This sleep disorder is characterized by profuse sweating that occurs during sleep—and may or may not occur during waking hours. The sweating during sleep can cause an awakening because of discomfort due to the wet sleepwear, and the patient may have to arise to change into another set of nightwear. Excessive night sweats can be due to a chronic or febrile illness. Other patients appear to be healthy but can have a subtle and unrecognized autonomic disorder. This can occur at any age but is most commonly seen in early adulthood.

This disorder has been reported in association with diabetes insipidus, hyperthyroidism, pheochromocytoma, hypothalamic lesions, epilepsy, cerebral and brain stem strokes, cerebral palsy, chronic paroxysmal hemicrania, spinal cord infarction, head injury, and familial dysautonomia. It also can occur in pregnancy and can be produced by the use of antipyretic medications. Excessive sweating can be seen in patients with obstructive sleep apnea syndrome, presumably because of the associated autonomic disturbance.