We breathe to fill our lungs with oxygen, the fuel for our bodies, and release waste products from inside. When we become stressed, our bodies demand considerably more fuel. Hence, we start to breath more rapidly and our heart rate increases. However, we also breathe more shallowly so there isn't a very efficient and healthy exchange between getting fresh oxygen in and getting waste gases out. Poor breathing contributes in this way to anxiety and fatigue, and makes stressors harder to cope with.
Relaxed breathing is an effective way to manage stress.
Be aware of your breathing. Expand your abdomen as you breathe to the count of four, hold the breath for a moment, and slowly exhale to the count of four. This is a good technique to use when you get a stressful phone call, for example. Put the person on hold, take your breath, release it slowly and thoughtfully, then return to the call. You can also take a few deep, relaxing breaths while in a washroom cubicle, waiting at a traffic light, waiting for a meeting to begin, etc.