It's free and incomparably easy to practice, deep breathing is a pretty miraculous exercise that most of us take for granted. It can reduce anxiety, bring you into the present moment through mindfulness, and even help you remember how to respond to your specific stressors. Deep breathing psychological effects help slow down our racing minds and calms us down, as well as many physiological benefits.
Before understanding deep breathing's physiological benefits, you first have to grasp how your body responds to stress. As most people have experienced, when you're worried, upset, or anxious, you can feel it viscerally — your heart starts to beat faster and faster, you can feel dizzy, and blood rushes toward your heart and your brain.
The system responsible for this is your sympathetic nervous system, better known as your fight or flight response.
Evolutionarily, you'll only develop this stress response if you're being attacked by a predator. Over time we've experienced so much chronic, low-level stress on our day-to-day life, you can have this low-level activation of the stress response all the time.