Melatonin — The Hormone of Darkness

The secretion of melatonin is stimulated by darkness. Darkness also characterizes our understanding of the physiology of melatonin in humans. (The name derives from the hormone's ability to lighten the skin of amphibians, a property it is not recognized to have in mammals.) The article by Puig-Domingo et al. in this issue of the Journal, describing a patient with hypogonadotropic hypogonadism, hypermelatoninemia, and pineal calcification, provides an opportunity to let in some light. 1 Melatonin is the predominant product of the still-mysterious pineal gland. This gland is probably the only source of circulating melatonin, since plasma melatonin concentrations after pinealectomy are . . .