Whether awake or asleep, nasal breathing is the way to go. While the nose can regulate temperature, provide humidity and smell, filter out harmful organisms, and protect the airway via reflex processes, its principal role is to operate as a resistor. To regulate the rate and duration of expiration when breathing, this resistor balances the impedance of the lower and upper airways. Pulmonary compliance and oxygen uptake are both enhanced by nasal airflow resistance. The nasal airway is a resistor, allowing for a more prolonged expiration during calm breathing. This gives the alveoli plenty of time to exchange gas, which includes recovering heat and water vapor. Conversely, breathing by the mouth is entirely voluntary, yet it reduces pulmonary compliance, ventilation, and oxygen uptake since there is less resistance to airflow. When the resistance to breathing through the nose becomes too high to provide enough oxygen during activity, oral breathing is believed to take its place (Morrison, Am Rev Resp Dis 148:606–611, 1993). To compensate for the decrease in nasal resistance, the accessory muscles of respiration increase pulmonary ventilation during exercise.