In a double-blind placebo-controlled cross-over study, 30 patients with Delayed Sleep Phase Syndrome (DSPS) were included, of whom 25 finished the study. Melatonin 5 mg was administered during two weeks in a double-blind setting and two weeks in an open setting successively or interrupted by two week of placebo. The study's impact was assessed by measurements of the 24-h curves of endogenous melatonin production and rectal temperature (n = 14), polysomnography (n = 22), actigraphy (n = 13), sleep log (n = 22), and subjective sleep quality (n = 25). Mean dim light melatonin onset (DLMO) (+/- SD), before treatment, occurred at 23.17 hours (+/- 138 min). Melatonin was administered five hours before the individual DLMO. After treatment, the onset of the nocturnal melatonin profile was significantly advanced by approximately 1.5 hour. Body temperature trough did not advance significantly. During melatonin use, actigraphy showed a significant advance of sleep onset and polysomnography, a significant decreased sleep latency. Sleep architecture was not influenced. During melatonin treatment patients felt significantly more refreshed in the morning. These results show that analysis of DLMO of patients suffering from DSPS is important both for diagnosis and therapy. These results are discussed in terms of the biochemistry of the pineal.