Hypoventilation syndrome in very obese persons with excessive adipose tissue around the abdomen and diaphragm is characterized by diminished to absent ventilatory chemoresponsiveness; chronic hypoxia; hypercapnia; polycythemia; and long periods of sleep during day and night (hypersomnolence). It is a condition often related to obstructive sleep apnea but can occur separately. The disease is MONDO_0009763 (obesity-hypoventilation syndrome).