The pineal gland is an obscure, mysterious pea-sized organ tucked deep inside our skulls. It helps regulate our sleep patterns by secreting a chemical known as melatonin. Although other organs play a role in what's called circadian (daily) rhythm, the pineal gland sets the pace. But exactly how it does this, and its other functions, are a bit maddening to unravel.

Melatonin signals the body to start preparing for slumber. It also appears to have a role in starting some of the body's reparative or growth functions during sleep, such as building bone. Because of melatonin's function in setting the body's internal clock, the hormone is sometimes helpful for treating insomnia, especially in older people. Melatonin production declines with age. It's also useful for coping with jet lag. Melatonin can be purchased over the counter.

Melatonin is made from serotonin, a neurotransmitter whose lack is associated with depression. This implies a link between the level of light one sees and depression. One form of depression, called seasonal affective disorder, or the "winter blues," is believed to be caused by a lack of sunlight. Sunlight is thought to stimulate serotonin production.

Scientists have reported many other physiological effects from melatonin, or its absence. But these findings are still not entirely accepted, and details from the studies are still being worked out.

Certain kinds of cancers, such as breast or prostate malignancies, are associated with low levels of melatonin production. Some laboratory tests show that adding melatonin reduces the growth of cancer cells. And some studies have found that melatonin helps reduce seizures in children with epilepsy, although the Mayo Clinic cautions that at least one study found that melatonin actually increases the risk of seizures.

Light inhibits the pineal gland from secreting melatonin, and darkness stimulates its production. This happens via a convoluted system. The retina signals the pineal gland through a chain of intermediary structures, such as the hypothalamus, another structure in the brain, and the signal goes from there to the pineal gland. It seems as if it would be more efficient if the retina directly secreted melatonin; however, the chemicals needed to make melatonin are damaging to the retina. Hence, melatonin production is segregated from the retina.

How did this system arise? A theory advanced a few years ago postulates that melatonin was originally junk material produced by primitive creatures about 500 million years ago. These creatures were the ancestors of all vertebrates, that is, mammals, reptiles and fish.

David Klein, a neuroendocrinologist, proposed that melatonin was manufactured in the eye as a chance by-product when toxic substances were inactivated. Around 500 million years ago, animals started to use melatonin as an internal indicator of when it was dark. That posed a difficulty, because increased melatonin production was necessarily associated with the toxic precursors. So the pineal gland evolved to make melatonin. If you're interested in the details, they are on the Web at tinyurl.com/2kdgeb.

Klein pointed to a number of facts to support this theory, such as the strong resemblance between pineal cells and the retina's photoreceptor cells. Some other animals, such as mice, fish and frogs, have retinas that make low amounts of melatonin. Primate retinas have lost this ability, in keeping with Klein's theory.

If much of this seems tentative and heavily qualified, it's because of the pineal gland's obscurity. It's not an obviously vital structure such as the heart or lungs, so ideas of its function have been often rather fanciful. Mystics claim the pineal gland plays a role in supposed psychic abilities.

The 17th-century French philosopher Rene Descartes picked the pineal gland as "seat of the soul." However, his reasons have been disproven. Descartes thought that only humans have pineal glands; but many animals have them. And he reasoned that since the soul is one substance, and the pineal gland is unique among brain structures in not being divided into halves, ergo, the pineal gland houses the soul. But closer examination has found the elusive structure does have left and right hemispheres.

Contact staff writer Bradley J. Fikes at (760) 739-6641 or bfikes@nctimes.com.