Monitoring of carbon dioxide (CO2) is employed in the identification of various hypoventilation syndromes during sleep.  Hypoventilation is defined as a level of ventilation that is lower than that necessary to meet the metabolic demands of the body.  Hypoventilation results in an increase in the CO2 level of the blood.  Increases in CO2 result in arousals from sleep to breathe more deeply so as to decrease/help expel the elevated CO2.  Increased CO2 during wakefulness is typically associated with increased sleepiness/fatigue.

CO2 monitoring is employed in overnight studies of children to detect prolonged intervals of obstructive hypoventilation (mild obstructive events accompanied by abnormal increases in CO2) not generally seen in adults, as well as in conditions afflicting children and adults in which nocturnal hypoventilation is suspected, e.g., in the muscle weakness associated with Amyotrophic Lateral Sclerosis, polio, Muscular Dystrophy, etc.); in various lung diseases (COPD, asthma, pulmonary fibrosis, etc.); in morbid obesity; and in restrictive thoracic disorders (e.g., kyphoscoliosis).