Endocrinology. Hormone produced by the pineal gland and released mainly at night in the absence of light on the retina. Changes of melatonin concentration regulate the circadian rhythm in humans and animals. Levels of melatonin decline with age. Melatonin is a potent endogenous free radical scavenger. It also stimulates the gene expression and activity of several important endogenous antioxidant enzymes, including superoxide dismutase, glutathione peroxidase and catalase.