The pineal gland is a pine cone shaped structure located in the diencephalon whose main function is the secretion of melatonin, a hormone that is best known for its role in regulating circadian rhythms. The pineal gland secretes melatonin throughout the 24-hour cycle, with secretion being highest in the middle of the night and lowest during daylight hours. In this video, I discuss the pineal gland and melatonin secretion, including 24-hour patterns of melatonin secretion and how the pineal gland uses signals from the retina about how much light is in the environment to determine what the time of day is.