Light alerts the biological clock to slow down the secretion of melatonin hormone that promotes sleep. Melatonin hormone is a hormone that is produced by the pineal gland in animals and regulates sleep and wakefulness. . The hormone is used to regulate night and day cycles or sleep-wake cycles, darkness causes the body to produce more melatonin, which signals the body to prepare for sleep. Light decreases melatonin production and signals the body to prepare being awake.