as hemispheric preference.
The qualities assigned to each nostril are postulated to correspond with brain hemispheric functioning. In particular, Yogis have stated that when a particular nostril is dominant, the contralateral hemisphere is also more active: activity in the right hemisphere is coupled with airflow through the left nostril. This tight relationship has been demonstrated in several physiological and psychological studies. Physiologically, electrocortical activity in one hemisphere (measured by greater EEG power) relates to contralateral nostril dominance (Werntz D. , Bickford, Bloom, & Shannahoff-Khalsa, 1983). This relationship has been most thoroughly explored using Forced Unilateral Nostril Breathing (FUNB). For example, participants perform better in cognitive tasks that require rational and logical thinking, tasks associated with the left hemisphere, when they are breathing through the right nostril. They perform better in spatial tasks, associated with the right hemisphere, when breathing through the left nostril (Shannahoff-Khalsa, Boyle, & Buebel, 1991).
The nostril cycle also has a marked effect on neurotransmitters and hormones. In particular, FUNB has an effect on involuntary blink rates, which are directly related to dopaminergic activity. Under left FUNB, blink rates significantly increased, which suggests a relationship between right hemispheric preference and a possible lateralization of dopamine (Backon & Kullock, 1989). Studies have also shown that plasma catecholamines, such as norepinephrine, epinephrine, and dopamine, are related with patterns of nostril airflow. The ratio of plasma catecholamines from samples taken in each arm closely matches nostril dominance (Kennedy, Ziegler, & Shannahoff-Khalsa, 1986). Furthermore, the effects of nostril cycle are also manifest in the endocrine system. In one study, levels of pituitary hormones (adrenocorticoids, luteinizing hormones) as well as catecholamines varied according to the nasal cycle (Shannahoff-Khalsa & Yates, 2000).
In addition to the effects of the nostril cycle in the brain, research has also explored physiological effects, with an emphasis in autonomic activity, such as that of the human heart. Studies have shown that heart rate and mean arterial pressure in dogs varies in a cycle that lasts approximately 1.5 hours (Zehr, 1989; Shimada & Marsh, 1979). From this, researchers have concluded that the sympathetic nervous system drives the internal variations of the heart (Shannahoff-Khalsa & Yates, 2000). FUNB studies have shown that the nostril cycle affects intraocular blood pressure: right nostril breathing reduces it, and left nostril breathing increases it (Mohan, Chandrasekhara, & Yoong, 2001). Similarly, nostril dominance has been shown to affect blood glucose level: right FUNB increased blood glucose levels, while left FUNB decreased the levels. Homeostasis of glucose levels is regulated by the autonomic nervous system (Backon, 1988).
With modern advances in brain imagining and autonomic assessment measures, yogic observation provides a tool for deepening our understanding of brain and nostril dominance and their connections to the autonomic nervous system, thereby closing the gap between metaphysical energetic structures and accepted biological anatomical structures.
The Boundaries of Science
Studies on the nostril cycle have focused on immediate changes that result after the transitioning from dominance of one nostril to the other by using the technique of Forced Unilateral Nostril Breathing. Most of these studies take measurements over a short period of time, ranging from a few minutes to half an hour, whereas the conclusions reached by Yogis are based on observations of changes that result from long term patterns of dominance. Current studies have not addressed patterns of nostril dominance over long periods of time (there have been inconclusive studies at Yoga Ashrams), and the limitations of Forced Unilateral Nostril Breathing methodology in understanding nostril dominance have not been sufficiently recognized. The Yogis are aware that changing the nostril dominance has immediate effects on the mind and the body, but the emphasis of the practice is on cycling regularly, thereby preventing unconscious periods of dominance for extended periods of time. Based on the observation that individuals rely on one hemisphere more than the other, as well as the close relationship between the nostril cycle and brain hemispheric activity, I hypothesize that these long-term cognitive differences also manifest in the breathing patterns. Although all the short-run differences may seem small, they can result in very large and stable individual differences when extended across the lifespan.
There are some conceptual problems in relating the nasal cycle to both the parasympathetic and sympathetic NS and to the Ida and Pingala. For instance, Yogis state that ideally, the right nostril should be active when eating and evacuating, while the left nostril should be active during urination and drinking. This means that the sympathetic NS should be active when eating, which directly contradicts the known role of sympathetic NS to inhibit digestion (Rosenzweig, Breedlove, & Watson, 2004). Similarly, defecation a function with extrinsic parasympathetic nervous connections (Shafik, El-Sibai, & Ahmed, 2002), is recommended by Yogis to be performed under right nostril dominance (or sympathetic NS activity). Very likely there is an explanation to this apparent contradiction, but such examples lead me to believe that the Yogic understanding of the nervous system is highly complex. Another limitation to the scientific study of the nostril cycle is based on the metaphysical underlying structures thought to exist. Yogis state that the physiological correlates are only a small part of a very complex system. In addition, there are other methodological barriers to the study of Swara Yoga. Specifically, Yogis explain that the flow of the breath varies greatly depending on spiritual progress as well as on daily activities. Taking continuous measurements of the flow of the breath over an extended period of time is methodologically difficult.
There are also conceptual limitations that arise from the yogic texts. Being an ancient art, Yoga teachings were not written down explicitly and were passed on to a very select group of people through an initiation. Furthermore, not many texts are available in English, and the few that are can be easily misinterpreted if not translated by someone knowledgeable in Yoga. The references included in this text are mostly from modern gurus who base their descriptions on the knowledge passed through them by their teachers and in the yogic scriptures.
Suárez‐Rubio S. From Eastern Metaphysics to Western Psychology: The Brain, the Heart and the Breath. Cornell University (thesis). Ithaca, NY: Cornell University. 2007.