BACKGROUND:The hormone melatonin plays a key role in the proper functioning of the circadian timing system (CTS). Exogenous melatonin has been shown to be beneficial in cases of CTS dysfunction and sleep disturbances. The aim of our study was to relate 24-h melatonin excretion to objective sleep measures. METHODS:A total of 67 individuals were included in the study: 29 healthy subjects (16 women, 13 men; mean age 62.4 y, range 24-86) and 38 outpatients with neuropsychiatric sleep-related disturbances (25 women, 13 men; mean age 46.5 y, range 21-69). Over two consecutive nights in the sleep laboratory, polysomnographic (PSG) recordings were made and urine samples were collected at predefined intervals. RESULTS:Our data failed to show any age-controlled partial correlation between 6-sulphatoxymelatonin (aMT6s) parameters and PSG parameters in either of the two groups. CONCLUSION:Measuring endogenous melatonin does not seem to be an adequate way to evaluate sleep quality. This could be due to the fact that the size of the pineal gland and the amount of melatonin produced vary 20-fold between individuals.