In his book titled Sadhana, the Path to Enlightenment, Swami Rama says:
"The language of the breath is very profound. The breath is the barometer of the body and mind, and registers any changes that take place in either. If you want to understand your mind, study your breath flow. The breath follows the mind, and the mind follows the breath. Physiological and biological changes occur because of disturbances in your thinking pattern. This is why negative thoughts, uncontrolled thoughts or passive thoughts, can all be injurious.
For example, if you are in agony, the breath will become jerky and irregular. Or if somebody gives you some sad news, your breath will become shallow and you may create long pauses. You are not able to control these disturbances because you have lost touch with the autonomic nervous system. But with breathing practices and pranayama you can directly experience and affect the pranic sheath, and consequently gain control of your body and mind.” (Pp. 261)