In vertebrates, melatonin's functions extend to synchronizing sleep-wake cycles, encompassing sleep-wake timing and blood pressure regulation, as well as controlling seasonal rhythmicity (circannual cycle), which includes reproduction, fattening, molting, and hibernation.[4] Its effects are mediated through the activation of melatonin receptors and its role as an antioxidant.[5][6][7] In plants and bacteria, it serves as a defense mechanism against oxidative stress, indicating its evolutionary significance.[8]Mitochondria, key organelles, are the main producers of melatonin,[9] underscoring its "ancient origins" and its fundamental role in protecting the earliest cells from reactive oxygen species.[10][11]
In addition to its endogenous functions as a hormone and antioxidant, melatonin is also administered exogenously as a dietary supplement and medication. Melatonin is used medically primarily for sleep-related problems: for example, prolonged-release melatonin (Circadin) is approved in several countries for short-term treatment of insomnia in people aged 55 years of age or older.[12] It is used in the treatment of sleep disorders, including insomnia and various circadian rhythm sleep disorders.
Numerous studies suggest that disruption of melatonin production may underlie the association between light exposure before and during sleep and impaired sleep quality.[13][14] It has also been suggested that light-induced disruption of melatonin production may potentially affect cognitive, emotional, cardiovascular, and metabolic functions.[15]
Visible light entering the eye and the cascading positive and negative signalling pathways to neuronal structures in the mammalian brain that may follow: When the eyes are exposed to sunlight, the pineal gland's melatonin production is suppressed, resulting in the secretion of hormones that promote wakefulness. Conversely, in the absence of light, the pineal gland synthesizes melatonin unabated, leading to feelings of drowsiness and facilitating the onset of sleep.
In mammals, melatonin is critical for the regulation of sleep–wake cycles, or circadian rhythms.[20] The establishment of regular melatonin levels in human infants occurs around the third month after birth, with peak concentrations observed between midnight and 8:00 am.[21] It has been documented that melatonin production diminishes as a person ages.[22] Additionally, a shift in the timing of melatonin secretion is observed during adolescence, resulting in delayed sleep and wake times, increasing their risk for delayed sleep phase disorder during this period.[23] In adults, approximately 30 μg of melatonin is synthesized per day, nearly 80% of which occurs at night.[24][25]
The antioxidant properties of melatonin were first recognized in 1993.[26]In vitro studies reveal that melatonin directly neutralizes various reactive oxygen species, including hydroxyl (OH•), superoxide (O2−•), and reactive nitrogen species such as nitric oxide (NO•).[27][28] In plants, melatonin works synergistically with other antioxidants, enhancing the overall effectiveness of each antioxidant.[28] This compound has been found to be twice as efficacious as vitamin E, a known potent lipophilic antioxidant, at scavenging peroxyl radicals.[29] The promotion of antioxidant enzyme expression, such as superoxide dismutase, glutathione peroxidase, glutathione reductase, and catalase, is mediated through melatonin receptor-triggered signal transduction pathways.[16]
Melatonin's concentration in the mitochondrial matrix is significantly higher than that found in the blood plasma,[17][19] emphasizing its role not only in direct free radical scavenging but also in modulating the expression of antioxidant enzymes and maintaining mitochondrial integrity. This multifaceted role shows the physiological significance of melatonin as a mitochondrial antioxidant, a notion supported by numerous scholars.[17][19][18]
Melatonin's interaction with the immune system is recognized, yet the specifics of these interactions remain inadequately defined.[30][31][32][33] An anti-inflammatory effect appears to be the most significant.[32][33] The efficacy of melatonin in disease treatment has been the subject of limited trials, with most available data deriving from small-scale, preliminary studies. It is posited that any beneficial immunological impact is attributable to melatonin's action on high-affinity receptors (MT1 and MT2), which are present on immunocompetent cells. Preclinical investigations suggest that melatonin may augment cytokine production and promote the expansion of T cells,[34][35] thereby potentially mitigating acquired immunodeficiencies.[36]
Weight regulation
Melatonin's potential to regulate weight gain is posited to involve its inhibitory effect on leptin, a hormone that serves as a long-term indicator of the body's energy status.[37][38]
In bacteria, protists, fungi, and plants, the synthesis of melatonin also involves tryptophan as an intermediate but originates indirectly from the shikimate pathway. The pathway commences with D-erythrose 4-phosphate and phosphoenolpyruvate, and in photosynthetic cells, additionally involves carbon dioxide. While the subsequent biosynthetic reactions share similarities with those in animals, there are slight variations in the enzymes involved in the final stages.[41][42]
The hypothesis that melatonin synthesis occurs within mitochondria and chloroplasts suggests an evolutionary and functional significance of melatonin in cellular energy metabolism and defense mechanisms against oxidative stress, reflecting the molecule's ancient origins and its multifaceted roles across different domains of life.[43]
Mechanism
Mechanism of melatonin biosynthesis
The mechanism of melatonin biosynthesis initiates with the hydroxylation of L-tryptophan, a process that requires the cofactortetrahydrobiopterin (THB) to react with oxygen and the active site iron of tryptophan hydroxylase. Although the complete mechanism is not entirely understood, two main mechanisms have been proposed:
The first mechanism involves a slow transfer of one electron from THB to molecular oxygen (O2), potentially producing a superoxide (O−2). This superoxide could then recombine with the THB radical to form 4a-peroxypterin. 4a-peroxypterin may either react with the active site iron (II) to create an iron-peroxypterin intermediate or directly transfer an oxygen atom to the iron, facilitating the hydroxylation of L-tryptophan.
Alternatively, the second mechanism proposes that oxygen interacts with the active site iron (II) first, forming iron (III) superoxide. This molecule could then react with THB to form an iron-peroxypterin intermediate.
Following the formation of iron (IV) oxide from the iron-peroxypterin intermediate, this oxide selectively attacks a double bond to yield a carbocation at the C5 position of the indole ring. A subsequent 1,2-shift of the hydrogen and the loss of one of the two hydrogen atoms on C5 would restore aromaticity, producing 5-hydroxy-L-tryptophan.[44]
The decarboxylation of 5-hydroxy-L-tryptophan to produce 5-hydroxytryptamine is then facilitated by a decarboxylase enzyme with pyridoxal phosphate (PLP) as a cofactor.[45] PLP forms an imine with the amino acid derivative, facilitating the breaking of the carbon–carbon bond and release of carbon dioxide. The protonation of the amine derived from tryptophan restores the aromaticity of the pyridine ring, leading to the production of 5-hydroxytryptamine and PLP.[46]
Serotonin N-acetyltransferase, with histidine residue His122, is hypothesized to deprotonate the primary amine of 5-hydroxytryptamine. This deprotonation allows the lone pair on the amine to attack acetyl-CoA, forming a tetrahedral intermediate. The thiol from coenzyme A then acts as a leaving group when attacked by a general base, producing N-acetylserotonin.[47]
The final step in the biosynthesis of melatonin involves the methylation of N-acetylserotonin at the hydroxyl position by SAM, resulting in the production of S-adenosyl homocysteine (SAH) and melatonin.[46][48]
Blue light, especially within the 460–480 nm range, inhibits the biosynthesis of melatonin,[51] with the degree of suppression being directly proportional to the intensity and duration of light exposure. Historically, humans in temperate climates experienced limited exposure to blue daylight during winter months, primarily receiving light from sources that emitted predominantly yellow light, such as fires.[52] The incandescent light bulbs used extensively throughout the 20th century emitted relatively low levels of blue light.[53] It has been found that light containing only wavelengths greater than 530 nm does not suppress melatonin under bright-light conditions.[54] The use of glasses that block blue light in the hours preceding bedtime can mitigate melatonin suppression.[55] Additionally, wearing blue-blocking goggles during the last hours before bedtime is recommended for individuals needing to adjust to an earlier bedtime since melatonin facilitates the onset of sleep.[56]
An extended-releasepharmaceutical formulation of melatonin is approved under the brand name Circadin for the treatment of insomnia in certain settings, such as in people aged 55years of age or older.[61][62][63][64] It is approved in the European Union, Israel, Australia, and countries in Asia and elsewhere in the world, but not in the United States (where it reached phase III clinical trials but was not approved).[63][64] The medication has been licensed since 2007.[63][64]
The 2023, European Insomnia Guideline recommended use of prolonged-release melatonin for treatment of insomnia in people age 55 year of age or older for up to three months.[12] It recommended against fast-release or over-the-counter melatonin for treatment of insomnia.[12] These recommendations were based on several meta-analyses published in 2022 and 2023.[12]
Melatonin is known to reduce jet lag, especially in eastward travel. However, if it is not taken at the correct time, it can instead delay adaptation.[67]
Melatonin appears to have limited use against the sleep problems of people who work shift work.[68] Tentative evidence suggests that it increases the length of time people are able to sleep.[68]
Meta-analyses, published between 2005 and 2017, appear to show different results as to whether melatonin is effective for circadian rhythm sleep disorders or not.[69][70][71][72] Some found that it was effective,[69][70][72] while others found no evidence of effectiveness.[71] Meta-analyses of melatonin for delayed sleep phase syndrome that found it effective have reported that it improves time to sleep onset by about 40minutes (0.67hours) and advances onset of endogenous melatonin secretion by about 1.2hours (72minutes).[70][72] One meta-analysis found that melatonin was notably more effective in improving sleep onset latency in people with delayed sleep phase syndrome than in people with insomnia (improvement of 39minutes vs. 7minutes, respectively).[72] One meta-analysis found that melatonin was probably effective for jet lag syndrome.[73]
A 2020 Cochrane review found no evidence that melatonin helped sleep problems in people with moderate to severe dementia due to Alzheimer's disease.[78] A 2019 review found that while melatonin may improve sleep in minimal cognitive impairment, after the onset of Alzheimer's disease it had little to no effect.[79] Melatonin may, however, help with sundowning (increased confusion and restlessness at night) in people with dementia.[80]
Pain
A 2026 meta-analysis concluded that people taking melatonin supplements may experience relief from musculoskeletal pain similar in magnitude to the effects of typical analgesics.[81] Although melatonin may be considered as an adjunct option for pain relief, it had only modest effects, and the quality of evidence for efficacy was low to moderate.[81]
Available forms
A bottle of melatonin tablets. Melatonin is also available in timed-release and in liquid forms.
A prolonged-release 2mg oral formulation of melatonin sold under the brand name Circadin is approved for use in the European Union in the short-term treatment of insomnia in people aged 55 years of age or older.[61][62][82][83]
Melatonin is also available as an over-the-counterdietary supplement in many countries. It is available in both immediate-release and less commonly prolonged-release forms. The compound is available in supplements at doses ranging from 0.3mg to 10mg or more. It is also possible to buy raw melatonin powder by weight.[84] Immediate-release formulations of melatonin cause blood levels of melatonin to reach their peak in about an hour. The hormone may be administered orally, as capsules, gummies, tablets, oral films, or as a liquid.[85] It is also available for use sublingually, or as transdermal patches.[86] Several inhalation-based melatonin products with a wide range of doses are available but their safety remains to be evaluated.[85]
Melatonin's discovery is linked to the study of color changes in some amphibians and reptiles, a phenomenon initially observed through the administration of pineal gland extracts.[87][88] In 1917, Carey Pratt McCord and Floyd P. Allen found that feeding extracts from the pineal glands of cows caused the skin of tadpoles to lighten by contracting the dark epidermalmelanophores.[89][90]
The hormone melatonin was isolated in 1958 by Aaron B. Lerner, a dermatology professor, and his team at Yale University. Motivated by the possibility that a substance from the pineal gland could be beneficial in treating skin diseases, they extracted and identified melatonin from bovine pineal gland extracts.[91] Subsequent research in the mid-1970s by Lynch and others demonstrated that melatonin production follows a circadian rhythm in human pineal glands.[92]
The etymology of melatonin stems from its skin-lightening properties. As detailed in their publication in the Journal of the American Chemical Society,[94] Lerner and his colleagues proposed the name melatonin, derived from the Greek words melas, meaning 'black' or 'dark', and tonos, meaning 'labour',[95] 'colour'[96] or 'suppress'.[97] This naming convention follows that of serotonin, another agent affecting skin color, discovered in 1948 as a modulator of vascular tone, which influenced its name based on its serum vasoconstrictor effect.[98] Melatonin was thus aptly named to reflect its role in preventing the darkening of the skin, highlighting the intersection of biochemistry and linguistics in scientific discovery.[94]
Occurrence
Animals and Humans
In vertebrates, melatonin is produced in darkness, thus usually at night, by the pineal gland, a small endocrine gland[99]
located in the center of the brain but outside the blood–brain barrier. Light/dark information reaches the suprachiasmatic nuclei from retinal photosensitive ganglion cells of the eyes[100][101] rather than the melatonin signal (as was once postulated). Known as "the hormone of darkness", the onset of melatonin at dusk promotes activity in nocturnal (night-active) animals and sleep in diurnal ones including humans.[102]
In humans, ~30 μg of melatonin is produced daily and 80% of the total amount is produced in the night (W). The plasma maximum concentration of melatonin at night are 80–120 pg/mL and the concentrations during the day are between 10–20 pg/mL.[103][104]
Many animals and humans use the variation in duration of melatonin production each day as a seasonal clock.[105] In animals including humans,[106] the profile of melatonin synthesis and secretion is affected by the variable duration of night in summer as compared to winter. The change in duration of secretion thus serves as a biological signal for the organization of daylength-dependent (photoperiodic) seasonal functions such as reproduction, behavior, coat growth, and camouflage coloring in seasonal animals.[106] In seasonal breeders that do not have long gestation periods and that mate during longer daylight hours, the melatonin signal controls the seasonal variation in their sexual physiology, and similar physiological effects can be induced by exogenous melatonin in animals including mynah birds[107] and hamsters.[108] Melatonin can suppress libido by inhibiting secretion of luteinizing hormone and follicle-stimulating hormone from the anterior pituitary gland, especially in mammals that have a breeding season when daylight hours are long. The reproduction of long-day breeders is repressed by melatonin and the reproduction of short-day breeders is stimulated by melatonin. In sheep, melatonin administration has also shown antioxidant and immune-modulatory regime in prenatally stressed offspring helping them survive the crucial first days of their lives.[109]
Cetaceans have lost all the genes for melatonin synthesis as well as those for melatonin receptors.[110] This is thought to be related to their unihemispheric sleep pattern (one brain hemisphere at a time). Similar trends have been found in sirenians.[110]
Plants
Until its identification in plants in 1987, melatonin was for decades thought to be primarily an animal neurohormone. When melatonin was identified in coffee extracts in the 1970s, it was believed to be a byproduct of the extraction process. Subsequently, however, melatonin has been found in all plants that have been investigated. It is present in all the different parts of plants, including leaves, stems, roots, fruits, and seeds, in varying proportions.[8][111] Melatonin concentrations differ not only among plant species, but also between varieties of the same species depending on the agronomic growing conditions, varying from picograms to several micrograms per gram.[42][112] Notably high melatonin concentrations have been measured in popular beverages such as coffee, tea, wine, and beer, and crops including corn, rice, wheat, barley, and oats.[8] In some common foods and beverages, including coffee[8] and walnuts,[113]
Although a role for melatonin as a plant hormone has not been clearly established, its involvement in processes such as growth and photosynthesis is well established. Only limited evidence of endogenous circadian rhythms in melatonin levels has been demonstrated in some plant species and no membrane-bound receptors analogous to those known in animals have been described. Rather, melatonin performs important roles in plants as a growth regulator, as well as environmental stress protector. It is synthesized in plants when they are exposed to both biological stresses, for example, fungal infection, and nonbiological stresses such as extremes of temperature, toxins, increased soil salinity, drought, etc.[42][114][115]
Herbicide-induced oxidative stress has been experimentally mitigated in vivo in a high-melatonin transgenic rice.[116][117][118] Studies conducted on lettuce grown in saline soil conditions have shown that the application of melatonin significantly mitigates the harmful effects of salinity. Foliar application increases the number of leaves, their surface area, increases fresh weight and the content of chlorophyll a and chlorophyll b, and the content of carotenoids compared to plants not treated with melatonin.[118]
Melatonin is produced by α-proteobacteria and photosynthetic cyanobacteria. There is no report of its occurrence in archaea which indicates that melatonin originated in bacteria[11] most likely to prevent the first cells from the damaging effects of oxygen in the primitive Earth's atmosphere.[10]
In 2022, the discovery of serotonin N-acetyltransferase (SNAT)—the penultimate, rate-limiting enzyme in the melatonin biosynthetic pathway—in the archaeon Thermoplasma volcanium[124] firmly places melatonin biosynthesis in all three major domains of life, dating back to ~4 Gya.[125]
Food products
Naturally occurring melatonin has been reported in foods including tart cherries to about 0.17–13.46 ng/g,[126]bananas, plums, grapes, rice, cereals, herbs, olive oil, wine,[127] and beer.[128] The consumption of milk and sour cherries may improve sleep quality.[129] When birds ingest melatonin-rich plant feed, such as rice, the melatonin binds to melatonin receptors in their brains.[130] When humans consume foods rich in melatonin, such as banana, pineapple, and orange, the blood levels of melatonin increase significantly.[131]
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