Melatonin

What Is Melatonin?

Melatonin is a methoxyindole hormone synthesized and secreted principally by the pineal gland at night under normal light-dark conditions. Its primary physiological role is to convey information about the daily cycle of light and darkness to the body’s tissues.^[1]

The molecule is not confined to the pineal gland or to vertebrates. Melatonin is produced by bacteria, protozoa, plants, fungi and invertebrates, and in vertebrates also at extrapineal sites such as the gut, skin, Harderian gland and leukocytes, with biosynthetic pathways that appear to be identical.^[2] In humans, the retina, lymphocytes, bone marrow, gastrointestinal tract and thymus contribute smaller amounts.^[3]

Melatonin acts through two routes. Receptor-dependent actions include circadian rhythm regulation and sleep, while receptor-independent actions relate to its ability to neutralize free radicals in experimental models of oxidative stress.^[4]

Chemical Structure and Molecular Identity

Melatonin is N-[2-(5-methoxy-1H-indol-3-yl)ethyl]acetamide, an indole ring carrying a methoxy group at position 5 and an N-acetylated ethylamine side chain at position 3. PubChem lists it under CID 896 with the formula C13H16N2O2, a molecular weight of 232.28 g/mol, CAS number 73-31-4 and InChIKey DRLFMBDRBRZALE-UHFFFAOYSA-N.

It is synthesized from the amino acid tryptophan.^[5] Structurally it is a tryptamine derivative: the same indole-ethylamine skeleton as serotonin, with the 5-hydroxy group replaced by a methoxy group and the terminal amine acetylated. The molecule has no stereocenters, two hydrogen-bond donors and a computed XLogP of 0.8, consistent with a small, moderately lipophilic compound that penetrates tissues readily.^[3]

Reviewers describe melatonin as a small, highly conserved indole, reflecting its presence across very different organisms.^[4][2]

Metabolism

Circulating melatonin is mostly 6-hydroxylated by hepatic cytochrome P450 enzymes and excreted as 6-sulfatoxymelatonin. In other tissues, especially the brain, cleavage of the pyrrole ring is more important and yields N1-acetyl-N2-formyl-5-methoxykynuramine (AFMK), formed through enzymatic, photocatalytic and free-radical reactions. A secondary metabolite, N1-acetyl-5-methoxykynuramine (AMK), has been reported to support mitochondrial function and downregulate cyclooxygenase 2.^[2]

Research History and Development

Melatonin was first characterized as the hormone of the pineal gland, and research has since expanded from endocrinology to receptor pharmacology, sleep science and redox chemistry. A 1985 review described it as the epiphyseal hormone, noting antigonadotropic actions in animals and inhibitory effects on several endocrine functions.^[5][2]

Early clinical work was largely descriptive. Once assay methods became available, researchers found melatonin in urine, blood and cerebrospinal fluid, with low daytime and high nighttime concentrations. Sex had no influence on this pattern, while secretion was reported to be highest in summer and winter and lowest in spring and autumn. Beta-blockers inhibited the nocturnal rise, implicating adrenergic control, and abnormal secretion patterns were reported in several conditions, including depressive syndromes. At that time, the hormone’s physiological function in humans was considered obscure.^[5]

Later work identified the mammalian MT1 and MT2 receptors and led to a class of synthetic melatonin receptor agonists, including agomelatine, ramelteon and tasimelteon, as well as a slow-release melatonin preparation.^[6] Structural biology has since resolved the molecular architecture of human MT1 and MT2, and molecular simulations are used to identify new ligands.^[7]

Mechanisms Studied in Research

Melatonin’s best-characterized mechanism is activation of the MT1 and MT2 receptors, with additional receptor-independent antioxidant activity studied in experimental systems. Its effects are also shaped by the timing of its secretion.^[7][4]

MT1 and MT2 receptors

MT1 and MT2 are members of the G protein-coupled receptor superfamily. They preferentially couple to Gi/o proteins but can engage other G proteins depending on cellular context.^[7] Melatonin’s actions are also mediated by nuclear receptors and other binding sites.^[2]

Circadian signaling

The rhythm of melatonin secretion is generated by the suprachiasmatic nuclei and entrained to the light-dark cycle; light can either suppress or synchronize its production.^[1] Melatonin synchronizes the central clock and peripheral oscillators in tissues such as the adrenal gland, pancreas, liver, kidney, heart and gut, and its measurement is considered the best peripheral index of human circadian timing.^[8]

Sleep regulation

Studies with selective ligands and MT1 or MT2 knockout mice suggest that MT1 receptors primarily regulate REM sleep and MT2 receptors regulate non-REM sleep. A 2024 review argued that melatonin may influence the homeostatic process of sleep and sleep architecture, not only its circadian timing.^[9]

Free-radical chemistry

Melatonin and its kynuramine metabolites take part in free-radical reactions, and antioxidative protection and neuroprotection have been demonstrated in many experimental systems.^[2] Reviews describe protection of critical molecules in models of ischemia-reperfusion injury, ionizing radiation and drug toxicity.^[4]

Key Findings From Published Studies

The published literature on melatonin is dominated by physiological and mechanistic research, with many findings drawn from animal and cellular models rather than controlled human trials. The reviews cited here summarize the following themes.

• Human circadian physiology: evidence suggests melatonin stabilizes and strengthens the coupling of circadian rhythms, especially core temperature and sleep-wake rhythms. Seasonal effects in humans in temperate regions remain of limited evidence.^[1]
• Measurement: the nightly rhythm can be assessed by repeated plasma or saliva melatonin measurements or by urinary 6-sulfatoxymelatonin.^[1]
• Breadth of studied effects: reviews cover research on antioxidant activity, bone formation, reproduction, and cardiovascular, immune and body-mass regulation.^[8]
• Receptor-selective pharmacology: discovery of ligands selective for MT1 or MT2 is a stated goal for developing more targeted agents.^[6]

A 2023 review noted that deficiencies in melatonin production have been associated with conditions such as breast cancer and neurodegenerative disorders. These are associations, not demonstrations of cause.^[3]

Research in Alzheimer’s disease illustrates the gap between mechanism and clinical evidence. A 2025 review discussed possible effects on amyloid-beta and tau burden, antioxidant defense, immune modulation and circadian regulation, but concluded that additional randomized clinical trials and better characterization of long-term side effects are needed before clinical translation.^[10]

Reviewers also note that the distinction between physiological and pharmacological effects of melatonin is not always clear, and that many functions attributed to the hormone in humans rest on correlations between clinical observations and melatonin secretion rather than on direct experimental tests.^[1]

Because melatonin degrades through photocatalytic and free-radical reactions, researchers typically confirm identity and purity with HPLC and mass-spectrometry certificates of analysis.

Regulatory and Research Status

Melatonin itself is not an FDA-approved drug in the United States, although two synthetic melatonin receptor agonists, ramelteon and tasimelteon, are FDA-approved medicines. Melatonin products sold in the US have not gone through the FDA drug-approval process for any indication.

Outside the US, a slow-release melatonin preparation has been developed and characterized pharmacologically as a medicinal product.^[6] Agomelatine, another synthetic agonist, is discussed in the receptor literature but is not approved in the United States.

Melatonin remains a widely used research tool in chronobiology, as both an endogenous hormone and a marker of internal clock timing.^[8] Open questions include the specific roles of MT1 and MT2 in different brain regions and the long-term effects of exogenous melatonin, which reviewers identify as gaps for future study.^[9][10]