Heart rate variability measures the relative balance between the sympathetic and the parasympathetic systems. When we inhale, we stimulate the SNS, which results in an increase in heart rate. Exhalations stimulate the PNS, which decreases how fast the heart beats. In healthy individuals inhalations and exhalations produce steady, rhythmical fluctuations in heart rate: Good heart rate variability is a measure of basic well-being.
