Pineal Gland

The pineal gland (or pineal body) is an important endocrine gland. It is a small, oval structure descending from the roof of the diencephalon, a section of the brain that relays sensory information between the brain's different regions. Although it's very tiny-only about six millimeters long-the pineal gland produces several important hormones. The most significant of these is melatonin, a hormone which regulates the circadian rhythm, or sleep cycle. In some of the lower vertebrates this...

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Full Pineal Gland Description

[Continued from above] . . . gland grows into an eyelike structure; in others, although it isn't a fully developed eye, it is still able to act as a light receptor. Because of this, the pineal gland is also known as the third eye.

Along with secreting melatonin, the pineal gland also regulates other endocrine functions and converts signals from the nervous system into endocrine signals. Melatonin production (or the lack of it) can contribute to a person feeling awake or becoming sleepy, and the pineal gland's endocrine function regulation can also influence sexual development.