The Pineal Gland

The pineal gland is a small, cone-shaped (hence its name, from the Latin for 'pine cone') endocrine structure attached by a short stalk to the roof of the diencephalon, deep within the brain, close to the point where the third ventricle narrows toward the midbrain. Unlike the pituitary, the pineal gland secretes essentially a single hormone of major physiological importance, melatonin, synthesised from the amino acid tryptophan via the intermediate compound serotonin.

The principal, best-established role of melatonin is the regulation of the body's circadian rhythm — the roughly twenty-four-hour internal cycle that governs the timing of sleep and wakefulness, along with a number of other physiological processes that fluctuate predictably across the day. Melatonin secretion is not constant through the day; it is driven by information about ambient light reaching the pineal gland via a dedicated neural pathway that begins with light-sensitive cells in the retina of the eye, distinct from the visual (rod and cone) photoreceptors used for seeing. During the hours of darkness, this pathway is not activated by incoming light, and the pineal gland's secretion of melatonin rises sharply, typically peaking in the middle of the night; as daylight returns and the retinal pathway is activated, melatonin secretion is suppressed once again, falling to a low daytime level. This nightly rise and daytime fall in circulating melatonin functions as the principal internal timing cue that entrains the sleep-wake cycle, and several other rhythmic physiological processes, to the external twenty-four-hour day-night cycle; because artificial light exposure late in the evening, from screens or indoor lighting, can suppress this natural rise, disrupted evening light exposure is understood to interfere with the sleep-wake cycle melatonin would otherwise help regulate. …