The pineal gland is a (relatively) tiny organ, part of the overall composition of the brain. In vertebrates such as humans, it sits directly in between the two hemispheres, within the epithalamus. Biologically, the pineal gland has a fairly straightforward purpose: to produce a hormone (melatonin) that signals the body when it is time to sleep or awaken. The pineal gland works in conjunction with light signals received from the eyes. In many reptiles, there is a small spot on the top of the skull that is sensitive to light; this spot is called the pineal eye or parietal eye, and it sends similar signals to the reptilian brain to the trigger wake/sleep cycles.
Because of its unique characteristics, the pineal gland is a popular subject in mysticism and the occult. The organ is concealed within the brain but not divided into halves as are most brain structures. Certain religions believe in a non-physical “third eye” located roughly between the eyes. This happens to coincide, vaguely, with the placement of the pineal gland. And, since the gland is related to the parietal eyes (light-sensitive skull sensors) of reptiles, it has been commonly linked to the concept of having a “sixth sense.”
Perhaps the strongest influence on the pineal gland’s mystical reputation is the work of René Descartes, who proposed that the pineal gland was the physical location of the soul. More recent suggestions are that the pineal gland secretes a hormone that causes an altered state of mind that doubles as a “religious” experience. Such was the contention of Rick Strassman’s Spirit Molecule book. Neither the pineal gland’s link to mystical experience nor its association with the soul has been verified or accepted by the scientific community at large.
Biblically, there is no reason to think of the pineal gland as anything other than a physical organ. The Bible does not give an explanation for how the body, soul, and spirit are connected. There certainly are no scriptural passages supporting the wilder claims about the pineal gland’s purpose. Attempts to connect this organ to spirituality come from pseudoscience, mysticism, and various versions of the occult. There are no scientific or scriptural reasons to make any such a link.