Humans born with the condition of central hypoventilation during non-rapid eye movement sleep, termed congenital central hypoventilation syndrome (CCHS), invariably have absent or greatly diminished central hypercapnic ventilatory chemosensitivity. Genetic and pathological studies of CCHS may enable identification of the genes or areas of the central nervous system involved in the syndrome and thus implicated in central hypercapnic ventilatory chemosensitivity. Functional studies of CCHS permit a more quantitative assessment of the importance of ventilatory chemosensitivity in the regulation of breathing during wakefulness and sleep. The experimental evidence suggests that central hypercapnic ventilatory chemosensitivity is crucial in regulating alveolar ventilation during non-rapid eye movement sleep but not during rapid eye movement sleep or during many of the behaviors occurring during wakefulness. Presumably, other neural drives to breathe supervene to enable adequate ventilation. However, although physiological studies in CCHS subjects have been greatly instructive, their accurate interpretation will have to await future determination of the potential genetic and/or neuroanatomic basis of the syndrome.