Take a deep breath.
The odds are good that you have been on the receiving end of the aforementioned recommendation more than once during your life. The odds are also good that, at least initially, this respiratory admonishment had the opposite effect from its calming intention.
No matter how you feel about the deep breath it is a folk remedy that has stood the test of time. But is there any scientific basis for the proposed stress-relieving effects of a slow, deep breath?
As you have likely surmised from the title of this article, there is robust scientific evidence for the benefits of mindful breathing. Let’s begin with an exercise and then delve into the physiology behind your breathing.
Find somewhere comfortable to sit. Sit in an erect posture and bring your chest forward, allowing your shoulders to fall back. Relax you gaze and rest your hands on your lap. Breathe with your abdomen, allowing your belly to expand on inhalation and contract on expiration.
Inhale through your nose for a count of four. Then exhale through your nose for a count of six. Repeat this process six times.
You have just slowed your respiratory rate to six breaths per minute and emphasized your exhalations. Adults breathe at an average of 15 breaths per minute so this exercise asks you to reduce your basal rate by a little more than half (1). Rather than getting caught up in the numbers, however, it is best to just try to take slow, deep breaths, emphasizing your exhale.
What is going on in your body during each phase of the exercise?
Your body contains two opposing peripheral nervous systems that act as a sort of gas and brake, speeding up and slowing down various functions of the body (heart rate, digestion, etc.). The parasympathetic nervous system (PSNS) is the brake in this analogy while the sympathetic nervous system (SNS) is the gas. (2)
For reasons beyond the scope of this article, your heart rate is primarily modified by the PSNS/"Brake" through the vagus nerve (3). This fact will become important in a little bit, for the time being, let’s proceed.
Your body is constantly trying to maintain equilibrium. When you inhale, blood is drawn from your heart into the vasculature of your lungs. This creates a relative deficit of blood for the rest of your body. Your heart compensates by increasing the heart rate and pushing more blood to your body. The increase in heart rate is made possible by decreased PSNS/"Brake" drive. (4)
When you exhale, blood returns to your body from your lungs and the heart slows back down as the PSNS/"Brake" drive increases. The increase in heart rate during inhalation and decrease during exhalation is known as respiratory sinus arrhythmia and is a sign of a healthy heart.
While your heart is busy fiddling with throttle, the lungs are hard at work as well. The lungs contain slow adapting pulmonary stretch receptors (SARs). SARs are activated by the expansion of your lungs during inhalation. This activation causes an inhibitory signal to travel to your brainstem and suppress the SNS/"Gas" (as well as the PSNS/"Brake"). This decrease in SNS/"Gas" drive effectively makes more room for the relaxing effects of the PSNS/"Brake" than the body would normally experience during more rapid, shallow breathing. (5)
There are a number of other processes that contribute to the respiratory balancing act between the PSNS/"Brake" and the SNS/"Gas," but they would create a novella out of a short story. So for the purposes of today’s discussion we will stop here and review.
The SNS/"Gas" is suppressed by the expansion of your lungs during inhalation and the PSNS/"Brake" is enhanced during exhalation as blood returns from your lungs to your body. By slowing your respiratory rate, you allow more time for your body to emphasize the two aforementioned mechanisms of self-regulation. And by emphasizing the exhalation you increase the amount of PSNS/"Brake" outflow.
The sum total of slow, deep breathing results in a relative increase in PSNS/"Brake" activity. And in a state of petroleum-fueled anxiety there is no better remedy than a biological brake. In fact, high PSNS/"Brake" tone has been associated with trait happiness, resilience in the face of stress, and childhood cognitive performance (6).
So the next time someone tells you to take a deep breath, smile, breathe deep, and heed their wise advice.