The pineal gland is a small cone-shaped gland believed to function as a body clock. The pineal is located deep in the brain just below the rear-most portion of the corpus callosum (a thick stretch of nerves that connects the two sides of the brain). The pineal gland, also called the pineal body, has mystified scientists for centuries. The seventeenth century philosopher, Rene Descartes, speculated that the pineal was the seat of the soul. However, its real function is somewhat less grandiose than that.
The pineal secretes the hormone melatonin, which fluctuates on a daily basis with levels highest at night. Although its role is not well understood, some scientists believe that melatonin helps to regulate other diurnal events, because melatonin fluctuates in a 24-hour period. Exactly what controls melatonin levels is not well understood either; however, visual registration of light may regulate the cycle.
Science EncyclopediaScience & Philosophy: Electrophoresis (cataphoresis) to EphemeralEndocrine System - History Of Endocrinology, Basic Endocrine Principles, The Pituitary, The Pineal, The Thyroid, The Parathyroids