It sounds simple, and it is. So let's review the mechanics of deep breathing and how it helps us to relax. Their high level of tension was a means of preparing their bodies for optimum performance. However, since we're not running or fighting, our tension has no release, so our stress response builds. One way to counteract the stress response is to learn how to breathe deeply and slowly - the opposite of how we breathe when under stress.
Most adults breathe from the chest so less oxygen is take in with each breath.
Deep breathing can reverse these effects. Take some time to practice this kind of breathing each day, especially when you're under stress. You can be sitting, standing, or lying down, but it helps to wear loose, comfortable clothing. Begin by breathing in through your nostrils. Do this breathing for two minutes or more each time.