Polysomnographic sleep patterns and melatonin secretion were investigated in 10 patients (age: 41.3 +/- 9.5 years) who suffered from chronic primary insomnia and complained predominantly about difficulties in maintaining sleep and in five healthy controls (age 27.2 +/- 0.7 years). Nocturnal plasma melatonin concentrations were obtained hourly, measured by direct radioimmunoassay and statistically compared between insomniacs and controls with age as a covariate. Plasma melatonin levels in the patient group tended to begin increasing earlier in the evening and were significantly (P < or = 0.01) lower during the middle of the night (peak value 82.5 +/- 26.5 pg/ml) than in the healthy controls (peak value 116.8 +/- 13.5 pg/ml). Among the patients, the most severely reduced nocturnal plasma melatonin levels were found in those patients with a history of sleep disturbance lasting for longer than five years (N = 6; age 41.8 +/- 11.7 years; duration 15.3 +/- 5.9 years; peak value 72.1 +/- 25.0 pg/ml); whereas those chronic insomniacs affected for fewer than five years had relatively higher nocturnal levels (N = 4; age 40.6 +/- 6.5 years; duration 3.8 +/- 1.5 years; peak value 98.2 +/- 23.9 pg/ml). These results show that the circadian rhythm of melatonin secretion is disturbed in patients with chronic primary insomnia, and that the nocturnal plasma melatonin secretion is increasingly more affected the longer the patients are unable to maintain a regular sleep pattern.