The nervous system has two branches: sympathetic, which is responsible for alertness, and the parasympathetic, which helps with relaxation. The vagus nerve is the parasympathetic nervous system’s main informant, letting it know when to activate, which it does by slowing the heart rate, increasing digestive motility, and lowering blood pressure and inflammation.
I’ve written a lot about the amygdala and its relationship with the fight, flight, or freeze response (the sympathetic nervous system). In the same way that oxygen from deep breaths cues the amygdala that it’s safe to stop sounding the alarm, deep breathing also signals the vagus nerve to activate the parasympathetic nervous system.
The vagus nerve takes clues from our breathing patterns and sends that information to the heart and the brain. Breathing slowly also slows the heart rate, which relaxes the body. The healthiest heart has a constantly varying beat, in a measure known as heart rate variability, or HRV. If you want to increase your HRV (yes, this is a good thing!) and reduce your stress, deep breathing with a longer exhale than inhale can help.
Here’s a technique to help you do it. Bonus: It’s quick and easy enough that even my 5-year-old students like to use it: