Data on the circadian melatonin secretion in sleep disordered patients and effects of sleep medication on melatonin are still missing. We studied plasma melatonin concentration, sleep, and effects of some hypnotics in 15 patients and 10 controls. Nocturnal melatonin levels were significantly decreased in patients with a more than five years history of sleep complaints compared to controls or patients with a shorter duration of illness. Independent of their sleep promoting properties drugs increased or decreased nocturnal melatonin in controls and patients. Patients with chronic sleep-wake rhythm disorders showed altered relations between their circadian melatonin secretion pattern and sleep. We conclude that nocturnal melatonin secretion is primarily independent of sleep regulation but represents a neuroendocrine feature of chronically disturbed sleep.