The pineal gland is a small endocrine gland located between the two hemispheres of the brain. It is about the size of a pea and shaped like a cone. It is responsible for producing the serotonin derivative melatonin. Melatonin is responsible for the sleep/wake cycle that we experience daily. The sleep wake cycle is effected by light that is sensed through the eyes, specifically through the retina. The pineal gland may play a role in sexual development. "The higher melatonin levels in children may inhibit sexual development." Tumors in the pineal gland have been associated with precocious puberty. This is when someone develops sexually before the typical age of puberty.
The pineal gland sits deep in the cranium posteroinferior to the hypothalamus and posterior to the brain stem. It is classified as being part of the endocrine system but some authors describe it as a "gland like" structure because it's exact function is idiopathic. Most agree though that it secretes a substance called melatonin. Melatonin "may" contribute to the bodies "internal clock" or circadian rhythm also called the bodies "biological clock". This internal clock is thought to be responsible for events that take place at approximately the same time everyday. For example, when you were a child, if you ate lunch at 11a.m. everyday, and you did that for years, when you reach adulthood your internal clock will signal hunger at 11a.m. This is also relevant to sleep/wake patterns.
Historically, the pineal gland was thought to have many different functions. For example, Rene' Descartes was a 17th century French philosopher and mathematician that concluded that the pineal gland was the seat of the soul.
The pineal gland is very small, weighing slightly more than 0.1 gram. It is located between the two cerebral hemispheres in the brain. The only hormone secreted by the pineal gland is melatonin. The amount of melatonin produced depends on the time of day and age. Melatonin levels are higher at night than during the day. In addition, children under the age of seven have much higher melatonin levels than adolescents and adults.
A rare disorder of the pineal gland are tumors. They are most commonly germ cell tumors that are malignant and can be life-threatening.