Pineal Gland and Melatonin Physiology

The pineal gland is a small midline structure in the brain that synthesises and secretes melatonin, a hormone whose release follows a daily rhythm: high at night and low during the day. Through this light-entrained pattern, melatonin conveys information about the time of day and the length of the night to the rest of the body, linking the pineal gland to the regulation of circadian and seasonal physiology.

Definition

Melatonin (N-acetyl-5-methoxytryptamine) is an indoleamine hormone synthesised primarily by the pineal gland from serotonin and secreted in a light-suppressed, night-time-high circadian rhythm that serves as an internal time signal.

Scope

The entry covers the synthesis of melatonin from serotonin, the neural pathway that links environmental light to pineal secretion, the hormone's circadian rhythm, and its role as a timing signal. It treats melatonin as a topic in endocrine and circadian physiology and does not provide dosing or treatment guidance.

Core questions

• How does environmental light control melatonin secretion?
• What is the biochemical pathway from serotonin to melatonin?
• How does the night-time melatonin signal convey circadian and seasonal timing?

Key concepts

• Pineal gland (epiphysis cerebri)
• Melatonin synthesis from serotonin
• Light-dark entrainment
• Suprachiasmatic nucleus (circadian clock)
• Circadian and seasonal rhythms
• Chronobiotic timing signal

Mechanisms

Light detected by the retina is relayed through the retinohypothalamic tract to the suprachiasmatic nucleus, the master circadian clock, which in turn controls the pineal gland via a multisynaptic sympathetic pathway. At night, in the absence of light suppression, pineal cells convert serotonin to melatonin through the sequential action of arylalkylamine N-acetyltransferase and a methyltransferase, releasing melatonin into the blood and cerebrospinal fluid. The resulting night-time peak acts on melatonin receptors to signal darkness, and the duration of secretion encodes night length, supporting both daily and seasonal timing. Melatonin is also produced at extrapineal sites, where additional functions have been proposed.

Clinical relevance

Melatonin physiology is central to understanding circadian rhythms, sleep timing, jet lag, and seasonal biology, and the pineal hormone is widely studied as a marker of circadian phase. This entry is for educational orientation and does not offer guidance on using melatonin or on diagnosing or treating sleep or circadian disorders.

Evidence & guidelines

Melatonin was isolated and chemically identified from the pineal gland by Lerner and colleagues in 1958. Subsequent physiology has characterised its light-entrained rhythm and its role as a chronobiotic, while reviews have examined extrapineal sources and antioxidant properties. The topic rests on this primary and review literature rather than on a single clinical guideline.

History

Aaron Lerner and colleagues isolated melatonin from bovine pineal glands in 1958 while seeking a factor that lightened amphibian skin, giving the hormone its name. Later work established the retina-suprachiasmatic-pineal pathway and the light-suppressed, night-time-high rhythm, framing melatonin as the principal hormonal output of the circadian system.

Key figures

• Aaron B. Lerner
• Russel J. Reiter
• Josephine Arendt

Seminal works

• lerner-1958

Frequently asked questions

Why is melatonin higher at night?

Light suppresses pineal melatonin synthesis through the retina-suprachiasmatic-pineal pathway, so secretion rises in darkness and falls in light, producing a night-time-high rhythm.

What does melatonin signal to the body?

Its timing and duration convey information about the time of day and the length of the night, acting as an internal clock signal that helps synchronise circadian and seasonal physiology.