The fourth stage of raja yoga is breath control, or pranayama. Patanjali describes pranayama as “stopping the motions of inhalation and exhalation. The breath may be stopped externally, or internally, or checked in mid-motion, and regulated according to place, time and a fixed number of moments, so that the stoppage is either protracted or brief.” The purpose of pranayama is to control the mind. It also purifies the body and promotes longevity.
- Patanjali, Yoga Sutra 2:49-50, in Prabhavananda and Isherwood, How to Know God, p. 162.