Heart rate variability or HRV measures how your body responds to stress by measuring the beat to beat timing of your heart rhythm. The variation in heart rhythm is controlled by the Autonomic Nervous System, which also controls all of the “automatic” functions of the body such as digestion, hormonal regulation, blood pressure, and breathing. The Autonomic Nervous System works regardless of our desire and works without us having to think about it. The ANS is subdivided into two large components, the sympathetic, also known as the fight-or-flight mechanism and the parasympathetic nervous system, known as the relaxation, rest, and heal response.