Melatonin is a derivative of serotonin best known for its role as a hormone. It is produced in a small endocrine gland in the brain called the pineal gland, as well as in the retina and a number of other organs and cells. The melatonin produced in the pineal gland, however, is thought to contribute most to circulating levels of melatonin in the bloodstream. This video discusses various aspects of melatonin, including its synthesis, functions, and receptors.