Melatonin Explained: What It Actually Does (and Doesn't Do)

Walk into any pharmacy and you will find melatonin sold in doses of 5mg, 10mg, even 20mg — marketed as a natural sleep aid. The framing implies that more melatonin means more or better sleep. This is not how the hormone works.

Melatonin is a circadian signal, not a sedative. Its role is to tell the body "it is dark" — not to induce unconsciousness. The physiological nightly peak of melatonin in most adults is somewhere between 0.1mg and 0.3mg (100 to 300 micrograms). A 10mg supplement delivers roughly 30 to 100 times the amount your body would produce naturally.

What Melatonin Is and Where It Comes From

Melatonin (N-acetyl-5-methoxytryptamine) is a hormone synthesized primarily in the pineal gland — a small endocrine organ situated near the center of the brain. It is also produced in smaller amounts by the retina, gut, skin, and immune cells, where it likely serves local paracrine functions.

The pineal gland synthesizes melatonin from serotonin, which is itself derived from the amino acid tryptophan. The synthesis pathway is regulated by the suprachiasmatic nucleus (SCN), the brain's master circadian clock, which receives light information directly from the retina via the retinohypothalamic tract.

The relationship between light and melatonin is inverse and direct: light suppresses melatonin synthesis; darkness permits it. This makes melatonin a reliable biochemical marker of night — which is precisely its function.

The Nightly Melatonin Rhythm

In a person living in typical light-dark conditions, melatonin secretion follows a highly consistent pattern:

• During the day, melatonin levels are very low — often undetectable in blood plasma.
• Approximately two hours before habitual sleep onset (a precisely measurable timepoint called Dim Light Melatonin Onset, or DLMO), the pineal gland begins releasing melatonin.
• Levels rise steeply through the early night, peaking around 2–3am.
• From roughly 3am onward, melatonin declines — a process accelerated by the morning light signal.
• By the time most people wake, melatonin is at or near its daytime nadir.

DLMO is one of the most precise, reproducible markers of circadian phase available — researchers use it to measure exactly where someone's biological clock is set. The fact that it occurs two hours before sleep onset reflects that melatonin does not cause sleep; it prepares the body for the biological night, of which sleep is one component.

What Melatonin Actually Does

Melatonin acts on MT1 and MT2 receptors throughout the body, communicating the timing of night to tissues that use this information to coordinate their own circadian programs.

In the SCN itself: Melatonin feeds back onto the master clock, helping stabilize and reinforce circadian timing. MT2 receptor activation in the SCN appears to be particularly important for the phase-shifting effects of exogenous melatonin.

In peripheral organs: Melatonin signals "nighttime programming" — reducing core body temperature, shifting metabolic activity, modulating immune function (immune activity is higher at night in many systems), and coordinating the release of growth hormone and other nocturnal hormones.

On sleep: Melatonin does produce a modest reduction in sleep onset latency (the time to fall asleep) in most people, but the mechanism is indirect — it promotes the physiological conditions associated with night and facilitates the transition to sleep rather than directly inducing unconsciousness. It is meaningfully weaker as a sleep inducer than the adenosine-driven sleep pressure that builds throughout the waking day.

How Light Suppresses Melatonin

The photoreception pathway for melatonin suppression runs through specialized retinal ganglion cells containing the photopigment melanopsin. These cells are maximally sensitive to short-wavelength light in the range of 460–480 nm — what we perceive as blue-to-cyan light.

Research by Charles Czeisler's group at Harvard demonstrated that even ordinary indoor room light (100–200 lux) is sufficient to suppress nighttime melatonin by 50% or more in many people. Bright office lighting or screen use in the hours before bed delays melatonin onset — effectively delaying the body's experience of night and pushing the circadian clock later.

The practical consequence: if you want melatonin to rise at its natural time (allowing you to feel sleepy at a reasonable hour), you need to reduce indoor light significantly in the 1–2 hours before bed. Blue-light-blocking glasses can help, but reducing overall light intensity is more effective and does not require any equipment.

Melatonin Supplementation: When It Helps and When It Doesn't

Melatonin supplements are most useful for situations involving circadian phase misalignment — where the body's clock is set to the wrong time relative to the desired sleep window.

Jet lag: The best-supported use case. Taking low-dose melatonin at the destination bedtime (not before flying) helps accelerate re-entrainment of the circadian clock to the new time zone. The effect is more pronounced for eastward travel (advancing the clock) than westward. Timing is important — melatonin taken at the wrong biological clock time can worsen jet lag.

Shift work: Melatonin taken before the desired sleep period helps shift workers fall asleep during what their clock codes as daytime. The evidence is modest, and the benefit is greatest in workers with completely reversed schedules rather than those on rotating shifts.

Delayed sleep phase syndrome (DSPS): People with DSPS have a circadian clock that runs significantly later than social norms demand. Low-dose melatonin taken 5–6 hours before the current sleep time — combined with morning bright light exposure — is one of the most effective treatments for gradually advancing the clock phase.

General insomnia in people with a normally-timed clock: The evidence here is weaker. For people who have no circadian phase problem — who simply have difficulty sleeping at an already-appropriate time — the additional melatonin provides minimal benefit beyond what the body is already producing. The modest sleep-onset benefit documented in some trials may partly reflect a placebo component.

Older adults: Melatonin production decreases with age — often dramatically after age 60. For older adults with genuinely low nocturnal melatonin, supplementation may provide meaningful benefit. Low-dose melatonin (0.5–1mg) is considered relatively safe in this population and lacks the cognitive side effects of sedative medications.

The Dosing Problem

Standard doses sold in pharmacies — 5mg, 10mg — are pharmacological, not physiological. They are many times higher than what the body produces naturally and have measurable effects beyond simply replicating the endogenous signal.

Note: Dose recommendations should be discussed with a clinician. This table reflects general patterns in the published literature, not personal medical advice.

The mismatch between commercial dosing and physiological dosing exists largely because melatonin is classified as a dietary supplement in the United States, where it is not subject to FDA approval requirements for dosing efficacy. Regulatory bodies in Europe and many other countries classify melatonin as a prescription medication, which has resulted in more conservative dosing practices in those markets.

Timing Is Everything

Melatonin has phase-shifting effects — it moves the clock. The direction and magnitude of the phase shift depend entirely on when relative to the current circadian phase you take it.

• Taken in the late afternoon or early evening (5–7pm for most people): advances the clock — makes the phase earlier, promoting earlier sleepiness.
• Taken in the morning after waking: delays the clock — pushes the phase later (worsening jet lag or making things worse for delayed sleep phase).
• Taken at the current biological sleep time: minimal phase shift, primarily adds to ongoing melatonin signal.

This is why the instruction "take melatonin 30 minutes before bed" is oversimplified. For someone going to bed at their natural sleep time, it makes sense. For someone trying to advance their clock (shift an evening person earlier, or recover from jet lag after eastward travel), taking it 4–6 hours before the target sleep time is more effective — and taking it just before bed is largely redundant.

Safety and Long-Term Use

Melatonin is considered safe for short-term use at appropriate doses in most adults. Side effects are generally mild and include next-day drowsiness (more common at higher doses), headache, and dizziness. Unlike sleeping pills, melatonin does not cause physical dependence or rebound insomnia when stopped.

Long-term data on chronic melatonin use at the doses sold commercially are limited. There is theoretical concern that sustained supraphysiological melatonin exposure could desensitize MT1/MT2 receptors, reducing the effectiveness of endogenous melatonin over time — but this has not been definitively demonstrated in humans.

In children, the evidence for melatonin use is growing but should involve clinician oversight. Melatonin is increasingly used for sleep difficulties associated with ADHD and autism spectrum disorder in pediatric populations, where some evidence supports modest benefit.