The doctrine of collateral activation was taken a bit farther by the ancient sages, to include arousal of the brain hemispheres. Yogic sages thought that forced lateralized breathing through one nostril, would effect a selective activation of one brain hemisphere over another. It would appears that nostril dominance originates from the brain itself.
The nasal cycle is an ultradian rhythm involving alternating breathing of the left and right nostrils,. It is known to have a cycle of two to eight hours (Keuning, 1968; Shannahoff-Khalsa, 1991). The nasal cycle is controlled by sympathetic/parasympathetic innervation of the nasal mucosa. When sympathetic activity to one side dominates, the result is vaso-constriction and thus decongestion on that side, while the enhanced parasympathetic activity on the other side simultaneously results in congestion (Keuning, 1968; Stocksted, 1953). Hence while the nasal cycle is regulated by the autonomic nervous system, it in turn influences the autonomic nervous system mechanism
Researchers at Nepal Medical College in Kathmandu measured the physiological effects of alternate nostril breathing (Nadi Sodhana ). They found significant increases in peak expiratory flow rate (exhale) and pulse pressure and decreases in heart rate, respiratory rate, and diastolic blood pressure.
There is no doubt that alternate nostril breathing can be a powerful way to quickly relax the nervous system, shifting the balance from sympathetic side to the more restorative parasympathetic. By slowing the breath, lengthening the exhalation, and pausing briefly after the exhalation, all tend to shift the balance towards the parasympathetic side.
In other words regular practice of alternate nostril breathing increases parasympathetic activity.Yoga Journal's medical Editor, Dr Timothy McCall talks about this in Part two of a three part series on "Yoga for chronic Pain"http://www.yogajournal.com/for_teachers/2561
My physiotherapist had suggested a long time ago that I do this style of breathing. At the time I was very hypersensitive and touching my face to close over a nostril was extremely uncomfortable. I know realize that I should have disregarded the unpleasant feeling for two reasons. One, the more I touched my face the less uncomfortable it would be and two, activating the parasympathetic nervous system is very beneficial for someone with CRPS/RSD. It calms and reduces pain.
My Intergrative Medical practitioner recently again suggested I practice this breathing, especially when agitated and in more pain. He explained that one nostril works on the sympathtic nervous system and the other the parasympathetic nervous system. He told me that there is a cycle of about 3 hours. Every 3 hours or so they switch sides sothat the side that was sympathetic became parasympathetic and so on. This style of breathing is very helpful for those, like sufferers who have a disturbance of the sympathetic nervous. This style of breathing helps to bring about balance. In so doing, it's calming. Calming, reducing stress lessens the perception of pain.