Studies of the 24 hour rhythm of melatonin in man

In a series of four separate studies of the 24-hour pattern of melatonin secretory function in man, the following results were obtained. Sequential measurement of the concentration of melatonin in plasma and urine demonstrated a 24-hour rhythm in which more melatonin is secreted during the sleep-lights off as compared to the waking-lights on period. However, during "free-running" and after an acute phase shift of the sleep-wake cycle, a melatonin rhythm can be dissociated from the sleep-lights out rhythm. A radioimmunoassayable plasma melatonin substance was found in significant amounts throughout the entire 24-hour day using a frequent sampling technique (every 20 min). Melatonin appears to be secreted into the blood in discrete brief episodes superimposed on a maintained "baseline" concentration. The half-life appears to be less than 30 min.