A once-repeated study of nocturnal plasma melatonin patterns and sleep recordings in six normal young men.

The concentration of melatonin was determined, using a sensitive and reliable radioimmunoassay, in plasma samples obtained at 20 min intervals during a 12 hr period (from 20.00 to 8.00) from six normal men. Polygraphic sleep recording was simultaneously performed. Each subject was studied twice at a 1 week interval. For each session, the plasma melatonin profile showed an episodic secretion: a mean frequency of 4.5 peaks and 4.0 troughs per night in the first study and a mean frequency of 4.0 peaks and 3.5 troughs in the second study. The two nocturnal melatonin profiles obtained from each subject were very similar. However, considerable interindividual variation was found (areas under the curve [AUC] from 15.3 to 125 pg X hr/ml). No relationship could be obtained between AUC and body weight. Apparent melatonin half-life calculated from the semilogarithmic plots of the melatonin pattern was 57 +/- 34 min. Chi-square testing revealed that the nocturnal pattern of melatonin levels was not related to sleep stages. Our data do not favor a direct relationship between melatonin secretion and the sleep-waking cycle in humans.