The pineal gland is considered to be the smallest organ in the human body. Located deep inside the brain, the gland exists to produce a hormone vital in the regulation of the body clock.
Melatonin in the only hormone that is secreted by the pineal gland. Controlled by light, its function is suppressed during the daytime yet increased at night as we prepare for sleep. The full purpose of the pineal gland has yet to be understood. In the past, the pineal gland was believed to be "the third eye" because of its regulation by light. It was once regarded as the "seat of the soul, where all thoughts are formed," according to the philosopher Descartes, but this claim has long since been rejected.