The pineal gland is considered to be the smallest organ in the human body. Shaped like a pinecone, the pineal gland is reddish-gray and about the size of a grain of rice (5-8mm) in humans. Located deep inside the brain, this endocrine gland (a gland that secretes its product or hormone directly into the blood) exists to produce a hormone vital in the regulation of the body's clock.

Melatonin, derived from seratonin, a mood neurotransmitter, is 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. Melatonin affects how we sleep, wake up and react to seasonal changes. 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 Rene Descartes, but this claim has long since been rejected.

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