Left/right breathing is a powerful yogic practice. As we age, so does our anxiety, so having tools to calm the mind and body are extremely beneficial. In addition to balancing both hemispheres of the brain, this practice balances both the sympathetic and parasympathetic branches of the autonomic nervous system.
Breathing through different nostrils has different effects:
Right-nostril breathing: Stimulates the sympathetic nervous system (fight or flight), stimulates the left brain, increases verbal performance, increases blood sugar, increases blinking, decreases intraocular (eye) pressure, increases heart rate, and inflates the right lung.
Left-nostril breathing: Stimulates the parasympathetic nervous system (relaxation), stimulates the right brain, increases spatial performance, lowers blood sugar, reduces blinking, increases intraocular pressure, decreases heart rate, and inflates the left lung.
Yoga masters believe the breath should flow evenly through both nostrils during meditation. This is why alternate nostril breathing precedes meditation in most yoga and meditation classes.
Not feeling balanced or feeling stressed? Try alternating your breath through both nostrils and see if it makes a difference in how you feel.
Reference: Yoga As Medicine, Timothy McCall