" Whenever you inhale, the sympathetic nervous system is activated slightly, creating a slight increase in heart rate. Exhaling does just the opposite: turning on the parasympathetic nervous system and activating your vagus nerve slows the heart as you exhale. This is why many breathing techniques practiced in yoga are built around extending exhalations. The breathing technique in which one gradually makes the out breath longer works by progressively slowing the heart and thus aiding relaxation. "

― , How Yoga Really Works