Breathing is the one vital body function that we can control. Consciously taking slow, deep, controlled breaths activates the parasympathetic nervous system, sending an “all clear” signal to the brain and body. The sympathetic nervous system is the “fight or flight” response, while the parasympathetic system can be thought of as “rest and digest”.
Coherent breathing is a specific breathing method at a rate of five breaths per minute. This puts your breathing into the middle of the resonant breathing range which maximizes the heart rate variability, a way of measuring the activation state of the parasympathetic nervous system.