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Melatonin receptor agonist

Melatonin

Melatonin is an endogenous neurohormone and exogenous supplement that acts as a melatonin receptor agonist, primarily used for sleep-wake cycle disorders. The most testable fact is that melatonin regulates circadian rhythm via MT1 and MT2 receptors in the suprachiasmatic nucleus without causing dependence or significant next-day sedation.

Practice questionsHigh-yield facts
Drug classMelatonin receptor agonist, neurohormone
MechanismAgonist at MT1 and MT2 receptors in suprachiasmatic nucleus
Main usesInsomnia, jet lag, delayed sleep phase syndrome
Key adverse effectDaytime drowsiness, headache, dizziness
7 min readFor medical studentsUpdated 2026-10-14

Mechanism of action

Melatonin is a neurohormone naturally synthesized from serotonin in the pineal gland. Endogenous melatonin secretion follows a circadian pattern, with peak levels occurring during darkness to promote sleep onset and maintenance.

Exogenous melatonin acts as an agonist at two G-protein coupled receptors:

  • MT1 receptors: Located in the suprachiasmatic nucleus (SCN) of the hypothalamus; activation suppresses neuronal firing and promotes sleep initiation
  • MT2 receptors: Also in the SCN; mediate phase-shifting of circadian rhythms, allowing adjustment of the sleep-wake cycle

Unlike traditional sedative-hypnotics such as benzodiazepines or Z-drugs, melatonin does not act on GABA receptors. It primarily regulates the timing of sleep (chronobiotic effect) rather than directly inducing sedation. This distinction makes melatonin particularly useful for circadian rhythm disorders rather than acute insomnia due to anxiety or pain.

Melatonin also has antioxidant properties and modulates immune function, though these effects are not the basis for its clinical use in sleep disorders.

High-yield for exams
  • Melatonin acts on MT1 and MT2 receptors in the suprachiasmatic nucleus to regulate circadian rhythm, not GABA receptors.
  • Metabolized primarily by CYP1A2; fluvoxamine dramatically increases melatonin levels and should be avoided.
  • Does not cause dependence, tolerance, withdrawal, or rebound insomnia unlike benzodiazepines and Z-drugs.
  • Most effective for circadian rhythm disorders (jet lag, delayed sleep phase, shift work) rather than acute insomnia.
  • Short half-life of 40 to 60 minutes limits next-day sedation with immediate-release formulations.
  • Preferred hypnotic in elderly patients due to favorable safety profile and lack of anticholinergic or fall risk.
  • Timing is critical: take 30 to 60 minutes before desired bedtime for sleep onset insomnia.
  • Higher doses do not necessarily improve efficacy; physiologic doses of 0.3 to 1 mg are often sufficient.

Indications

Melatonin is used for various sleep and circadian rhythm disorders:

Primary indications:

  • Insomnia: Particularly in adults over 55 years (prolonged-release formulations are licensed in some regions for this age group)
  • Jet lag: Helps resynchronize circadian rhythm after rapid travel across time zones
  • Delayed sleep-wake phase disorder: Common in adolescents and young adults with constitutionally delayed sleep phase
  • Shift work sleep disorder: Assists adaptation to non-standard sleep schedules
  • Non-24-hour sleep-wake disorder: Especially in blind individuals who lack light-mediated circadian entrainment

Pediatric uses:

  • Sleep onset insomnia in children with neurodevelopmental disorders (autism spectrum disorder, ADHD)
  • Sleep disturbances in children with visual impairment

Other uses:

  • Preoperative anxiolysis and sedation (investigational)
  • REM sleep behavior disorder adjunct
  • Sleep disturbances in dementia (limited evidence)

In many countries, melatonin is available as an over-the-counter dietary supplement, though prescription formulations exist for specific indications. Dosing follows product labeling and clinical context.

Pharmacokinetics

Absorption: Melatonin is rapidly absorbed after oral administration, with peak plasma concentrations reached within 30 to 120 minutes. Bioavailability is low (approximately 15 percent) due to extensive first-pass hepatic metabolism. Food can delay absorption but does not significantly affect overall bioavailability.

Distribution: Melatonin is lipophilic and crosses the blood-brain barrier readily. It is approximately 60 percent protein-bound in plasma. Volume of distribution suggests wide tissue distribution.

Metabolism: Hepatic metabolism is the primary route of elimination, predominantly via cytochrome P450 enzymes, especially CYP1A2 (major pathway) and CYP2C19. Melatonin is hydroxylated to 6-hydroxymelatonin, then conjugated with sulfate or glucuronide before renal excretion.

Excretion: Metabolites are excreted primarily in urine. The elimination half-life of immediate-release melatonin is short, approximately 40 to 60 minutes, contributing to its limited duration of effect. Prolonged-release formulations extend the duration by delaying absorption.

Special populations:

  • Elderly: May have reduced CYP1A2 activity, leading to higher plasma levels and prolonged half-life
  • Hepatic impairment: Clearance is reduced; caution is advised
  • Renal impairment: Minimal effect on melatonin clearance since metabolism is hepatic

Adverse effects

Melatonin is generally well tolerated, with a favorable safety profile compared to traditional hypnotics. Adverse effects are typically mild and transient.

Common adverse effects:

  • Daytime drowsiness or somnolence: Especially if taken at inappropriate times or in excessive doses
  • Headache: One of the most frequently reported effects
  • Dizziness: May impair balance or coordination transiently
  • Nausea: Gastrointestinal discomfort is occasional

Less common effects:

  • Vivid dreams or nightmares
  • Mood changes: Irritability, depressive symptoms (rare)
  • Hypothermia: Mild decrease in core body temperature
  • Abdominal discomfort, diarrhea

Rare effects:

  • Hypersensitivity reactions: Rash, pruritus
  • Transient hypertension or palpitations (case reports)
  • Seizures (very rare, causality unclear)

Important safety considerations:

  • No dependence or withdrawal: Unlike benzodiazepines and Z-drugs, melatonin does not cause physical dependence, tolerance, or withdrawal syndrome
  • Minimal next-day impairment: Short half-life limits hangover effect, though prolonged-release formulations may cause morning drowsiness
  • Hormonal effects: As an endocrine substance, theoretical concerns exist regarding reproductive hormones, though clinical significance is unclear at typical doses

Melatonin does not suppress REM sleep or alter normal sleep architecture, which is advantageous over traditional sedative-hypnotics.

Contraindications and warnings

Absolute contraindications:

  • Hypersensitivity to melatonin or any formulation component

Relative contraindications and cautions:

  • Autoimmune disorders: Melatonin may stimulate immune function; use cautiously in systemic lupus erythematosus, rheumatoid arthritis, or other autoimmune conditions
  • Hepatic impairment: Reduced metabolism may lead to elevated plasma levels
  • Seizure disorders: Limited data; some case reports suggest potential for lowering seizure threshold, though evidence is inconsistent
  • Depression: May exacerbate depressive symptoms in susceptible individuals

Pregnancy and lactation:

  • Pregnancy: Safety has not been established. Melatonin crosses the placenta, and high doses in animal studies show reproductive effects. Avoid unless benefit clearly outweighs risk.
  • Lactation: Melatonin is excreted in breast milk. Endogenous levels are naturally elevated at night. Exogenous supplementation should be approached cautiously.

Pediatric use: While increasingly used off-label for sleep disorders in children with neurodevelopmental conditions, long-term safety data are limited. Concerns exist regarding effects on pubertal development due to melatonin's role in reproductive hormone regulation.

Activities requiring alertness: Patients should be cautioned about driving or operating machinery within several hours of taking melatonin due to potential drowsiness.

Surgery: Some practitioners recommend discontinuing melatonin before surgery due to theoretical bleeding risk (antiplatelet effects) and interaction with anesthetics, though evidence is limited.

Drug interactions

Melatonin has a moderate potential for drug interactions, primarily via CYP1A2 metabolism and additive CNS effects.

CYP1A2 interactions:

  • Fluvoxamine (strong CYP1A2 inhibitor): Dramatically increases melatonin levels (up to 17-fold); concomitant use should be avoided
  • Ciprofloxacin, other fluoroquinolones: Inhibit CYP1A2, potentially increasing melatonin exposure
  • Smoking (tobacco): Induces CYP1A2, reducing melatonin levels and efficacy
  • Caffeine: Metabolized by CYP1A2; high intake may compete with melatonin metabolism

CNS depressants (additive sedation):

  • Benzodiazepines, Z-drugs (zolpidem, eszopiclone)
  • Alcohol
  • Opioids
  • Sedating antihistamines
  • Antipsychotics, particularly sedating agents

Anticoagulants and antiplatelets:

  • Warfarin: Case reports suggest melatonin may increase INR; monitor coagulation parameters
  • Antiplatelet agents: Theoretical increased bleeding risk

Antihypertensives:

  • Melatonin may reduce effectiveness of antihypertensive medications (particularly nifedipine, other calcium channel blockers) due to potential transient blood pressure effects

Immunosuppressants:

  • Theoretical concern that melatonin's immune-stimulating properties could antagonize immunosuppressive therapy in transplant recipients

Other interactions:

  • Antidiabetic agents: Melatonin may affect glucose metabolism; monitor blood glucose
  • Oral contraceptives: May increase endogenous melatonin levels

Overall, clinically significant interactions are uncommon, but awareness of CYP1A2 inhibitors and additive sedation is important.

Clinical pearls

  • Timing is critical: For sleep onset insomnia, melatonin should be taken 30 to 60 minutes before desired bedtime. For circadian phase shifting (e.g., jet lag, delayed sleep phase), timing relative to the current circadian phase determines whether melatonin advances or delays the rhythm.
  • Dose-response is not linear: Higher doses do not necessarily produce better sleep outcomes. Typical effective doses range from 0.5 to 5 mg, with physiologic doses (0.3 to 1 mg) often sufficient. Supraphysiologic doses may increase adverse effects without added benefit.
  • Not a traditional hypnotic: Melatonin works best for circadian misalignment, not acute stress-related or pain-related insomnia. Patients expecting immediate sedation similar to benzodiazepines may be disappointed.
  • Jet lag strategy: For eastward travel (phase advance needed), take melatonin in the evening at the destination. For westward travel (phase delay), melatonin is less useful; morning light exposure is preferred.
  • Blind patients: Melatonin is particularly effective for non-24-hour sleep-wake disorder in blind individuals who lack light-mediated circadian entrainment.
  • Formulation matters: Immediate-release formulations help with sleep onset; prolonged-release formulations assist with sleep maintenance. Quality and bioavailability vary widely among over-the-counter supplements.
  • Blue light exposure: Evening blue light (screens, LED lighting) suppresses endogenous melatonin secretion. Advising patients to minimize blue light exposure enhances melatonin efficacy.
  • No rebound insomnia: Unlike benzodiazepines and Z-drugs, discontinuing melatonin does not cause rebound insomnia or withdrawal, making it a safer long-term option for appropriate indications.
  • Exam tip: Melatonin is the preferred agent for sleep disorders in elderly patients due to lack of anticholinergic effects, fall risk, or cognitive impairment associated with traditional hypnotics.

Practice questions

1. A 62-year-old man with a history of major depressive disorder controlled on fluvoxamine presents with new-onset insomnia. He requests melatonin supplementation after reading about its benefits online. Which of the following best explains why melatonin should be avoided in this patient?

Answer: B. Fluvoxamine is a potent CYP1A2 inhibitor, and since melatonin is primarily metabolized by CYP1A2, concurrent use can increase melatonin levels up to 17-fold, leading to excessive sedation and adverse effects. This combination should be avoided. Melatonin does not directly interact with serotonergic pathways in a way that causes serotonin syndrome, nor does it induce CYP2D6 or antagonize SSRI mechanisms.

2. A 28-year-old flight attendant complains of difficulty adjusting to frequent transatlantic flights from New York to London. She asks about melatonin for jet lag. At what time should she take melatonin to optimally adjust her circadian rhythm when traveling eastward?

Answer: C. For eastward travel requiring a phase advance of the circadian rhythm, melatonin should be taken in the evening at the destination bedtime to promote earlier sleep onset and help shift the sleep-wake cycle forward. Taking melatonin in the morning would delay the circadian phase, worsening jet lag after eastward travel. The timing relative to the light-dark cycle at the destination is critical for appropriate phase shifting.

3. A 70-year-old woman with insomnia and mild cognitive impairment is currently taking diphenhydramine 50 mg nightly. Her daughter is concerned about fall risk and cognitive side effects. Which property makes melatonin a superior alternative in this patient?

Answer: B. Melatonin is preferred in elderly patients because it lacks anticholinergic effects (which can worsen cognition and cause urinary retention) and has a lower risk of falls and next-day sedation compared to diphenhydramine. The short half-life of melatonin actually limits hangover effects, and it does not act on GABA receptors but rather on melatonin receptors in the suprachiasmatic nucleus. This favorable safety profile makes it ideal for older adults with cognitive impairment.

Frequently asked questions

How does melatonin work?

Melatonin works by activating MT1 and MT2 receptors in the suprachiasmatic nucleus of the hypothalamus, which regulates the body's circadian rhythm and promotes sleep onset. Unlike traditional sedatives, it does not act on GABA receptors but instead helps synchronize the sleep-wake cycle with environmental light-dark cues. This makes it particularly effective for circadian rhythm disorders such as jet lag and delayed sleep phase syndrome rather than general insomnia.

What are the side effects of melatonin?

Melatonin is generally well tolerated with mild and transient side effects. The most common adverse effects include daytime drowsiness, headache, dizziness, and nausea. Unlike benzodiazepines and other hypnotics, melatonin does not cause dependence, tolerance, withdrawal symptoms, or significant next-day impairment. Rare effects include vivid dreams, mood changes, and hypersensitivity reactions. It does not suppress REM sleep or alter normal sleep architecture.

Is melatonin a controlled substance?

No, melatonin is not a controlled substance in most countries including the United States. It is widely available as an over-the-counter dietary supplement without prescription requirements. However, in some countries such as the United Kingdom and Australia, certain melatonin formulations are prescription-only medications for specific indications. Unlike benzodiazepines and Z-drugs, melatonin has no abuse potential and does not cause physical dependence or withdrawal.

Melatonin vs diphenhydramine — what is the difference?

Melatonin is a hormone that regulates circadian rhythm via MT1 and MT2 receptors, making it ideal for jet lag and delayed sleep phase disorder, whereas diphenhydramine is a sedating antihistamine that works through H1-receptor blockade and anticholinergic effects. Diphenhydramine causes more side effects including dry mouth, urinary retention, confusion (especially in elderly), and next-day sedation, while melatonin has a cleaner side effect profile. Melatonin does not cause tolerance or dependence, whereas diphenhydramine loses efficacy with repeated use and carries higher risk in older adults.

Can you take melatonin every night?

Yes, melatonin can be taken nightly for extended periods as it does not cause tolerance, dependence, or withdrawal symptoms unlike benzodiazepines and Z-drugs. Long-term safety studies support its use for months to years in adults, particularly those over 55 years with insomnia. However, it is most effective for circadian rhythm disorders rather than chronic insomnia from other causes. In children, long-term safety data are more limited, and concerns exist about potential effects on pubertal development, so ongoing monitoring is recommended.

Conditions it is used for

InsomniaJet lagDelayed sleep-wake phase disorderShift work sleep disorderNon-24-hour sleep-wake disorderSleep disorders in autism spectrum disorderREM sleep behavior disorder
RamelteonTasimelteonZolpidemEszopicloneDiphenhydramineTrazodoneDoxepinSuvorexant

Sources and further reading

DailyMed (FDA labels)StatPearls (NCBI Bookshelf)MedlinePlusWHO ATC/DDD index: N05CH01

Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.