Respiratory changes that occur during sleep are dependent on the staging of the sleep, during rapid eye movement sleep versus non rapid eye movement sleep. During sleep, the wakeful drive to breathe goes away and only the metabolic drive initiates ventilation. Central sleep apneas are commonly observed at sleep onset. Upper airway muscle tone is decreased, and there is increased resistance to the airflow. From wakefulness to non-REM sleep, resistance increases and then further increases to the highest level during REM sleep. Because of the reduced tidal volumes, the minute ventilation decreases, as does alveolar ventilation. In sleep, the functional residual capacity declines. Breathing is observed to be regular during non-REM sleep, but it becomes irregular during REM sleep. In wakefulness, both high CO2 and low PO2 increase the ventilation. Tese responses are dampened during sleep. Oxygen concentration in the blood, as well as the oxygen saturation, decreases slightly; however, total oxygen consumption and the production of carbon dioxide are decreased accordingly. In non-REM sleep as well as in REM sleep, cough reflexes are diminished.