endocrine (endocrine system, human) gland found in vertebrates that is the source of melatonin, a hormone derived from tryptophan that regulates circadian rhythm ( sleep cycle). The pineal gland develops from the roof of the diencephalon, a section of the brain. In some lower vertebrates the gland has a well-developed eyelike (eye, human) structure; in others, though not organized as an eye (eye, human), it functions as a light receptor.

The pineal gland is located at the posterior end of the third cerebral ventricle in the midline of the brain. Its name is derived from its shape, which is like that of a pine cone (Latin pinea). In adult humans it is about 0.8 cm (0.3 inch) long and weighs approximately 0.1 gram (0.004 ounce). The gland is relatively large in children and begins to shrink with the onset of puberty. In adults small deposits of calcium often make the pineal body visible on X-rays.

The endocrine function of the gland is to produce melatonin. In several vertebrate species the pineal plays a major role not only in sexual development but also in hibernation and seasonal breeding. In humans, however, its function is less well understood. Melatonin production is high in infancy and childhood and declines with age. High levels of melatonin in children may contribute to delay of sexual development. Melatonin secretion increases during darkness and is inhibited by light. Disordered melatonin secretion may contribute to sleep disorders, jet lag, and depression. However, administration of melatonin has very limited efficacy in the treatment of these problems.