Melatonin: what it is, what the evidence really says, and what gets overclaimed

What it actually is: the hormone of darkness

Melatonin is a hormone produced by the pineal gland, a pea-sized structure at the center of the brain. Its secretion follows a very simple rule: it rises when darkness falls and shuts off with light. That's why chronobiologists call it 'the hormone of darkness' — it isn't a sleep signal in itself, but the circadian clock's way of telling the rest of the body that it's biological night. In fact, nocturnal animals also secrete melatonin at night — precisely when they're most active — which makes clear the hormone marks the dark phase of the day, not the act of sleeping. Josephine Arendt, the researcher who has done more than anyone to understand this hormone in humans, describes it as a 'chronobiotic': a substance capable of shifting the phase of the internal clock (Arendt and Skene, Sleep Medicine Reviews, 2005). That distinction is the key to this whole article: melatonin doesn't work like a sedative that knocks you out, but as a time signal telling your body where it stands in the circadian day. Understanding that explains both what it can do and what it can't.

What the evidence actually shows: about 7 minutes

Here comes the number almost no supplement label will tell you. The reference meta-analysis on melatonin and primary sleep disorders, published by Ferracioli-Oda, Qawasmi, and Bloch in PLoS ONE (2013), pooled results from 19 placebo-controlled trials and found that melatonin reduces sleep-onset latency — the time it takes you to fall asleep — by about 7 minutes on average, and increases total sleep time by about 8 minutes, with a modest improvement in subjective sleep quality. The effect is real and statistically solid, but small: for comparison, hypnotic drugs and cognitive behavioral therapy for insomnia produce clearly larger effects on those same measures. The authors themselves note that melatonin may have a role because of its relatively benign side-effect profile, not because it competes on potency. If you currently take 45 minutes to fall asleep and you're hoping for 10, the evidence doesn't support that expectation. This honesty matters: much of the disappointment with melatonin comes from asking it to be what it isn't — a sleeping pill — instead of using it for what the evidence does support, which is what comes next.

Where it genuinely shines: shifting the clock, not sedating the brain

If melatonin is a time signal, its natural territory is problems where the internal clock is misaligned with the schedule you want to keep. The best-studied example is jet lag: the Cochrane review by Herxheimer and Petrie (2002) concluded that melatonin, taken close to the target bedtime at the destination, is remarkably effective in preventing or reducing jet lag, and found it reasonable to recommend to adult travelers crossing five or more time zones, especially eastward. Here the effect isn't 'fall asleep faster tonight' — it's helping your circadian clock realign sooner with the new time zone. The same principle applies to delayed sleep phase syndrome — people, often teenagers and young adults, whose clock runs systematically late so they can't fall asleep until the small hours: a low dose taken hours before the desired bedtime can advance the clock's phase, and sleep-medicine guidelines contemplate its use for this disorder, ideally guided by a professional who knows how to time it correctly. Notice the pattern: in both cases melatonin's value lies in when it's taken relative to the internal clock, not in how much it sedates. It's a chronobiotic doing chronobiotic work.

The dose misconception: more is not better

The intuition of 'if it's not working, take more' fits painkillers, but not a time signal. The Cochrane review on jet lag already observed that doses between 0.5 and 5 mg were similarly effective against jet lag (with one caveat: on 5 mg participants fell asleep somewhat faster): beyond a certain point, more melatonin doesn't shift the clock further. It helps to remember the scale: the pineal gland secretes minuscule amounts, and many supplements on the market produce blood levels far above physiological nighttime concentrations. With melatonin, the variable with the most leverage isn't dose but timing: the same amount taken in the evening tends to advance the clock, while taken in the early morning it tends to delay it — this is what chronobiology describes with so-called phase response curves, and the reason reviewers in the field like Nachum Zisapel (British Journal of Pharmacology, 2018) stress that melatonin's effect depends critically on the circadian time at which it's taken. Taking it 'whenever I remember,' or already in bed at 2 a.m., can in the worst case push your clock in the opposite direction from the one you want. One added practical detail: independent analyses of commercial supplements, such as Erland and Saxena's (Journal of Clinical Sleep Medicine, 2017), have found that the actual melatonin content of many products differs enormously from what the label declares.

Supplement or medicine? It depends on the country

Part of the confusion around melatonin comes from its regulatory status, which changes depending on where you live. In the United States it's sold freely as a dietary supplement, in widely varying doses and formats, without the manufacturing controls required of a drug. In Spain and much of the European Union a dual track coexists: low-dose melatonin (below 2 mg) is sold as a food supplement — available over the counter in pharmacies and supermarkets — while a 2 mg prolonged-release melatonin medicine is authorized in the EU that requires a prescription and carries a specific indication. In other countries, such as the United Kingdom, melatonin is generally a prescription-only medicine. This disparity isn't a technicality: it explains why the same compound is presented simultaneously as a shelf product and as a drug with a specific indication, and why European food authorities have only accepted narrowly-scoped health claims for the supplement (related to time to fall asleep and to jet lag), far from the broad promises circulating in marketing. If you're unsure about the specific product in front of you and whether it suits you, the right person to ask is your pharmacist or doctor, not the label.

Light rules the system: it makes no sense to take it under bright light

One actor dominates this entire system above any supplement: light. The classic experiment by Lewy, Wehr, Goodwin, Newsome, and Markey (Science, 1980) demonstrated that bright light suppresses melatonin secretion in humans — a foundational finding of modern chronobiology, because until then it was doubted that the human circadian system was as sensitive to light as that of other animals. The practical consequence is direct: if you spend the last hour of your day under intense light, or with a bright screen a few centimeters from your eyes, you're braking your own melatonin exactly when it should be rising — and no tablet compensates well for a contradictory signal. The right mental image isn't 'I'm low on melatonin, I'll top it up with a pill,' but 'my light environment is telling my brain it's still daytime.' That's why the light-related sleep hygiene measures — dimming lights in the evening, seeking natural light in the morning — act on the very same system as exogenous melatonin, for free and with no dose to get right. For many people with mild trouble falling asleep, sorting out light exposure first is the step with the best effort-to-result ratio, before considering any supplement.

Safety, limits, and when to talk to a professional

In healthy adults and in the short term, melatonin is generally considered well tolerated, with adverse effects that are usually mild and transient, such as drowsiness, headache, or dizziness. But 'generally well tolerated' doesn't mean 'for everyone and in every circumstance.' There are situations where the conversation with a doctor or pharmacist should come before the purchase: pregnancy and breastfeeding, where safety data are insufficient; children and adolescents, in whom its increasingly widespread use should always go through the pediatrician — because this is a hormone in a developing body, and because behind a child's poor sleep there may be causes a pill only masks; and anyone taking other medication, since melatonin can interact with, among others, anticoagulants, immunosuppressants, diabetes medication, contraceptives, and some antidepressants, and its metabolism is affected by common drugs. This article is educational, not personalized medical advice: Ensuenia does not prescribe doses or regimens, and decisions about how much, when, and whether to take melatonin in your specific case belong to a professional who knows your history. And one cross-cutting warning sign: if you've needed melatonin every night for weeks in order to sleep, that isn't maintenance — it's a symptom, likely of insomnia or another sleep problem that deserves its own evaluation, not more supplement.

Frequently asked questions

Sources

• Eduardo Ferracioli-Oda, Ahmad Qawasmi, Michael H. Bloch, Meta-analysis: Melatonin for the treatment of primary sleep disorders (2013) — PLoS ONE 8(5):e63773; metaanálisis de 19 ensayos controlados: la melatonina reduce la latencia de inicio del sueño en ~7 minutos y aumenta el tiempo total de sueño en ~8 minutos de media — efecto real pero modesto.
• Andrew Herxheimer, Keith J. Petrie, Melatonin for the prevention and treatment of jet lag (2002) — Cochrane Database of Systematic Reviews; concluye que la melatonina es notablemente eficaz para prevenir o reducir el jet lag y razonable de recomendar a viajeros adultos que cruzan cinco o más zonas horarias; dosis de 0,5 a 5 mg resultaron similarmente eficaces contra el jet lag, aunque con 5 mg se conciliaba el sueño algo más rápido.
• Josephine Arendt, Debra J. Skene, Melatonin as a chronobiotic (2005) — Sleep Medicine Reviews 9(1):25-39; revisión de referencia que caracteriza la melatonina como cronobiótico: una señal capaz de ajustar la fase del reloj circadiano, no un sedante.
• Alfred J. Lewy, Thomas A. Wehr, Frederick K. Goodwin, David A. Newsome, Sanford P. Markey, Light suppresses melatonin secretion in humans (1980) — Science 210:1267-1269; experimento fundacional que demostró que la luz brillante suprime la secreción de melatonina en humanos.
• Nachum Zisapel, New perspectives on the role of melatonin in human sleep, circadian rhythms and their regulation (2018) — British Journal of Pharmacology 175(16):3190-3199; revisión de la fisiología y farmacología de la melatonina que subraya la dependencia crítica de su efecto respecto a la hora circadiana de administración.
• Lauren A. E. Erland, Praveen K. Saxena, Melatonin Natural Health Products and Supplements: Presence of Serotonin and Significant Variability of Melatonin Content (2017) — Journal of Clinical Sleep Medicine 13(2):275-281; análisis independiente de suplementos comerciales que encontró que el contenido real de melatonina difiere de forma muy notable del declarado en la etiqueta.