Melatonin is a hormone that is secreted by the pineal gland to regulate sleep in response to light and darkness. The rhythm of melatonin secretion is regulated by the body?s master internal clock, or ?biological clock,? a group of nerve cells in the brain?s hypothalamus known as the suprachiasmatic nucleus (SCN). Melatonin type 2 receptors of the SCN receive information from the eyes about the amount of light present in the environment. With darkness, more melatonin is released; eighty percent of melatonin production occurs at night.