What is the pineal gland?

    The pineal gland is located in the lower central part of the brain. It may be all that remains of a third eye that ancestors of vertebrates (animals with backbones) once had. An ancient-type lizard in New Zealand (sphenodon) has a light-sensitive place on its head above the pineal body. Scientists now know that the pineal is a light-sensitive gland that makes a hormone.
    The pineal gland is rich in serotonin, a substance secreted by the nervous system. Recently a new hormone, melatonin, and an enzyme found only in the pineal were isolated. Research showed that the enzyme acted on serotonin to form melatonin, Mela­tonin acts on the sex glands to inhibit (stop) the sexual cycle.
    Light controls the amount of hormone produced through the sympathetic nervous system. The concentration of melatonin shows a 24-hour rhythm, decreasing during the day and increasing at night.

Pineal gland