in DSM–IV–TR, a sleep disorder characterized by difficulty in initiating or maintaining a restorative sleep to a degree in which the severity and persistence of the sleep disturbance causes clinically significant distress, impairment in a significant area of functioning, or both.

The disorder is not caused by a general medical condition or the effects of a substance and is not exclusively an aspect of another sleep disorder or mental disorder. It is termed insomnia disorder in DSM–5. See dyssomnia. Compare primary hypersomnia.