Melatonin

Melatonin (N-acetyl-5-methoxytryptamine) is a peptide hormone and neurotransmitter most known for its regulation of sleep, where darkness causes increased synthesis and secretion of melatonin induces sedation and initiate the sleep cycle. Due to its structure, it is also classified as an indoleamine.

Melatonin is also found in a variety of foods, and is universally found in plants, although possibly in minute, bioactive quantities.^[1] Good sources of melatonin that are commonly consumed include:^[2][3]

• Tomato at 3–114 ng/g^[4][5]
• Walnuts at 3-4ng/g^[6]
• Cereal (barley, rye) at 300–1,000 pg/g^[1]
• Strawberries at 1–11 ng/g^[4]
• Olive oil at 53–119 pg/ml^[7]
• Unprocessed cow milk at 3–25 pg/ml^[8][9]
• Night-time milk (functional food product) at 10–40 ng/ml^[10]
• Wine (from grapes) at 50–230 pg/ml^[4][11]
• Beer (from hops) at 52–170 pg/ml^[12]
• Cherries at 2.06-13.46ng/g^[13]

Common supplements (or herbs not commonly used as food products) that contain melatonin are (64 total, not all listed):

• Periostracum cicadae (Chantui) at 3.7mcg/g^[14]
• Babreum coscluea (Shiya Tea-Leaf), Uncaria rhynchophylla (Gouteng), and Viola philippica Cav (Diding) at 2.1-2.3mcg/g^[14]
• Phellodendron amurense (Huangbo) at 1.235mcg/g and Mori Albae Cortex (Sangbaipi) at 1.11mcg/g^[14]
• Coptis chinensis (Huanglian) at 1mcg/g^[14]
• Angelica sinsensis (Danghui) at 698ng/g^[14]
• Ziziphus jujuba (Suanzhaoren) at 256ng/g, or 146ng/g in the variant referred to as Dazao^[14]
• Salvia miltiorrhiza (Danshen) at 187ng/g^[14]
• Panax notoginseng (Sangqui) at 169ng/g^[14]
• Curcuma aeruginosa (Erzhu) at 120ng/g^[14]
• Schisandra Chinensis (Wuweizi) fruits at 86ng/g^[14]

Melatonin is produced naturally in the body, and is also a functional food component, similar to its amino acid precursor L-tryptophan, and some intermediate metabolites like serotonin, which is found in some foods.

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Melatonin is made de novo in the human body in multiple locations, with the pineal gland in the brain being the most well known. Other locations include bone marrow cells,^[15] the retina,^[16] and the gastrointestinal tract.^[17]

The biosynthetic pathway of melatonin starts from the dietary amino acid L-tryptophan, which is converted to 5-hydroxytryptophan, or 5-HTP, by tryptophan-5-hydroxylase. 5-HTP is converted to the active hormone serotonin by the enzyme aromatic L-amino acid decarboxylase, further converted to N-acetylserotonin by the enzyme serotonin-N-acetyltransferase (sometimes called arylalkylamine-N-acetyltransferase), and finally converted to N-acetyl-5-methoxytryptamine (melatonin) by the enzyme hydroxyindole-O-methyltransferase (HIOMT).^[18] Increases in melatonin from supplemental L-tryptophan have been noted in non-human species to varying degrees.^[19] Sometimes, melatonin failed to increase.^[20] This may be due to the first rate limiting enzyme tryptophan-5-hydroxylase (regulating conversion of dietary tryptophan to serotonin). Thus, supplementing melatonin appears to circumvent this rate limit. The 'true' rate-limiting enzyme is arylalkylamine-N-acetyltransferase, which mediates conversion from serotonin to N-acetylserotonin^[21] and is the control point for external regulation by light and darkness, being suppressed with light-induced signals from the retina.^[22]

Melatonin is made from serotonin, which is made from dietary L-tryptophan. It is regulated by sunlight, mostly in the retina.

Given how the suprachiasmatic nuclei (SCN) of the pituitary expresses melatonin receptors,^[23] it is thought that supplemental melatonin can suppress endogenous secretion via negative feedback.

Oral melatonin supplementation at 500mcg, over a period of a week in shift workers, did not influence basal secretion, as cessation for one day prior to measurements did not show differences when compared to secretion status prior to supplementation.^[24] Twenty-four hour melatonin levels in this study, when graphed, essentially overlapped, suggesting next to no variance.^[24] These results, indicating a lack of negative feedback, have been replicated with 2mg^[25] and 5mg of melatonin.^[26]

When a blind person supplemented a dose of 50mg in one case study (blind people being an example of a population with no sunlight-mediated melatonin production), this dose being 100-fold higher than the standard 500mcg, did not significantly influence basal secretion status.^[24] In this population, lower doses of 500mcg are also effective and without apparent negative feedback.^[27]

Results in blind people suggest that an override of regulation from sunlight/darkness may not be the factor behind the lack of negative feedback, as some studies do control for persons with no conscious light perception.^[27]

Regulation of melatonin secretion from the pineal gland does not appear to be negatively influenced by melatonin supplementation over the long term (multiple days) and no negative feedback from melatonin supplementation (less natural secretion after a period of supplementation) has been observed.

Beyond just fluctuating throughout every day (higher in evening, lower during waking hours), melatonin may fluctuate throughout the seasons in humans, following a bimodal distribution with peaks in January and July.^[28]

Some alterations of circulating melatonin level depend on the season. Clinical relevance is currently unknown.

The circadian rhythm of melatonin appears to shift toward earlier clock hours later on in the aging process.^[29]

When investigating rats divided into three age groups (correlating with youth, adult, and elderly), it appears that overall melatonin levels are highest in adults (only due to youth having smaller pineal glands; youth has the highest density of melatonin) and lower levels of melatonin are apparent in elderly rats.^[30] Similar trends were seen in N-acetylserotonin, another pineal hormone.^[30] This higher density in rat pups is seen in human youth, where spikes in melatonin occur around age 2-4 and then decline until puberty, where they remain constant, ^[31] before gradually declining over the rest of the lifespan. The most drastic drop in nightly melatonin levels appears to occur around the ages of 41-60, where the melatonin levels of 41-50 year-olds is significantly higher than the age bracket of 51-60.^{[31][32][33][34][35]}

Melatonin appears to fluctuate with age, decreasing steadily after puberty, but undergoing a significant decrease in average nightly melatonin levels around the ages of 41-60.

Melatonin levels appear to correlate with one's dietary lifestyle.^[2] Consumption of plant-based products may influence circulating melatonin levels due to plants containing melatonin in possibly physiologically relevant levels, if a sufficient amount of the plant is consumed.^[3] At least in survey research, the highest quartile of vegetable intake (relative to the lowest quartile) is associated with a 16% higher urinary melatonin level.^[36]

Studies on fasting people (or food restriction to less than 300 calories daily) note that circulating melatonin levels can decrease by up to 20%, with no change of urinary secretion rates over a period of 2-7 days.^[37][38][39] This may be secondary to transient glucose deprivation, as supplementation with glucose at 0.5g/kg during these periods restores circulating levels of melatonin, and suggests that the pinealocytes that secrete melatonin may require glucose for optimal functioning.^[39][38] However, when investigating rats undergoing caloric restriction (40% of basal metabolic rate) for the purposes of longevity, melatonin levels in serum are increased.^[40]

Melatonin levels appear to fluctuate with overall caloric and food intake, as well as some food selections. Melatonin may be relevant for explaining some of the health benefits of vegetables, and this could potentially extend to alcohol-containing products.

In people with major depressive disorder, serum melatonin disorders appeared to be lower than age-matched controls, with no difference in this sample during depressive episodes and during periods of remission.^[41]

In people with bipolar disorder, serum melatonin disorders appeared to be lower than age-matched controls.^[41]

Some alterations in circulating melatonin levels depend on clinical states, like depression or bipolar disorder.

When comparing active smokers of tobacco against non-smoking controls, smokers appear to have approximately twice the circulating levels of melatonin when measured during the daytime (11-12h): 17.44+/-1.8 pg/ml in smokers relative to 9.77+/-1.4 pg/ml in non-smokers,^[42] while a study on 21 young female smokers taking measurements of serum melatonin at night (23-24h) found that, relative to non-smokers, smokers had reduced levels of melatonin (47.9+/-14.5pg/mL) and nonsmokers 47% higher levels (70.5+/-18.9pg/mL).^[43] A later study on smokers found endogenous levels of melatonin in smokers just above 2nmol/L/h, which is not significantly different from the nonsmokers in the previous study.^[44] This latter study did note, however, that smokers were about half as responsive to blood increases in melatonin from supplements,^[44] due to induction of the aromatase enzyme from Polyaromatic Hydrocarbons (PAHs) in cigarette smoke.^[42][45]

Alterations in melatonin status and smoking are complex and understudied. There may be a normalization of the circadian rhythm, which needs to be confirmed with further studies. Smokers appear to have less response to melatonin supplementation than do nonsmokers.

Epithalamin is a hormone secreted from the pineal gland (similar to melatonin) that has been implicated in prolonging lifespan in Drosophilia, certain strains of mice, and rats. No influence was found in spontaneously hypertensive (SHR) mice.^[46][47] For those that saw an improvement in lifespan, the values ranged from 10-35%, with rats experiencing a 57% reduction in mortality. In these tested animals, epithalamin administration increased melatonin secretion.^[46]

One study administering epithalamin (6 courses of treatment of 10mg every third day for 15 days, spanning 3 years) to people aged 60-69 with aged cardiovascular systems and low serum melatonin noted that, 12 years after cessation of epithalamin treatment and during follow-up, some parameters associated with aging appeared to be either attenuated or reversed. The epithalamin group outperformed placebo on cycling power, improved lipid and glucose metabolism, and appeared to normalize the suppressed nightly spike of melatonin in the aged control.^[48]

Epithalamin is a peptide hormone related to melatonin that seems to be able to prolong lifespan in research animals, and improve biomarkers in elderly humans.

Melatonin exerts many of its effects vicariously through melatonin receptors, similar to how insulin affects the insulin receptor. The melatonin receptors are named MT1 and MT2, and are G-protein coupled receptors (GPRCs) coupled to G_i proteins (a heterotrimer of α, β, and γ that dissociates into α and βγ when the receptor is activated^[49]).^[50] These two receptors are quite different from each other, as they structure^[51][52] pharmacological characteristics^[53] and chromosomal location,^[54] yet both have high affinity for melatonin.^[55][56] A third 'receptor' exists, known as MT3, but it is not a GPRC like MT1/2. Due to the cytoplasmic protein quinone reductase II having the same melatonin binding properties as 'MT3',^[57] and deletion of quinone reductase II causing 'MT3' to disappear, MT3 may just be quinone reductase II.^[58][59]

Two G-protein coupled receptors, with different actions, mediate the actions of melatonin, MT1 and MT2. Quinone reductase II may bind to melatonin and was thought to be a third receptor (MT3), but it is a cytoplasmic protein. Quinone reductase II may still be relevant, however.

An orphan receptor on the nuclear membrane, named RZR/ROR, can mediate melatonin-like effects.^[60][61]

A fourth receptor, located on the nucleus rather than the cytoplasm, appears to be intimately involved with nuclear melatonin signalling via cytoplasmic receptor cross-talk.

Expression of melatonin receptors are in^[50] the Suprachiasmatic Nuclei (SCN) of the Pineal gland, where MT1 and MT2 both exist and MT1 activation suppresses neuronal firing, the hypothalamus, where both receptors suppress gonadotropin releasing hormone release, the retina, where MT2 reduces dopamine release and an MT3 receptor reduces ocular pressure, the pars tuberalis of the pituitary gland,^[62] the kidneys (MT1),^[63] the pancreas and beta-cells of the pancreas (both MT1 and MT2),^[64] the adrenal cortex, where MT1 activation suppresses cortisol secretion, the testes, where MT1 suppresses testosterone, the pituitary, where MT1 suppresses Follicle Stimulating Hormone (FSH), Luteinizing Hormone (LH), and prolactin. Expression of melatonin receptors is also present in vasculature, where each main receptor mediates either vasoconstriction (MT1) or vasodilation (MT2) and on some adipocytes where MT1 negatively regulates adipose tissue proliferation and increases leptin secretion (there is no MT2 on white adipose tissue, but there is expression of MT2 on brown adipose suppresses glucose uptake^[65]).

Melatonin is most heavily localized in the brain and associated neuro-organs. It also has a heavy presence in sex organs. It is a fairly ubiquitous compound, with expression on fat cells, immune cells, cardiac cells as well as the bloodstream cells

GPR50 is an orphan receptor (belonging to the same GPRC family as melatonin receptors^[66]), which appears to play a role in adaptive thermogenesis^[67] and is expressed in humans, specifically in the dorsomedial nucleus of the hypothalamus and tanycytes that line the third ventricle.^[68] Knockout of this receptor (abolishing its effects) appears to confer resistance to diet-induced obesity, yet paradoxically reduces the amount of weight lost in a fasted state. Knockout also reduced night time thermogenesis, despite 25% higher locomotion during waking hours.^[69] GPR50 appears to interact with leptin signalling, as administration of leptin can improve GPR50 nuclear activity in obese mice (with seemingly suppressed levels of GPR50^[67]) but not in GPR50^-/- mice, suggesting leptin acts vicariously through this receptor, but only on matters related to thermogenesis (as feeding patterns appear unaltered).^[67][70] In fact, after leptin was administered to rats with the standard leptin receptor but no GPR50 receptor, the amount of genes activated by leptin in control mice (2,705) is reduced by just over 50% (to 1,327).^[67] It should be noted that melatonin is not a direct ligand of this receptor.^[71][66]

GPR50 is an orphan receptor, which seems to mediate the aspects of leptin related to thermogenesis, but not appetite. It may mediate up to half of the effects of leptin as well.

GPR50 appears to heterodimerize with the melatonin receptor MT1, which results in reduced efficacy of MT1 signalling, by preventing the binding of agonists to MT1.^[72] GPR50 has the capacity to heterodimerize with MT2, but does not influence the functions of MT2, similar to how MT1 and MT2 heterodimerization does not influence binding of ligands.^[72] Melatonin normally has a K_i of 0.73±0.26nM, yet the heterodimerization has a K_i of 0.37±0.28nM.^[72] Secondary to this, GPR50 appears to antagonize the effects of MT1.^[72] Co-expression of GPR50 alongside MT1 does not affect basal MT1 actions, but reduces the maximal response of MT1 by melatonin agonism by 50%. Reducing the expression of GPR50 reverses these effects.^[72]

Expression of GPR50 appears to reduce the actions of MT1, and may alter the signals sent through MT1 via melatonin.

In a model of delayed sleep phase syndrome, 500mcg of melatonin appears to be as effective as ten-fold the dose (5mg).^[73] A comparative study between 300mcg and 3mg in age-related insomnia noted that 300mcg was more effective than 3mg.^[74]

A comparative study between 500mcg and forty-fold the dose (20mg) showed that 500mcg was more effective at regulating the circadian rhythm of a blind person (with no external melatonin regulation from darkness).^[75] A lower dose of 10mg in blind people appears to be more effective than 20mg, although insignificantly different from 500mcg.^[76]

Although differences between very low (300-500mcg) and higher (3-5mg) doses tend to go back and forth in regard to efficacy, both seem to be more potent than superloaded doses (20mg).

Oral doses of 500mcg melatonin (seen as the lowest active dose) in shift workers resulted in a C_max of 1,580+/-329 pg/ml after a T_max of 1+/-0.14 hours,^[24] noting that this high dose was above physiological ranges for up to three hours, but after the fourth hour levels returned to the physiological range of 24-198pg/mL.^[24] Other studies that use 500mcg note serum levels of 3,054+/-3,022pmol/L (709.4+/-701.9pg/mL) with a half-life of 0.68 hours.^[77]

Studies that use 5mg of melatonin note that serum spikes of 1,638.0+/-61.9pg/mL are seen at 30 minutes,^[78] 23,982pmol/L (5,570.67pg/mL) with a half-life of 0.79 hours.^[77]

With 6mg of melatonin, in one hour serum levels of 1,171.3+/-235.2pg/mL are seen.^[79]

One study assessing 100mg melatonin (200-fold higher dose than the lowest active dose) noted serum peaks approximately 100 minutes after administration, or 652,310+/-82,456pmol/L (151,517+/-19,153pg/mL).^[80] This dose is able to stay in circulation longer than smaller doses, still affecting biology up to bedtime, when taken at 8am.^[80][81] The study noting the higher serum peak measured levels of 95+/-15pmol/L (22.06+/-3.48pg/mL) the next morning at 8am, which was still above baseline for these participants.^[80]

Serum peaks are quite unreliable, although all small doses tend to increase circulating melatonin around 8-10 times more than the highest physiologically relevant concentration. Higher doses increase melatonin levels even further, and delay the time to peak concentrations while also delaying the excretion rates.

When examining a large dose (80mg) of melatonin given to healthy people in the morning (7:30am), the various parameters recorded were: a 24m half-life, with stable levels 60-150 minutes after ingestion and a gradual decline in serum melatonin levels until 9pm, when they returned to physiologically relevant levels.^[82] The peak values were highly variable and significant in magnitude, 350-10,000fold higher than previously observed peak levels. Such variability has been seen elsewhere^[83]), and hourly dosing of 80mg melatonin was able to attenuate but unable to inhibit the decline throughout the day.^[82]

Fast absorption appears to be similar between higher doses (80mg),^[82] 100mg^[84]) and lower doses (6mg).^[83] One study testing four low doses (100mcg, 500mcg, 1mg, 5mg) also noted that the absorption rates appeared to be nonsignificantly different, all taking within 0.78-1.25 hours to reach C^max.^[77]

Melatonin appears to be rapidly absorbed and very rapidly excreted, with the concentration in the blood between these times being significantly higher than normal levels.

When administered intranasally to rabbits, melatonin had a bioavailability of 94% when paired with sodium glycocholate, but 55% without.^[85] This route of administration was accompanied by a 5 minute T_max and a 13 minute half-life, with 1.5mg of melatonin conferring a C_max of 493+/-290ng/mL at 5 minutes. These parameters mimicked intravenous injections.^[85]

Melatonin administered via the nasal route may be markedly more effective than oral supplementation.

At the stage of 6-hydroxylation of melatonin into its main urinary metabolite (6-hydroxymelatonin), aromatase (CYP1A1/2) appears to be very important, with some metabolism by CYP1B1 and CYP2C19.^[86][87][88] Metabolism of melatonin by CYP1B1 appears to be of greater relevance to central (neural) melatonin, due to it not being expressed much in the liver.^[89] Due to metabolism by aromatase, co-ingestion of aromatase inhibitors (in this case, fluvoxamine) can increase melatonin AUC and overall exposure.^[90][91] Habits that induce aromatase (such as tobacco smoking) appear to be correlated to reduced circulating melatonin.^[44]

In humans, 3mg of melatonin is able to attenuate a stress-induced rise in adrenaline and noradrenaline in young healthy men, but is unable to abolish it.^[92] Without a stressor, oral melatonin at low doses (1-2mg) appears to reduce circulating adrenaline by about 60-90 minutes after ingestion.^[93][94][95]

Melatonin may be able to reduce concentrations of circulating adrenaline and noradrenaline.

Melatonin appears to inhibit dopamine release in the ventral hippocampus, medulla pons, preoptic area, and the hypothalamus (posterior and median),^[96][97] yet no inhibition occurs in the cerebral cortex, striatum, cerebellum, or dorsal hippocampus.^[97] This inhibition appears to be mediated by inhibiting calcium influx into co-stimulated nerves.^[98][99] In accordance to this inhibition, active in physiologically relevant nM ranges (although maximal potency is at pharmacological mM ranges), dopamine experiences a diurnal rhythm of release, vicariously through melatonin suppression.^[100][101] This inhibition of dopamine release appears to apply to amphetamine-induced dopamine release, which may be of concern to ephedrine supplementation.^[102][103]

Mechanistically, melatonin appears to be a negative regulator of dopamine release in neurons.

Melatonin works through the MT1 receptor on the Suprachiasmatic Nuclei (SCN) to inhibit cAMP element response element-binding protein (CREB) phosphorylation, secondary to pituitary adenylate cyclase activating polypeptide (PACAP)^[104][105] and through the MT2 receptors of the SCN to facilitate changing of the circadian rhythm, a phenomena known as phase shifting. This appears to be mediated via protein kinase C (PKC).^[106] It is mostly through MT2, but to a lesser extent MT1, that melatonin acts to regulate sleep-wake cycles.^[107][108] MT2 works to regulate the phase shifts,^{[106][109][110]} while MT1 exerts general suppressive actions on cell activation.^[111][112]

A 500mcg dose of melatonin appears to be able to increase secretion of oxytocin and vasopressin within 40-60 minutes of oral ingestion. However, 5mg has no significant influence on its own, yet it is able to suppress an increase in vasopressin normally seen with exercise.^[113] Another study using 50mcg as well as 500mcg and 5mg noted that, in roughly the same population, 50mcg was not significantly different than placebo in regards to vasopressin and barely more significant in increasing oxytocin, while 500mcg significantly increased both neurohormones by 40 minutes after dosing, levels of which appeared to normalize by 150 minutes. A 5mg dose showed a suppressive effect once again.^[114] This suppression was noted in a third study using 5mg nightly for 4 days.^[115]

Melatonin is implicated in neuroprotection as an antioxidant compound and as a protector against the harmful effects of beta-amyloid pigmentation, by reducing levels of said pigmentation and by offering protection from downstream effects.^[116] Melatonin also seems to have preventive effects on hyperphosphorylation of the tau protein, which is a risk factor for Alzheimer's disease.^[116]

Melatonin may also confer neuroprotection via mediating the Akt/mTOR pathway, as it can induce the pathway in times of ischemia-reperfusion injury (when it should be suppressed),^[117] methamphetamine induced suppression of mTOR,^[118] and suppresses overexpression of this and the MAPK pathways via H202 stimulation.^[119]

Melatonin shows synergism with Resveratrol in regards to protection against beta-amyloid pigmentation in regards to AMPK phosphorylation and its downstream effects; although its effects on the glycogen synthase enzyme expression (GSK-1) and glutathione depletion (both risk factors neuropathy) were not synergistic in vitro.^[120]

Melatonin has been investigated for its usage in treating migraines^[121], due to migraines having a circadian rhythm^[122] and insomnia being correlated with morning migraines.^[123] One study that tested this hypothesis in people aged 18-65, with an attack frequency of 2-7 per month, noted that melatonin failed to exert more benefits than placebo when taken at 2mg one hour before sleep for 8 weeks, although a (nonsignificant) improvement in sleep quality (assessed by the Pittsburg Sleep Quality Index) was observed. This became significant when controlling for people with insomnia (correlated highly with those who had auras with their migraines).^[124] This study was criticized for its methodology, mainly because the combination of a crossover design and an 8 week duration limits the length observations could be made. Placebo response rate was much higher than expected, suggesting design flaws.^[125]

Melatonin has not yet been shown to reduce the frequency or intensity of migraines, but has not conclusively been shown to be ineffective. More evidence is required.

Melatonin supplementation exerts most of its benefits through decreased sleep latency, or a reduction of the time it takes to fall asleep.

Some studies do not note a significant decrease in sleep latency,^{[126][127][128]} although the majority tend to note a shortening of the time it takes to fall asleep in otherwise healthy subjects.^{[129][130][131][132][133][134][135]} Although some studies, measuring REM sleep (indicative of sleep quality), note improvements, this is not inherent to melatonin. One study using 10mg melatonin one hour prior to sleep for 28 days in people aged 28+/-5 years (n=30) noted that this decreased sleep latency could occur without benefit to REM latency/density or sleep architecture.^[136] Improvement in sleep may also not necessarily be dependent on circulating melatonin levels.^[137] The best predictor of response to melatonin could be age (55yrs or greater) or the status of insomnia.^[138]

A part of melatonin's pro-sleep mechanisms may be related to a decrease in body temperature, as the two are highly associated.^[133]

Melatonin appears to be quite reliable for shortening sleep latency, and inducing faster sleep. This may be related to the hypothermic (temperature lowering) effects of melatonin, and does not necessarily indicate better sleep quality.

In older people with primary insomnia, melatonin (2mg of a slow release formula) has shown efficacy in improving sleep quality.^[139][140] Melatonin has also shown efficacy for children suffering from insomnia that affects development.^[141] In this latter study, kids with an average age of 12 (8.6-15.7yrs), using melatonin in ranges of 0.3-10mg (average dose of 2.69mg), for an average of 3.1 years, did not significantly differ from a non-supplemented control when assessed by Tanner stages, three questions used to assess physical maturation of puberty.^[142][143] No differences in mental maturity were seen.^[141] Benefits to insomnia have also been noted for people who suffer from insomnia and also experience migraines with auras, which may be correlated.^[124]

Due to melatonin shortening the time it takes to fall asleep (sleep latency) it shows most efficacy in insomnia treatment and, despite being used in all age groups, is surprisingly free of withdrawal and other side-effects at the doses used.

Melatonin has shown some efficacy in improving sleep quality for people suffering from tinnitus.^[144]

At least one study has noted synergistic sedative effects of melatonin and Monoamine Oxidase (MAO-A) inhibitors. In this frog study, clorgyline and moclobemide were used.^[145]

Jet lag is a term used to refer to dysregulation between external regulators of time (light and darkness) and the internal clock located, in part, in the Suprachiasmatic Nuclei (SCN) of the brain.^[146] Jet lag is named after plane travel between time zones causing dysregulation of hours up to half a day (depending on time zones traversed). The general phenomena of dysregulation of the circadian rhythm also applies to shift-work (due to working at a time normally reserved for sleep), fluorescent lights in the late afternoon disrupting melatonin secretion. Blind people, due their having no influence from light or darkness, may also suffer.^{[146][147][148]}

Melatonin is investigated for its usage in solving jet lag due to its ability to 'fix’' the circadian rhythm and restore desynchronization via signalling the SCN through MT2 receptors.^[149]

In situations where external stimuli (sunlight and darkness cycles) and internal stimuli (the internal clock) are not in sync, supplemental melatonin is thought to help re-establish balance.

A Cochrane database meta-analysis of 10 studies that transversed at least 5 time zones found that melatonin was significantly more effective than placebo, when taken at the destination's bedtime, in normalizing the circadian rhythm and reducing the symptoms of jet lag.^[150] According to the studies reviewed, there is no significant difference between 500mcg and 5mg on the effects of melatonin in reducing jet lag. Some better sleep was noted with 5mg. It should be noted that some people still experienced jet lag, as the meta-analysis^[150] noted that in the two studies that reported individual statistics, about 18% of subjects still experienced jet lag after melatonin, with placebo at 67%. The one study that did not report benefits can be found here.^[151]

In studies comparing melatonin against other sedatives, it appears to be less effective than zolpidem (Ambien or Sublinox are the brand names) but also associated with less side-effects.^[152]

Other interventions using melatonin for jet lag (in regards to travel) indexed in Medline are found here,^{[153][154][155][156][157]} and this phenomena as it applies to shift work is noted here.^[158][159]

Melatonin, taken in the evening (sometimes 30 minutes before sleep, at times up to 4-5 hours before sleep with a higher dose) appears to normalize abnormal circadian rhythms. In order to fix jet lag, supplementation should be timed with the clock of the current time zone.

Interestingly, green light treatment in the morning combined with 3g of melatonin the night prior, additively,but not synergistically, benefits correction of abnormal circadian rhythms.^[160] Bright light in the morning also aids in normalizing the circadian rhythm^[161] or otherwise shifting it to another time.^[162] Bright light, when observed in the afternoon and combined with melatonin, partially abolishes the effects of melatonin.^[163] In this study, while light treatment at 21:00 and 24:00 delayed normalization of the circadian rhythm by 0.68 hours. Supplemental melatonin at 20:40 corrected it by 0.4 hours. The combination failed to be significantly different than placebo.^[163] These results also suggest that melatonin can negate the negative effects of light at night, as it applies to jet lag.

Melatonin at night works well with bright light therapy in the morning, for the purposes of lowering sleep latency, but is antagonistic with bright lights prior to sleep.

The first night effect is a delay in sleep onset due to sleeping in new settings, common during travel. Similar to Panax ginseng,^[164] melatonin is effective in reducing sleep latency (time to fall asleep) and as an aid against the first night effect, which is sometimes seen in any study assessing patients in clinical settings during sleep.^[165]

The opposite effect has been noted in people suffering from schizophrenia,^[166] despite improving sleep quality otherwise.^[167]

The phase-shift hypothesis states that seasonal depression in the winter months is associated with alterations in light-sleep cycles^[168] and melatonin, alongside bright light therapy (night and morning, respectively), is able to normalize disorders of the circadian rhythm,^{[169][170][171]} including seasonal depression.^[172]

A study on elderly people (86+/-6yrs) with mild cognitive impairment, given a combination supplement of melatonin (5mg), soy phospholipids (160mg), L-tryptophan (95mg), and fish oil (720mg DHA, 286mg EPA, vitamin E at 16mg) noted that nightly ingestion for 12 weeks significantly reduced the rating score of the MMSE and MNA (indicative of cognitive enhancement) without influencing short or long term memory parameters. The improved score appeared a minor trend to improve with supplementation, relative to a minor deterioration seen in control.^[173]

One study assessing memory with 3mg melatonin given to healthy young men noted that melatonin supplementation was associated with improved memory encoding under stress.^[92] Taking melatonin one hour prior to a combined learning and stress experience improved the amount remembered the next day, relative to control, but tests conducted 15 minutes after the stressor (when cortisol was highest) were not different between groups.^[92]

Adrenaline-mediated signalling (adrenergic) is a regulator of cardiac function and, in some clinical populations, can be seen as undesirable due to its hypertensive and pro-contractile properties.

Melatonin appears to have anti-adrenergic actions in heart tissue. One in vitro study using excised cardiac tissue from rats noted that melatonin (50uM, with 25uM having no effect) appears to reduce Cyclic Adenosine Monophosphate (cAMP) production in the heart by up to 34% via the melatonin receptors, although abolishing Nitric Oxide (NO), Protein Kinase C (PKC), or guanyl cyclase also abolished the effects of melatonin.^[174]

A dose of 1-2mg melatonin is able to acutely reduce blood pressure in men^[93] and women,^[95] possibly secondary to a reduction in adrenaline, which is also observed. The difference between groups appears to be reduced when subjects are standing and mobile,^[94] Not all studies note an acute decrease in blood pressure, ^[175] suggesting these acute effects (measured in a passive supine position) may not hold much practical relevance.

An intervention using 5mg of melatonin supplementation for 2 months, taken 2 hours before bed, noted that melatonin, in people with metabolic syndrome, was able to reduce blood pressure from 132.8+/-9.8 systolic to 126.3+/-11.5 (95.1% of baseline) and decrease diastolic from 81.7+/-8.7 to 76.9+/-9.2 (94.1% of baseline).^[176] These effects were independent of major changes in body weight, and may have been secondary to melatonin's antioxidative effects.^[176]

At least one study has investigated blood partitioning after ingestion of 3mg melatonin, in participants in a supine position. The researchers noted an increased blood flow to the forearms and less to the kidneys (without influencing cerebral blood flow) occurring 45 minutes after ingestion of melatonin, independent of changes in heart rate or blood pressure.^[175]

Although administration of 5mg of melatonin nightly for 2 months in people with metabolic syndrome does not appear to significantly influence triglyceride levels,^[176] triglycerides may be acutely affected as assessed by a study involving 6mg of melatonin taken prior to exercise. The expected decrease in triglycerides during exercise was exacerbated with melatonin.^[177]

When 5mg of melatonin is taken for 2 months, 2 hours before bed, by people with metabolic syndrome, there are no significant effects of melatonin on LDL cholesterol, HDL cholesterol, or total cholesterol.^[176]

A human study using melatonin supplementation and measuring serum leptin noted that, in a population of 11 people with idiopathic stomach ulcers, leptin increased from 6.2-7.0ng/mL to 12.2-16.2ng/mL after 7 days, maintaining this level up to 21 days, after 5mg of melatonin was taken twice a day, morning and evening.^[178] This same dose (10mg) in people with non-alcoholic fatty liver disease over 28 days with elevated leptin caused a further elevation of 33%.^[179]

These effects have also been seen in rats at 25mcg/mL in drinking water (about 500mcg daily) for 9 weeks with either a high fat (35% fat, 35% carbs) or a low fat (4% fat, 60% carbs) diet, where leptin area under curve was increased, but only when measured from early morning to early evening, with no significant difference at any point in the evening.^[180] Another study in rats using a lower dose of 10mcg/mL (ended up being 35mcg daily) of water also found influences on circulating leptin, where levels were increased to approximately 150% of control (data derived from graph) after one month. This study also found an increase in circulating zinc.^[181] Similar results have been seen in cases of excessive melatonin administration (3mg/kg in mice via I.V), where leptin increased to 127% of control over 6 months.^[182]

Melatonin supplementation over a period of longer than a week increases circulating leptin levels without changes in body fat or food intake. This increase does not appear to be dose-dependent. Due to interactions with the circadian rhythm, concrete numbers pertaining to the leptin increase may not be reliable.

When examining isolated fat cells (where most leptin is produced), the amount of leptin secreted is not significantly enhanced when incubated with 1nM melatonin.^[181] However, this may be due to the incubation with melatonin alone, as other studies pairing melatonin with insulin note that melatonin may augment the insulin-induced secretion of leptin, as neither induced leptin secretion in vitro by themselves, while the combination increased secretion by 120% and mRNA content by 50%. Adding dexamethasone to the mixture increased these levels to 250% and 100%, respectively.^[183] Melatonin was able to suppress a Cyclic Adenosine Monophosphate (cAMP)-induced suppression of leptin release and play a synergistic role in activating the insulin receptor and its target, Protein Kinase B (Akt). Its effects were abolished when the MT1 receptor was prevented from acting.^[183] These effects were later replicated by the same research group, with the same potency, when adipocytes were incubated on a 12 hour on/12 hour off protocol to mimic the circadian rhythm.^[184]

Melatonin appears to potentiate insulin-induced leptin secretion.

Please refer to the section on Mechanisms and the subhead 'Non-Melatonin Receptors' for discussion on GPR50, a leptin receptor that negatively influences melatonin signalling through MT1. The increase in leptin may be a mechanism of negative feedback due to melatonin signalling.

Melatonin has been investigated for its interactions with obesity, since rats lacking pineal glands that secrete melatonin (rats that have undergone a pinealectomy) experience increased lipogenesis and reduced lipolysis.^[185] The pairing of a lack of melatonin secretion and synthesis with weight gain suggests that melatonin may either be anti-obesogenic (reducing fat gain) or may induce fat loss.

Melatonin appears to be somewhat of a negative regulator of adipocyte physiology, being able to influence Mesenchymal Stem Cell (MSC) differentiation away from adipocytes and promote osteogenic cell growth secondary to Peroxisome Proliferator-Activated Receptor (PPAR) inhibition^[186] while suppressing proliferation of mature 3T3-L1 adipocytes, secondary to suppression of C/EBPbeta transcriptional activity.^[187]

When treated in 3T3-L1 preadipocytes, melatonin is able to induce proliferation. This appears to act through MT1 receptor activation.^[188]

Melatonin appears to influence mesenchymal cells (a cell that can develop into either adipose cells or bone cells) towards bone rather than fat, a process similar to that seen with resveratrol.

In a model of PAZ6 adipocytes (the human brown preadipocyte cell line^[65]), it was found that the mRNA for both melatonin receptors existed, and via MT2 receptors, suppressed GLUT4 translocation and glucose uptake by approximately 25% over 14 days incubation, but failed to significantly reduce activity after one day.^[65] In this study, brown and white adipocytes were both tested and although white had less MT1 than brown adipose, white expressed no MT2.^[65] A study using luzindole (an agonist of mostly MT2 but with some affinity to MT1) demonstrated it was less effective than melatonin at creating these effects, supporting the lack or either relative absence of active MT2 on white adipocytes.^[188]

Activation of melatonin receptors appears to be associated with suppression of adenyl cyclase and a decrease in Cortisol-Induced Aromatase (cAMP) levels.^[188][187] This decrease in cAMP levels (associated with the G_i protein coupled to melatonin receptors) can suppress lipolysis induced by beta(2)adrenergic stimulation.^[189]

White adipose expresses MT1 and little-to-none MT2. Although glucose metabolism is heavily influenced in brown adipocytes, humans do not have much (relative to rats), which means it may not be practically relevant. Signalling via MT1 in white adipose may be the most practically relevant mechanistic pathway of melatonin for humans.

When investigating oxidation, incubation of preadipocytes with melatonin is associated with increased levels of copper, zinc, manganese and Superoxide Dismutases (SODs). An increase in catalase was observed after a 24 hour incubation. These trends reversed at the 48 hour mark of incubation.^[188]

When rats are fed 500mcg melatonin daily via drinking water and concurrently given a high fat (35%) diet, the rate of weight gain is attenuated, independent of changes in calories.^[180] This has been replicated with 0.4mcg/mL, where a 7% decrease in body weight and 16% lower intra-abdominal adipose mass was recorded.^[190]

When 5mg of melatonin is administered 2 hours before bedtime to a sample of people with metabolic syndrome, a small but statistically significant reduction of BMI has been observed (from 29 to 28.8) over two months, which correlates with improvements in blood pressure and antioxidant profile.^[176]

Though melatonin is unlikely to be a potent weight loss or anti-weight gain agent, it does seem to beneficially influence these parameters.

When 6mg of melatonin is taken by youth (18-20) immediately (30 minutes) prior to exercise, it is able to reduce the amount of lipid peroxidation induced by exercise (serum malondialdehyde (MDA)) and appears to significantly preserve, and perhaps slightly elevate, levels of endogenous antioxidant enzymes.^[177] In this study, plasma triglycerides from exercise were also significantly reduced upon melatonin supplementation, relative to control.^[177]

After rats are subject to crush injury, melatonin injections at 10mg/kg bodyweight were associated with better muscle function (tetanic and twitch force) at 4, 7, and 14 days after injury relative to control, exhibiting about 1.2 to 1.3-fold better recovery.^[191] This appears to be secondary to an increase in satellite cells (2-fold) and a decrease in apoptotic cells, measured after the first dose, up to the 4 day marker (50% decrease) with no further influence. This increase in satellite cells was not observed in uninjured tissue.^[191] A reduction in apoptosis and mitochondrial dysfunction has also been observed with ischemia/reperfusion injuries to skeletal muscle.^[192] The mechanism of mitochondrial protection seems to be related to preserving membrane permeability, possibly secondary to melatonin's antioxidative capacity.^[193]

At least one study pertaining to athletes investigated whether nightly melatonin supplementation (5mg) hampered daytime physical activity. The study failed to note any harm to physical performance from sedation.^[194]

In a study where 5mg of melatonin was taken 3 hours prior to exercise, it was found that melatonin increased sedation and reduced reaction time, whereas it did not significantly affect physical performance as assessed by 4km cycling test. The authors suggest that the hindering effect of melatonin on physical performance during the day was more neural than physical.^[195]

When investigating postmenopausal breast cancer survivors, melatonin does not appear to have any influence on circulating estrogens (17b-estradiol measured) after 4 months of daily 3mg melatonin supplementation prior to sleep.^[196]

When examining the aromatase enzyme (CYP1A1/2, conversion of testosterone to estrogen), melatonin appears to interact with aromatase. In MCF-7 (breast cancer) cells conditioned to proliferate after testosterone administration (via estrogen), melatonin was found to slightly suppress proliferation and inhibit aromatase at physiological concentrations (approx. 58% of control levels at 1nM) as compared to pharmaceutical concentrations (75% at 10uM). It was able to suppress Cortisol-Induced Aromatase (cAMP) upregulation.^[197] These actions appear to be through activation of the MT1 receptor,^[198] secondary to downregulating aromatase-inducing genes.^[199] These effects have also been noted in fibroblast cells, a source of estrogen production in postmenopausal estrogen-responsive breast cancer.^[200]

Melatonin appears to regulate aromatase, but the concentrations at which it does this may be more likely to prevent an age-related deficiency, rather than act as a pharmacological intervention that would be used with testosterone supplements.

In the Leydig cells of hamster testes, melatonin appears to suppress androgen signalling via the MT1 receptor. Melatonin agonism of the MT1 receptor leads to downregulation of the StAR enzyme, as well as other steroidogenic enzymes, such as 3β-HSD and 17β-HSD.^[201] These effects appear opposite of D-aspartic acid and in parallel to the actions of Corticotropin-Releasing Hormone (CRH).^[201] Melatonin appears to increase intracellular corticotropin-releasing hormone levels^[201] which, paired with the passive diffusion of CRH from Leydig cells,^[202] led to researchers to pair melatonin with an antagonist of the CRH receptor, which completely abolished the inhibitory effects of melatonin on testosterone synthesis.^[203] Melatonin appears to work via MT1 to decrease phosphorylation of p38, which increases synthesis of CRH,^[204] after which CRH suppresses androgen synthesis.^[203]

Melatonin is an indirect negative regulator of testosterone in the testes.

When supplemented to otherwise healthy men, 6 mg of melatonin does not appear to significantly influence testosterone levels. It may trend to attenuate the exercise-induced decrease in testosterone.^[79] This same dose taken nightly for a month does not alter testosterone levels, luteinizing hormone levels or follicle stimulating hormone levels in otherwise healthy men.^[165]

Despite the mechanisms of negative regulation, melatonin does not appear to actually influence testosterone levels in healthy men.

Supplemental melatonin in young healthy men at the doses of 50mcg, 500mcg, and 5mg does not appear to influence circulating cortisol levels when taken at 2:30 and measured for 150 minutes afterward.^[114] When 5g of melatonin is taken at 5pm nightly, for four nights, in a similar demographic, and measurements are taken during sleep, the 24-hour area under curve (AUC) of cortisol is slightly increased.^[115]

In a clinic study on non-obese, postmenopausal women aged 54-62, a 100mg dose of melatonin was observed to be able to keep melatonin elevated for up to 12 hours.^[81]) Melatonin taken at 8am was able to increase cortisol 24-hour AUC slightly (from 219+/-17 to 229+/-14nmol/L, a 4.5% increase) but caused a significant increase during the hours of 20:00-01:00, which was suppressed by exogenous estrogen.^[80]

Melatonin supplementation appears to stimulate growth hormone secretion secondary to resensitizing the pituitary gland to Growth-Hormone-Releasing-Hormone (GHRH), as evidenced by augmenting the effects of single injected doses of GHRH and normalizing the magnitude of the second pulse (repeated doses of GHRH have an attenuated spike due to desensitization).^[205] It is thought that the mechanism is similar to that of pyridostigmine.^[205]

Both 500mcg and 5mg of melatonin appear to be similarly effective in increasing growth hormone levels one hour after ingestion during waking, exhibiting a trend to normalize by 150 minutes after ingestion, with the area under curve (AUC) until this point increasing by 16+/-4.5 to 17.3+/-3.7mUh/L. A 50mcg dose is not significantly different than placebo.^[114] A 5mg dose has been tested elsewhere and measured over 24 hours, but the overall increase was less (increasing basal levels from 3.4+/-1.3mU/l to 5.3+/-2.4mU/l) and not statistically significant.^[115]

A 500mg dose of melatonin may inhibit the release of growth hormone that is induced by serotonin, which appears to be exercise-related and insulin-induced hypoglycemia (low blood sugar).^[206]

Melatonin supplementation in the range of 500-5,000mcg is able to acutely increase growth hormone levels in otherwise healthy young males at rest, which is thought to be due to melatonin’s ability to sensitize the pituitary to the effects of GHRH, rather than through a direct stimulatory effect.

Interactions between melatonin and exercise in regard to growth hormone are somewhat mixed, as one study using 500mcg and 5mg of melatonin against placebo in young and otherwise healthy people experienced with resistance training, noted that, for 120 minutes after exercise, 5mg melatonin significantly increased growth hormone response, relative to placebo, in men, while 500mcg trended towards significance. This study has a research grant from Iovate Health Sciences.^[207] Other studies on the subject note that a 5mg oral dose of melatonin taken before anaerobic bicycle exercise can significantly increase the peak and overall exposure to growth hormone by approximately 72%.^[78] One other study in resistance-trained adult men undergoing full-body resistance training supplementing 6mg melatonin an hour before exercise noted that melatonin actually decreased the exercise-induced spikes in growth hormone, relative to placebo.^[79]

Melatonin at 50mcg, 500mcg, and 5mg does not appear to significantly influence prolactin levels over 150 minutes post-ingestion, a time frame during which melatonin influences other hormones.^[114] 5mg of melatonin taken for four days appears to positively influence 24-hour prolactin levels, however.^[115]

Melatonin is a multi-modal antioxidant, being implicated in increasing the concentration of certain antioxidant enzymes, such as catalase and the superoxide dismutases,^[188] as well as inherently having a structure that confers antioxidative properties per se.^[208] Melatonin has shown benefits for inhibiting oxidation, secondary to oxygen-based free radicals^[209][210] hydroxyl radicals,^[211][212] and reactive nitrogen species like peroxynitrite and nitric oxide.^[213][214]

Through antioxidant means, melatonin can inhibit mineral-induced damage to DNA (in this study, chromium III was used) in a dose-dependent manner, with 24+/-1% inhibition at 1uM and 80+/-3% at 100uM.^[215] Melatonin was the most protective, tested on a concentration dependent basis (outperforming green tea catechins, resveratrol, and alpha-lipoic acid). It has an IC₅₀ value of 3.6+/-0.1uM.^[215]

In at least one blinded intervention, 3mg of melatonin taken nightly also improved subjective ratings of Gastroesophageal Reflux Disease (GERD) and reduced heartburn. It was less effective than omeprazole, but the two were additive when used in combination.^[216]

In patients with stomach ulcers that test positive for Heliobactor pylori, twice daily dosing of 5mg of melatonin over 21 days (paired with omeprazole in all groups), complete healing of ulcers was seen in the melatonin group (n=7) and the L-tryptophan group, but only 3 of 7 had complete healing in control, given only omeprazole.^[217] These healing effects on ulcers have also been seen with H. pylori negative stomach ulcers, in combination with omeprazole,^[218] In a study on aspirin-induced stomach ulcers, melatonin in isolation showed protective effects.^[219]

Melatonin is able to influence eye health, as the eye expresses melatonin receptors (MT1 and MT2),^{[220][221][222]} where activation of retinal MT1 receptors decreases ocular pressure. This is seen after supplementation of melatonin in healthy men^[223] and when pre-loaded before cataract surgery.^[224] A 500mcg dose of melatonin, administered orally at 6 p.m., to otherwise healthy men was able to significantly reduce intra-ocular blood pressure by 9-10 p.m., and retained statistically insignificant suppression at 8 p.m. as well as 11-12 p.m. (latest measurement).^[223]

One study, investigating mechanisms, noted that incubation of retina with melatonin agonists was associated with modulation of adrenaline receptors, downregulation of the beta(2)adrenergic receptor and upregulation of the alpha(2)adrenergic receptor, which is suppressive of adrenaline's actions.^[225] Protein content of carbonic anhydrases is also reduced with melatonin agonists, which may also confer reductions in ocular blood pressure.^[226]

Melatonin appears to regulate ocular blood pressure, by reducing intraocular blood pressure through melatonin receptors. This effect may occur through suppressing the actions of adrenaline-mediated blood pressure increases. A 500mcg oral dose can reduce ocular blood pressure 2-3 hours after administration.

The concentration of melatonin in the eye (aqueous humour) appears to be roughly similar between glaucoma patients and normal people, when measured in the morning and early afternoon (800 and 1600h),^[227] a time when levels are lower than in the evening.^[228] It is not known if people with glaucoma have lower sleeping melatonin levels in the aqueous humour.^[229]

In an open-label pilot study using 3mg of melatonin nightly for 6 months on 55 people (110 tested eyes), melatonin was associated with either a slight improvement or stability of disease pathology in the majority of patients.^[230]

One study investigating melatonin and tinnitus found that 3mg of melatonin, taken nightly for 30 days (double-blind crossover with 1 month washout), was associated with improvements on at least 2 of 3 rating scales for tinnitus (tinnitus matching, tinnitus severity index, self-rated tinnitus), with 57% of the melatonin group experiencing benefits and only 25% of the placebo reporting benefit. Melatonin appeared to benefit men slightly more than women.^[144] This study was a similarly structured replication of a previous study, which reported 46.5% of the melatonin group experiencing benefit, and 20% of the placebo reporting the same.^[231] Only one other study has been conducted on melatonin and tinnitus thus far, and in this prospective open-label study, 3mg of melatonin taken for 4 weeks was also associated with decreased symptoms of tinnitus that appeared to be, overall, unrelated to the benefits of melatonin on sleep.^[232]

In another trial pairing, 3mg of melatonin, paired with the blood-flow enhancing drug Sulodexide at 250-500mg (with the other two groups being melatonin in isolation and control) during both combination therapy and melatonin in isolation were found to be more effective than placebo, while additive benefits against tinnitus were seen with combination therapy.^[233] Improvements were seen in 79.4% of combination therapy instances, and in 58.8% of people in the melatonin-only group, when assessed by both Tinnitus Handicap Inventory (THI) and acufenometry.^[233] This study is duplicated in Pubmed.^[234]

Melatonin is not a cure for tinnitus, but it appears to be more effective than placebo at suppressing tinnitus-related symptoms and improving sleep, secondary to a lessened subjective sensation of tinnitus.

Melatonin is one of the few supplements that demonstrates interactions with the telomerase enzyme (such as astragalus membranaceus), which is correlated with lifespan. In general, older subjects experience less telomerase activity relative to younger subjects, who still have higher telomerase activity.^[235][236] Telomerase is composed of two subunits, the catalytic TERT subunit and the RNA-containing TR subunit.^[237]

A study comparing young and old rats fed 10mg/kg melatonin of intraperitoneal injections, daily for 21 days, found that melatonin increase telomerase activity in gastric mucosa. When only 55% of rat pups had detectable levels of telomerase activity, melatonin increased the levels to 100%, while older rats went from no detectable telomerase to 45% of the rats having detectable levels (assessed by a telomerase PCR ELISA kit).^[238]

In vitro activation of the membrane receptor (MT1) was able to increase the RNA levels of the catalytic subunit of telomerase, TERT (observed in research as it correlates well with telomerase^[239]), in an MCF-7 cell line by about 50% at 1nM concentration with no dose-dependence.^[237] However, binding of an agonist to RZR/RORα (a nuclear receptor melatonin can bind with^[61]) can reduce expression of TERT in a dose-dependent manner by 30-40%, depending on concentration (1pM-1nM, the latter being the level of circulating melatonin).^[237] Neither receptor appears to influence the TR subunit.

Melatonin appears to be able to positively and negatively regulate the catalytic subunit of telomerase, which appears to be of equal potency at physiologically relevant concentrations of 1nM. At least one rat study noted that pharmacologically high levels of melatonin via supplementation increased telomerase expression.

Estrogens appear to be able to induce telomerase activity, due to an imperfect estrogen response element on the telomerase (TERT) promotor.^[240] Estradiol can upregulate telomerase via the ERα.^[241] Melatonin has the ability to regulate aromatase, and suppress excessive telomerase activity induced by estrogens and estrogenic compounds such as cadmium,^[242] which is useful for estrogen-responsive cancers that express higher levels of telomerase for cell viability.^[243][244] This inhibition of TERT expression in cancerous estrogen responsive cell lines has been observed in vivo with 0.1mg/mL melatonin in rat drinking water.^[245]

Melatonin appears to be able to suppress excessive expression of telomerase through environmental and endogenous estrogens, via regulation of TERT transcription. This is more of an anti-cancer mechanism, with its implications for longevity currently unexplored.

Melatonin has been noted in one meta-analysis to reduce the risk of death after one year in people with solid tumor cancers, with a relative risk (RR) of 0.66 and 95% CI of 0.59-0.73, suggesting approximately a quarter risk.^[246]

Melatonin may exert a general protective effect in cancer patients, which results in less death.

It has been hypothesized that melatonin levels, through markers such as circadian rhythm disturbances and urinary metabolite levels, is inversely correlated with breast cancer, and that a reduced melatonin status increases breast cancer risk.^{[247][248][249][250]}

A few mechanisms have been investigated in regard to melatonin's role in breast cancer, including its role as a modulator of aromatase enzyme protein content, modulation of the cell cycle via suppressing cyclin D1, and influencing transcriptional activity of nuclear receptors.^[250]

Melatonin at high doses (18mg) has been investigated in a trial on advanced gastrointestinal cancer, with or without high dose fish oil (4.9g EPA, 3.2g DHA).^[251] This study used treatment for the first 4 weeks and combination therapy for the next 4 weeks. No changes in circulating biomarkers or cytokines were observed, although both fish oil and melatonin had limited efficacy in stabilizing weight or inducing weight gain (38% of patients on fish oil, 27% on melatonin) while the combination had additive effects (68%), suggesting promise for cancer-related cachexia and anorexia.^[251]

Melatonin’s life-extending properties are being investigated for a few reasons. A study in rats involved actually removing the pineal gland from young rat pups and putting it into live, elder, male mice. This process induced a 12% increase in lifespan and is theorized to be secondary to melatonin.^[252] This hypothesis results are strengthed by the reverse study where old pineal glands into younger rats accelerates aging,^[253]. The only currently validated method of life extension (caloric restriction) is associated with up to twofold higher circulating levels of melatonin, when compared to control rats fed ad libitum.^{[40][254][255]} It is currently unknown if this is a biomarker of longevity or causative thereof.

A study comparing young healthy controls (n=20), clinically healthy elder people (n=24), and centenarians (n=24) found that those over 100 years of age had standard aging of thyroid parameters and dehydroepiandrosterone (decreases in accordance with age) but that the difference between daily and nightly melatonin excretion (urine) was more similar to the youthful control rather than the aged cohort, which experienced a normalization.^[256]

Melatonin and the pineal gland are highly associated with longevity. Since melatonin levels decline with age, supplementation can have restorative effects.

Melatonin has been shown (in rats) to alleviate the hyper-oxidative state forced upon mitochondria with aging.^[257] This can be seen as a protective effect, but it is not exerting a protective effect above what is observed in youth.

Administration of melatonin through drinking water to BALB/c (albino, laboratory-bred) female mice between the hours of 18:00 to 08:30, at a concentration of 10ug/mL, increased lifespan from an average of 715 days in control to 843 days in melatonin-treated groups. This is an 18% increase. A life extension effect was also observed when male mice started melatonin at 19 months of age (elderly status).^[258] Another part of the study, using NZB mice given melatonin either nightly or throughout the day, noted that nightly administration increased lifespan from 19 months to 23 months with statistical significance, but administration throughout the day only increased lifespan by one month and was not statistically significant.^[258]

Combination therapy of melatonin (1mg/kg) and growth hormone (2mg/kg) has been found to reverse some increases in inflammatory cytokines (TNF-α and IL-1) and increase others (IL-10) in cardiac tissue of rats. Therapy also abolished age-related changes in NF-kB distribution (cytosol and nuclear membrane).^[259] The reduction in mitochondrial potential seen with aging in cardiac cells (SAMP8 mice) appears to also be normalized^[260] and is seen in rats in a rehabilitative manner (30 days of supplementation to older rats).^[261]

Melatonin appears to confer a cardioprotective effect against aging.This is not abolished by growth hormone therapy, though the study in question was unable to assess synergism or additive effects.

In SAMP8 mice and Wistar rats, oral melatonin (1mg/kg) appears to reduce some effects of aging on the skin.^[262]

Melatonin administration of both 1 and 10mg/kg daily in rats is also able to prevent age-related changes in pancreatic tissue.^[263] and liver tissue.^[264]

A reduction in age-related changes associated with oral intake of melatonin at 1mg/kg in rats and mice (0.08-0.16mg/kg bodyweight humans, or 5.45mg for a 150lb person) appears to extend to all measured organs.

At least one study on human skin cells noted that glycolic acid, a small α-hydroxy acid commonly found in skin care products, reduced lipid peroxidation, as assessed by thiobarbituric acid reactive substances (TBARS), by up to 14% at 1mM concentration, yet melatonin at a 1:200 ratio (much less than glycolic acid) exerted 80% antioxidative synergism, as assessed by TBARS.^[265] More potent antioxidative synergism was observed between glycolic acid and vitamin E, however.

The pathophysiology of hair loss involves oxidative stress^[266] which seems to extend to androgenic hair loss, as hair follicles have increased sensitivity to oxidative stress.^[267] Due to the potency of melatonin as a direct antioxidant,^{[211][268][269]} and the fact that hair follicle cells express and produce melatonin,^[270][271] it has been investigated for its role in preventing hair loss.^[272] Topical application of melatonin to the scalp does not significantly increase serum concentrations of melatonin, instead providing a clinically irrelevant increase in peak values, with no change in 20-hour mean values.^[272]

In vitro, 1-5mM of melatonin has been found to accelerate the growth of hair (inhibitory at 30mM), which is thought to be secondary to the melatonin receptor, as the effect is abolished by receptor antagonists (study cannot be located online, mentioned indirectly through this paper^[272]). These growth enhancing effects were confirmed in a pilot study using 1mL of a solution of 0.1% melatonin, given to women with androgenic alopecia over 6 months, since the anagen phase was promoted,^[273] and a later study noted that there was a response rate of 54.8% of participants, in regard to increasing hair density and hair cell count over 3 months with topical melatonin (0.03%), with the increase being measured at 27.2% (3 months) and 42.7% (6 months) more than control.^[272]

Although the study cannot be found online (again, mentioned indirectly through a review^[272]), there appears to be a large open-label study of 1891 people with androgenic hair loss, which reported that the rate of no hair loss rose from 12.5% to 61.5%, but stimulated hair growth in 22.5% of the sample.

Melatonin receptors are expressed in hair cells. It appears to have a role in promoting hair growth. While melatonin does appear to be both effective and potent at inducing hair growth and stalling hair loss, even in androgenic hair loss, as is the case with a receding hairline, it appears to be somewhat unreliable and does not affect 100% of subjects.

Caffeine is an adenosine receptor antagonist with affinity towards A2(A), and is seen as anti-sleep due to its effects. It is also stimulatory, but this effect wanes with time.

Both caffeine and melatonin are metabolized by the same enzyme class, specifically, the aromatase enzyme (CYP1A1/2). Coingestion of the two appears to exert competition in metabolism, as the area under curve (AUC) of melatonin is increased by 120% when ingested with caffeine, without significantly affecting the half-life of melatonin.^[274]

When investigating mitochondrial function in mice and in vitro, caffeine appears to exert similar effects as melatonin in preventing Alzheimer's-related cognitive deficit, but it is less potent and actively inhibs melatonin's benefit when coingested.^[275] This effect does not appear to occur through the adenosine receptors per se, but through Phosphodiesterase-4 (PDE4) inhibition and increasing Cyclic Adenosine Monophosphate (cAMP) levels, which oppose melatonin's decrease of cAMP.^[275]

Alcohol is a component of some beverages, like beer and wine, that, surprisingly, contains melatonin. However, when investigating how alcohol consumption affects circulating melatonin levels during social drinking (10-100g ethanol), some studies suggest a resulting reduction of circulating or salivary melatonin levels,^[276][277] with inhibition up to 41%, measured at night.^[278] Some studies suggest an increase in melatonin, as according to one study, using beer (7.2%), where women had 330mL and men had 660mL, reported an increase in melatonin levels 45 minutes after ingestion.^[12]

When measuring urinary melatonin levels, however, there do not appear to be any significant influences from alcohol consumption,^{[278][279][276]} and one study suggests a decrease in urinary melatonin secretion between 9-17% (2-4 drinks, no effect seen from just one).^[280]

The effects of alcohol on melatonin are not clear at this time.

An in vitro study assessing melatonin (1uM) and vitamin C at 0.5uM demonstrated synergism in protecting DNA from oxidative damage induced by chromium III.^[215] The slightly pro-oxidative effects seen with vitamin C were reversed when melatonin was co-incubated.^[215]

An in vitro study assessing melatonin (1uM) and alpha-lipoic acid (5uM) found that, in protecting DNA from chromium III induced oxidative damage, the two were synergistic in their protective effects.^[215]

Tryptophan is the amino acid metabolized into 5-HTP, from which serotonin and subsequently melatonin can be produced.

Melatonin appears to inhibit the tryptophan 2,3-dioxygenase (TDO) enzyme, which directs tryptophan away from production into 5-HTP by enhancing its catabolism.^[281] Inhibition of TDO via melatonin can enhance the amount of bioavailable tryptophan, independent of supplementation.

Epigallocatechin Gallate (EGCG) is the main polyphenol referred to as green tea catechins. In an in vitro test on DNA-induced oxidation, it was found that co-incubation of melatonin and EGCG, both at 1uM, slightly suppressed each other's actions, demonstrating antagonism.^[215] It should be noted that the overall protection exerted with both was still greater than either in isolation, but that there was a less than additive benefit.^[215]

When tested in vitro, resveratrol, the wine polyphenol, did not show synergism or antagonism with melatonin in protecting DNA from oxidative damage.^[215] However, the slightly pro-oxidative effects of resveratrol on DNA were abolished when melatonin was added to the medium.^[215]

Resveratrol does display synergism with melatonin in regard to neuroprotection, where melatonin and resveratrol both showed dose-dependent protection from toxicity in hippocampal cells from beta-amyloid pigmentation (associated with Alzheimer's), since the combination required less of a dose to exert the same effects as either supplement in isolation.^[120] This appears to be mediated via antioxidative means, and superloading either in isolation overrides synergism.^[120]

Although not a nutrient, melatonin has shown synergistic effects with physical exercise in regard to accelerating neuronal repair after nerve injury,^[282] and in terms of general neuroprotective effects.^[283] The former study noted more motor neurons in the ventral horn and improved functional capacity in rats given 10mg/kg of melatonin daily, in conjunction with exercise. This was hypothesized to occur via a suppression of Nitric Oxide Synthase (iNOS) in neurons, which was observed, since iNOS tends to be increased during neuronal injury.^[282] The latter study noted that in mice prone to Alzheimer's disease,^[284] this same dose of melatonin, paired with free access to a running wheel, is able to preserve reflexes and memory function while synergistically decreasing neural oxidation and Alzheimer's pathology in the brain.^[283] Melatonin and exercise appeared to significantly increase CoQ₉ in brain mitochondria, the precursor to CoQ10.^[283]

A sodium selenate (source of selenium) and melatonin treatment, given to rats prior to ischemia injury (middle carotid artery occlusion) is able to confer protective benefits.^[285]

Galantamine is a cholinergic drug that can inhibit the acetylcholinesterase enzyme, and elevate levels of acetylcholine in the brain (a mechanism similar to huperzine-A). While subeffective levels of melatonin (0.3-10uM) and galantamine (10-300nM) are able to confer synergistic protection against rotenone-induced oxidative stress, 300nM galantamine and 10uM melatonin protects neurons by 56% and 50%, respectively, and 0.3uM melatonin and 10nM galantamine combined confer the same level of protection, despite being in concentrations 30 to 33-fold lower.^[286] This synergism may not be restricted to the molecules, but may be mediated through the nicotinic and melatonin receptors.^[286]

Up to 240mg^[287] and 500mg oral^[288] and intravenous injections^[289] of melatonin taken acutely is not associated with any toxicological effects.

In older people with insomnia, 6 months of melatonin at 2mg (long-release formula) is not associated with any overt harm^[137] and appears to be safe and effective at 5mg (according to a caretaker interview) in children with the average age of 6 (at onset of treatment) that suffer from sleep disorders as symptoms of other medical problems (autism, cerebral palsy, epilepsy). When the caretakers were interviewed 3.8 years after starting treatment, some caretakers had upped the dose to 10mg or 15mg.^[290]

A study assessing 6 months (n=112) and 12 months (n=96) of melatonin treatment of 2mg in a controlled release capsule, taken 1-2 hours prior to sleep, people aged 20-80 with primary insomnia failed to show any tolerance to the treatment. The authors noted a slight sensitization to the effects of melatonin at the 3-4 month period, which was attributed to better entrainment of the circadian rhythm.^[291] These results have been replicated in another study, lasting 6 months, with a sample of 791 people. No melatonin tolerance due to usage was observed,^[137] Another study, lasting 6 months with a sample size of 421 people also replicated these results.^[138]

A handful of large scale, 6-12 month studies, suggest continued administration of melatonin does not result in tolerance.

Since benzodiazepines are a popular sedative and known to have withdrawal effects, it is a common concern as to whether melatonin supplementation confers withdrawal or dependence in otherwise healthy people. This concern is somewhat backed by melatonin interacting with benzodiazepine receptors.^[292][293] Melatonin is sometimes used for prolonged periods in youth with clinically meaningful sleep disturbance problems.^[290][294]

In a study investigating children with sleep onset problems taking melatonin (1-5mg depending on individual efficacy) for 3 weeks, halving the dose for 1 week before stopping supplementation, the termination of effects was replicated where half-dosing for a week reduced the benefit to sleep latency nonsignificantly. Ceasing treatment removed the benefits of melatonin.^[295] This study also made note of an unpublished thesis, where the benefits of 3-week melatonin usage were abolished upon cessation. The thesis is not available online. One other study on people with rapid-cycling bipolar disorder given 10mg noted negative effects. In this sample of people (n=5), delayed sleep onset relative to baseline was observed.^[296]

In contrast to this, at least one large scale (n=791) double-blind study on insomniacs noted that there were no withdrawal symptoms associated with stopping melatonin usage after 6 months, at 2mg of a sustained release formulation. Withdrawal was assessed by the Tyrer questionnaire, with about 28% in both placebo and melatonin.^[137] Data from 6-12 months of melatonin usage in insomniacs noted that during a 2-week monitoring period after cessation of melatonin, there was a slight residual effect of better sleep, no tolerance during long-term melatonin treatment, and no noted withdrawal effects significantly different than placebo.^[291] An apparent absence of withdrawal or dependence is more common in older people with symptoms of insomnia. Melatonin for usage of up to 6-12 months is not associated with dependence or withdrawal symptoms, an example of a potential withdrawal symptom being exacerbation of insomnia.^{[297][139][140]}

There is a lack of solid evidence to suggest the presence of withdrawal or dependence from melatonin supplementation. Some evidence affirms that there is no adverse effect on drug dependence or withdrawal. Doses higher than 2mg have not been sufficiently studied.

It is possible that the termination of the benefits to sleep upon discontinuing melatonin may be seen as 'reactive insomnia', as sleep quality returns to the quality that it was prior to melatonin intervention.