A model for the study of the acute effects of melatonin in man

The role of the pineal hormone melatonin in human physiology is uncertain. Previous studies correlated plasma melatonin levels with several physiological parameters or determined the responses to pharmacological doses of melatonin during daylight hours. We established an acute model that is more rigorously physiological. Constant nocturnal bright light in sleep-deprived normal men resulted in low plasma melatonin levels throughout the night, in contrast to sleep in the dark and dim light sleep deprivation nights. Subsequently, melatonin was infused during bright light exposure to approximate physiological levels. Plasma GH and PRL measurements in these four conditions revealed an effect of sleep deprivation independent of the presence or absence of melatonin. A subsample of these men had an intermediate level of melatonin suppression with 500 lux light intensity, relative to those during sleep and bright light. The results suggest that melatonin has no acute modulatory effect on the secretion of these two sleep-related hormones.