The pineal gland is a miniscule gland located in the human brain which secretes melatonin in response to light. It is located at the top of the spinal cord where it ends inside the brain, just above the cerebellum. It is about the size of a grain of rice. A great deal of blood flows through it, similar to the kidneys, so that the different chemicals it puts out can circulate through the body.
This gland is also referred to as the "third eye." The name does not connote psychic abilities. Instead, certain similarities to the light-sensing retinas of the human eye have led evolutionary scientists to theorize that it evolved from the same organ. In some animals, there is even a small hole in the skull through which light can reach their equivalent of the gland. In human beings, receptors in the eyes lead directly to the gland.
The role of the pineal gland in the human body is still being explored. It is thought to send out varying levels of hormones in response to seasonal changes in light, and in some animals, it regulates the related breeding urges. In humans, tumors in the pineal gland can sometimes cause a too-early onset of puberty.
Primarily, the pineal gland secretes melatonin, without which a person cannot sleep. The 24-hour sleep-wake cycle in humans is regulated by the gland producing more melatonin in response to light. In nocturnal animals, the process is reversed so that they come awake in response to darkness. For this reason, melatonin supplements are prescribed for people who have severe insomnia and for those with jet lag.
It is thought that many other disorders may be related to the gland's melatonin production. Those with bipolar disorder, depression, or obesity have achieved some relief after being treated with melatonin, but results are so far inconclusive. Disruptions in melatonin production may even cause effects as diverse as headaches, cancer, and gallbladder stones.
The pineal gland has somewhat of a romantic history in medicine. Early doctors and anatomists dissected it, but were unable to discover its purpose. Its similarity to an eye was debated. The philosopher Descartes pointed out it had to be of importance because of its singularity. His reasoning was that in the symmetrical human brain, all parts are doubled, but the gland had no counterpart. Therefore, he theorized that it was the seat of all original thoughts in the human mind.