The pineal gland, also known as the pineal body, conarium or epiphysis cerebri, is a small endocrine gland in thevertebrate brain. It produces melatonin, a serotonin derived hormone, which affects the modulation of sleep patterns in both seasonal and circadian rhythms.^[1][2] Its shape resembles a tiny pine cone (hence its name), and it is located in theepithalamus, near the center of the brain, between the two hemispheres, tucked in a groove where the two halves of thethalamus join.

Nearly all vertebrate species possess a pineal gland. The most important exception is the hagfish, which is often thought of as the most primitive extant vertebrate. Even in the hagfish, however, there may be a "pineal equivalent" structure in the dorsal diencephalon. The lancelet Branchiostoma lanceolatum, the nearest existing relative to vertebrates, also lacks a recognizable pineal gland.^[3] The lamprey (considered almost as primitive as the hagfish), however, does possess one.^[3] A few more developed vertebrates, including the alligator, lack pineal glands because they have been lost over the course of evolution.^[5]

The results of various scientific research in evolutionary biology, comparative neuroanatomy and neurophysiology, have explained the phylogeny of the pineal gland in different vertebrate species. From the point of view of biological evolution, the pineal gland represents a kind of atrophied photoreceptor. In the epithalamus of some species of amphibians and reptiles, it is linked to a vestigial organ, known as the parietal eye which is also called the third eye.^[6]

René Descartes believed the pineal gland to be the "principal seat of the soul" (a mystical concept). Academic philosophy among his contemporaries considered the pineal gland as a neuroanatomical structure without special metaphysical qualities; science studied it as one endocrine gland among many. 

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