Around the spinal cord, yogis say we have two big nerve channels, ida and pingala. Ida begins and ends at the left side of our spine and is related to a cool, lunar, feminine energy.
Pingala begins and ends at the right side of the spine and is related to a hot, solar, masculine energy. Those channels correspond to the two branches of the autonomous nervous system: ida is the parasympathetic branch that promotes inactivity and relaxation (vagal nerve) and pingala is the sympathetic branch that sustains dynamic activity and alertness. A study on unilateral nostril breathing showed that when we breath in only from the left nostril, the parasympathetic system is more active and the heart beat slows down. When we breath in only from the right nostril the sympathetic system is more active and the heart beat speeds up. Therefore, if we alternate nostrils while breathing we manage to equalize the activity of those systems and attain a neutral state that arises upon equilibrium of both energies in the body.