in DSM–IV–TR, a sleep disorder characterized by excessive sleepiness (evidenced by prolonged episodes of sleep, daytime episodes of sleep on an almost daily basis, or both), the severity and persistence of which cause clinically significant distress or impairment in functioning. The disorder is not caused by a general medical condition and is not an aspect of another sleep disorder or mental disorder. In DSM–5, it is known as hypersomnolence disorder. See dyssomnia. See also disorders of excessive somnolence. Compare primary insomnia.