 nerve centers trigger an increase in the depth and frequency of breathing in an attempt to compensate for perceived high blood CO0 levels - although the blood CO0 levels have already dropped. By the time the respiratory control nerve centers detect the drop in blood CO0 levels and act to slow respiration, the blood CO0 levels have already increased. This cycle becomes increasingly unbalanced until respiration alternates between hypopnea and hyperpnea. The periods of hypopnea often become sufficiently severe that no breathing occurs between the periods of hyperpnea, i.e. periods of frank apnea occur between the periods of hyperpnea. The 