Pineal gland

The pineal gland (also called the epiphysis cerebri or conarium) lies deep in the brain, behind the third ventricle, and is composed of parenchymal cells and interstitial cells. Its sole known secretion is melatonin, an amine hormone whose central job is regulating the body's circadian rhythm — the roughly 24-hour cycle of biological activity (sleep-wake pattern, body temperature, appetite, and more) that tracks the natural cycle of daylight and darkness. Melatonin is secreted mainly at night; light falling on the retina actively suppresses its production, which is why melatonin secretion tracks the light-dark cycle so closely and why long-distance travel across time zones (jet lag) disrupts it. …