The breathing done in yoga calms the body and mind, through regulation of the autonomic nervous system; these effects have many positive impacts on health and mood. The effects are optimal when the breathing is deep, and maintained at 6 breaths per minute, for at least ten minutes. There are fundamental physiological effects underlying why practicing yoga makes you feel good; in addition, yoga-based practices may be useful in the treatment of a broad array of stress-related disorders.
The autonomic nervous system (ANS) controls the involuntary movements of the body, e.g. the beating of the heart, the pace of breathing, the actions of the digestive tract. The ANS has two parts working in balance: the sympathetic nervous system, and the parasympathetic nervous system. The sympathetic nervous system controls the ‘fight-or-flight’ response, energizing the body to prepare to deal with stimuli of danger (e.g. attack by a lion); among other things, it increases the heart rate, and stimulates the release of adrenaline from the adrenal glands, through nerves running parallel to the spinal cord. The parasympathetic nervous system controls calming effects, e.g. slowing the heart rate; it does this primarily via the vagus nerve, one of the largest nerves in the body running through the central part of the body. Retired Tufts professor J. W. Kimball observes “Although the autonomic nervous system is considered to be involuntary, this is not entirely true. A certain amount of conscious control can be exerted over it as has long been demonstrated by practitioners of Yoga and Zen Buddhism. During their periods of meditation, these people are clearly able to alter a number of autonomic functions including heart rate and the rate of oxygen consumption. These changes are not simply a reflection of decreased physical activity”.
Deep breathing stimulates the parasympathetic nervous system, and hence has a calming effect. It does this through the baroflex response, in which deep breathing increases lung chamber volume, in turn lowering blood pressure, which triggers pressure sensors in the aorta (baroreceptors) to send signals thru the parasympathetic nervous system to increase the heart rate. These variations in the heart rate (heart rate variability, HRV, also called respiratory sinus arrhythmia, RSA) can be easily measured. Studies have shown the optimal rate of breathing is 10 times per minute. (for a detailed review of HRV, see Grossman and Taylor)
This calming effect of breathing stimulating the parasympathetic nervous system is described as increasing ‘vagal tone’ (referring to the vagus nerve). A number of free iPhone or Android apps are available for measuring HRV, e.g. Stress Check from Azumio, which can also be used as biofeedback to improve breath control of the parasympathetic system.
Diminished parasympathetic function has been associated with a number of stress-related health conditions: epilepsy, depression, panic attacks, post-traumatic stress disorder, etc; yoga can help with these disorders. Chris Streeter, an NIH-funded investigator studying the health effects of yoga, states “It is proposed that as yoga-based interventions support the return towards optimal balance in the PNS and GABA system, function improves in regions of the brain that regulate response to threat, such as threat perception, interoception, fear processing, emotion regulation, and defensive reactions. As central regulatory systems become more balanced and flexible, allostatic load is reduced leading to health improvement”. (Streeter et al. See also the above citation of Grossman and Taylor). The abstract of that same paper concludes “This has far-reaching implications for the integration of yoga-based practices in the treatment of a broad array of disorders exacerbated by stress.”