Deep Breathing

Deep or Yogic Breathing is the single most important exercise because it helps us feel more energized, relaxed and attuned to ourselves. We inhale oxygen which sustains life and exhale carbon dioxide, but when we focus on deepening or lengthening each breath, we also become more present and less stressed. "The ancient yogis developed many breathing techniques to maximize the benefits of breath. Breath control helps clear and cleanse the body and mind".

http://www.yogasite.com/pranayama.htm