Notice, for example, how you breathe when you are concentrating on accomplishing a delicate task, such as threading a needle. Chances are that you hold your breath. Breath-holding may actually help you steady needle and thread, but for most tasks it's counterproductive. Notice whether you also hold your breath when you are frightened, confused or hurried. You need more oxygen, not less, to think and act appropriately, so train yourself to inhale energy and exhale tension.
Shallow breathing is a typical physical response to anger, stress, fear, sadness, and pain.
It takes a conscious effort to breathe deeply when you're flooded by emotion, but by doing so you can control the spiral of emotion and physical response. Calm, steady breathing is a lullaby for body and brain - a signal that everything really is okay. When practicing deep breathing, inhale through your nostrils, because they filter and warm the air before it enters your delicate lungs. Some people prefer to exhale through their nostrils, while others exhale through the mouth. This is a personal preference, and it doesn't matter.