The effect of clonidine and bright light on plasma melatonin

We studied the effect of the α₂-adrenoceptor agonist, clonidine, and bright artificial light (>2500 lux) on the nocturnal increase in plasma melatonin in normal subjects. Clonidine (1.5 μg/kg, intravenously) was without effect on plasma melatonin concentration. In contrast, bright light treatment abolished the increase in night-time melatonin. Bright light is a simple and effective means of altering melatonin secretion in humans.