Neurosciences of unilateral nasal breathing

If you only have a few seconds:

Nasal breathing follows a cycle of alternating congestion/decongestion between the two nostrils. Unilateral nasal breathing involves blocking one nostril to breathe through only one. Nasal breathing has an effect on brain activity, but the scientific understanding is not clear yet. A recent study suggests that unilateral nasal breathing through the left nostril may promote a meditative state. But it’s probably more complicated than that so…

…if you have a few minutes, read on…

Some breathing techniques involve forcing nasal breathing through one nostril instead of two. This is known as unilateral nasal breathing (UNB). Depending on the exercise, this can be maintained or alternated between the two nostrils. Such techniques are emblematic of yoga and more particularly its branch called pranayama. In the theoretical framework of pranayama, they allow work on prana, or “vital energy”. Not being an expert in this theoretical framework myself, I cannot say much more. On the other hand, as a curious person versed in the sciences and more particularly in biology, I have regularly asked myself how to interpret these breathing techniques from a strictly biological point of view. For want of a better word, I had a very basic interpretation: blocking a nostril increases the resistance to the air flow; in order to breathe a sufficient volume of air, the person compensates by breathing slower and deeper. This stimulates the use of the diaphragm and thus its strengthening. In addition, depending on the individual, this may reveal a sensitivity to CO2 and allow more specific work to be started on this aspect.

It is understandable that such exercises can help many people. But these benefits are already provided by other techniques. So, it is legitimate to ask whether UNB techniques offer more specific effects, or even benefits, that other conscious breathing techniques do not. Specifically, the yogic techniques of UNB claim to influence brain activity. Yoga offers a wealth of traditional knowledge, embodied in practices and texts that are sometimes ancient and sometimes recent. While this knowledge is clearly worthy of attention, it is not always easy to approach it through the scientific method. Typically, the movement of prana does not currently have a scientific definition, and is therefore outside the scope of current scientific research. But when yogis claim that UNB can modify brain activity, it is a different story. Indeed, brain activity is an object of scientific research, precisely an object for neuroscience. So, we can hope to find some answers on such a subject. Recently, fellow spirophile Yvan Cam, who is also scientifically inclined, wrote an article on the subject, which I recommend you read. In addition, a scientific paper on the subject was published in Scientific Reports quite recently, in January 2021 (Niazi et al., 2021). This piqued my curiosity and made me want to write about it, to share with you what I understood about it.

Let’s set the scene.

Research is looking into the relationship between UNB and brain activity: does breathing through the left or right nostril only induce a particular brain activity? This question is interesting for several reasons. Indeed, if a particularly desirable brain activity is inducible by breathing through this or that nostril, then one should be able to set up such an activity through the appropriate UNB exercise. The consequences could even be therapeutic. In this respect, some research teams are already advocating UNB exercises as a non-invasive treatment for certain psychiatric problems or to help regain the use of speech after a stroke (Price & Eccles, 2016). It should be noted, however, that these protocols need to be confirmed.

Let’s take some time to unpack this.

Nasal respiration follows an ultradian cycle.

Biological systems (cells, tissues, organisms, ecosystems) have many interlocking cycles, which biologists like to define in a function of the length of a day. Thus, there are cycles of about one day, the circadian cycles. In animals, such as humans, they set the pace for eating behaviour, sleep-wake alternation, internal temperature, etc. There are also infradian cycles, which last longer than one day (menstrual cycle, hibernation, etc.). Finally, the ultradian cycles last less than a day. There are many examples, including certain hormonal secretions, the blinking of the eyelids, the heartbeat, etc… and the nasal cycle! The latter was apparently described for the first time in 1895 in the medical literature. Briefly, the two nasal passages consist of independent mucosa. Vaso-constriction or -dilation of the blood vessels that irrigate these mucous membranes results in congestion or decongestion of the nasal passage, respectively. The decongested nasal passage is in the working phase: the air flow is important and the air is conditioned to the right temperature and humidity before going deeper into the respiratory system. The congested nasal passage is in the resting phase: there is less airflow. The duration of this cycle is variable, between 2 hours and 4.5 hours depending on whether you are awake or asleep. Moreover, the duration of the cycle, but also its amplitude, is disturbed by external factors (environmental stimulus, body position, physical activity, etc.) (Kahana-Zweig et al., 2016). A good example of the intertwining of biological cycles is the nasal cycle, which varies according to the waking state, which is regulated by the circadian cycle.

At the neurological level, the nasal cycle is under the control of the hypothalamus. This leads to an excitation of the sympathetic nervous system which, via an oscillator in the brainstem, will be asymmetric between the two nasal passages. There is vasoconstriction on one side and vasodilation on the other. And so one side is more congested than the other.

How important is it to understand this nasal cycle? In fact, forcing one-sided breathing by blocking one nostril is only exaggerating a phenomenon that occurs naturally. So, if this cycle has one or more precise biological functions, we can imagine triggering them simply… by blocking one nostril.

Nasal and cerebral activity

The hypothalamus is therefore the conductor of the nasal cycle. But is this cycle connected to more complex parts of the brain? Furthermore, is this nasal cycle under the influence of the central nervous system or does it influence the central nervous system? And in particular, can a relationship be established between the nasal cycle and the cerebral cycle, i.e. the alternation of activity in the two cerebral hemispheres? This is where both classical yoga texts and neuroscience studies come in. And it is also here that things get complicated.

Classical yoga texts can be difficult to interpret and therefore difficult to test in a neuroscience laboratory. Moreover, neuroscience studies have not yet reached a consensus. Let’s take a closer look.

In 2016, Price and Eccles synthesised the state of the art on the relationship between nasal and brain activity (Price & Eccles, 2016). They approach the issue from two perspectives: top-down – brain activity determines nasal activity; and bottom-up – nasal activity determines brain activity.

Regarding a top-down determinism (brain activity determines nasal activity), they propose that the asymmetry of brain activity between the two hemispheres could be transmitted to the brainstem oscillators which, in turn, would transmit this asymmetry to the nasal cavities. This model is based in particular on observations of left-handed people and people suffering from schizophrenia or autism. Overall, the level of evidence seems to be quite low. So, at this stage, we don’t really know.

On the side of bottom-up determinism (nasal activity determines brain activity), there is a little more certainty. Indeed, in the nasal cavities, the trigeminal nerve detects the cooling of the air during inhale and this increases brain activity. This is confirmed by specifically stimulating the trigeminal nerve, whereas it is not observed by stimulating the olfactory nerves. So we are beginning to see the beginnings of a bottom-up relationship between nasal breathing and brain activity. Similar conclusions have been drawn from studies of certain forms of epilepsy, in which a seizure can be triggered by overexciting a nasal cavity. In short, there seems to be a bottom-up link between nasal and brain activity. But what exactly? A series of studies measuring brain activity, notably by electroencephalogram, during UNB sessions such as can be done in yoga. The results of these studies seem very diverse and sometimes conflicting. They are often difficult to interpret and relate to each other, as the studies are often carried out on very small cohorts, use quite different methods to facilitate comparisons and neglect certain confounding factors. It would be tedious to report them all, but some studies report a change in active hemisphere after about two minutes of UNB. Others report that left UNB stimulates the right hemisphere while right UNB does not… But some authors report the opposite, namely that right UNB stimulates the left hemisphere while left UNB has no effect…

In the end, the authors of this literature review conclude, with caution because the data still lack robustness, that it is likely that UNB from one nostril enhances brain activity in the opposite hemisphere (Price & Eccles, 2016). In their recent study, Niazi et al. tested this so-called ‘contralateral activation bias’ hypothesis (Niazi et al., 2021).

Nose: left, right. Brain: left, right, front, back

But Niazi et al. describe that brain activity is not limited to an opposition between hemispheres. Indeed, for a given hemisphere, activity in the posterior region is associated with relative rest in the anterior region, and vice versa. According to this model, brain activity can therefore be analysed along two axes: the left-right lateral axis and the anteroposterior longitudinal axis. This team studied the impact of UNB on the brain activity, by electroencephalogram, of 19 women and 11 men, aged between 20 and 53 years, right-handed (except for one ambidextrous) and in good health. It should be noted that, while their brain activity was being recorded, the subjects watched a silent film and were informed of a possible examination at the end of the session. The aim was to distract them from thinking about their breathing and not to induce a “mindfulness relaxation” effect. During the session, the participants’ nostrils were supplied with air in such a way as to introduce an imbalance between left and right air intake. The test lasts about 25-30 minutes: time to determine the subject’s dominant (decongested) nostril, then to feed it mainly for 10 minutes while recording brain activity, then to change the airflow in order to feed mainly the non-dominant (=congested) nostril still while recording brain activity.

The main results confirm that UNB does have an effect on brain activity. But what are these effects?

First of all, this study did not confirm the hypothesis that UNB would stimulate the opposite brain hemisphere, the “contralateral bias” mentioned above. In fact, it was shown here not only that both hemispheres were globally equally activated during right UNB, but also that both hemispheres were globally equally calmed during left UNB. Generally speaking, this indicates that the right UNB favours greater brain activity while the left UNB allows for rest, introspection and recovery. If we look in more detail, the left UNB does decrease brain activity globally, but locally it increases activity in the posterior region of the brain. Such an increase in activity in these regions is typically observed during periods of relaxation, repair and meditation. The authors indicate that the left UNB could thus be a tool to induce such a state, which corresponds to the functioning of the brain “by default”, which would be the signature of our “inner world”.

Let’s conclude.

This recent study is clearly of interest because it confirms that UNB modifies brain activity. Thus, it is one more piece of evidence that one can modify one’s brain activity through breathing, specifically unilaterally though the nose . Moreover, let’s remember that the subjects simply breathed while their attention was riveted by a film. So brain activity is modified by nasal breathing without even a “conscious breathing” type of practice. This is, in my opinion, a very important point of the study: brain activity is modified solely by UNB in the absence of a psychological component. In practice, this means that one could modify brain activity simply by plugging one nostril (with cotton wool for example) while doing tasks that occupy one’s mind (reading, watching TV, writing etc). This aspect may sound like a matter of convenience, but it could be crucial for people with poor control of attention/focus. It is also conceivable that the addition of conscious breathing exercises to UNB could extend the control of brain activity. For this, it would seem that the state of knowledge does not yet allow us to determine with certainty which UNB breathing protocol induces which effect on the brain.

One point that is somewhat obscure, in my opinion, in this study concerns the establishment of the left or the right nostril as the determining factor for modifying brain activity. In fact, it seems that the question is posed in terms of dominant/non-dominant nostril. However, in almost all the subjects in the study, the non-dominant nostril turned out to be the left nostril. This raises the question of whether the determining factor is the dominant/non-dominant or right/left parameter. If the determining parameter is dominant/non-dominant, then the authors’ conclusions should be taken with caution when dominance is left. As we have seen, the nasal cycle modifies dominance during the day. In addition, dominance can vary between individuals, with a possible (but cautious) bias in left-handedness (Price & Eccles, 2016).

Overall, this study, by confirming some studies but contradicting others, underlines how far the neuroscience of UNB still has to go before a consensus is reached. The coming years will undoubtedly bring new knowledge.

🔥❄️🧠✌️

Sébastien.

References:

Kahana-Zweig R, Geva-Sagiv M, Weissbrod A, Secundo L, Soroker N, Sobel N. Measuring and Characterizing the Human Nasal Cycle. PLoS One. 2016 Oct 6;11(10):e0162918. doi: 10.1371/journal.pone.0162918. PMID: 27711189; PMCID: PMC5053491.

Price A, Eccles R. Nasal airflow and brain activity: is there a link? J Laryngol Otol. 2016 Sep;130(9):794-9. doi: 10.1017/S0022215116008537. Epub 2016 Aug 1. PMID: 27477330.

Niazi IK, Navid MS, Bartley J, Shepherd D, Pedersen M, Burns G, Taylor D, White DE. EEG signatures change during unilateral Yogi nasal breathing. Sci Rep. 2022 Jan 11;12(1):520. doi: 10.1038/s41598-021-04461-8. PMID: 35017606; PMCID: PMC8752782.