When people are under stress, the sympathetic nervous system takes over, which is part of the basic biology that helps you fight or flee. Heart rate goes up, and variability goes down. The heart gets “stuck” at a higher rate—contributing to the physical feelings of anxiety or anger that accompany the fight-or-flight response. In contrast, when people successfully exert self-control, the parasympathetic nervous system steps in to calm stress and control impulsive action. Heart rate goes down, but variability goes up. When this happens, it contributes to a sense of focus and calm.
