There is a yogic theory that we are born with an individual, set number of breaths. Once we reach the end of that number, we die. Some people obviously are given many more breaths than others. In many ways, this is something we have little control over. We can, however, control how quickly we go through our allotted number.
The pranayama work is one of the 8 limbs of yoga. Prana translates, roughly, to energy. Prana is your spirit, your stress, your drive and your sense of ease. Prana is your life force. Yama is the control of your prana. Therefore, pranayama is the control of your life force. Through breath work you are controlling and directing your life force.
From a more scientific perspective, slowing down your breath slows down your heart rate. The ability to control your breath sends a signal to your nervous system that you are in control. Being able to breath calmly makes you feel more calm. It prevents you from being trapped in that fight or flight, primal reactionary mode. Short, rapid, shallow breath is controlled by your sympathetic nervous system. Basically it tells your brain to release the signals to your body that you are in danger. Your heart rate increases, your ventricles constrict, you body gets ready to run or fight. Alternatively, deep, calm breaths are controlled by your parasympathetic nervous system, more specifically the Vagus nerve. To make a complex system overly simply, the Vagus nerve helps to calm us down (among other things). By choosing to take deep breaths even when we are feeling stressed out, we can begin to have more power over our reactions. We can slow down in the difficult moments, and choose a calmer, less primal reaction to whatever stresses us.
Through yoga, we put ourselves in difficult, even stressful, situations. We create moments where our heart rate will increase. And in the midst of that work, we ask our bodies to breathe more slowly. This rewires your nervous system to be more calm, in the face of stress. Through this practice, it becomes easier for your nervous system to be not jump into the fight or flight mode when you are stressed. This is (one of many reasons) why “yogis” seem so chill. This benefit does not have to only grow on your mat. Practice taking a few deep breaths the next time you’re stressed out, in traffic for example. Choose to calm your breath, and feel everything else slow down. You will be lengthening your life, controlling your prana, and rewiring your brain, one slow breath at a time.