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Does Melatonin Cause Weight Gain? What the Research Actually Shows

"Melatonin makes you gain weight" is one of those warnings that circulates on forums and Facebook groups without any specific source — yet it still effectively scares off people who might genuinely benefit from better sleep. A randomized clinical trial from 2021 tested this claim directly in overweight adults and found the opposite: in the second half of a 12-week intervention, the melatonin group kept losing body fat while the placebo group plateaued.

JWJulia WiśniewskaAugust 23, 202610 min read
Table of contents

TL;DR — key takeaways

Where the belief that melatonin causes weight gain comes from

In our knowledge-base entry on melatonin, we describe it primarily as a hormone regulating circadian rhythm, used mainly in the context of falling asleep and jet lag. So where does the popular warning that supplementing it leads to weight gain come from? Partly from confusing cause with effect — sleep disorders and obesity frequently co-occur, so it's easy to wrongly assume the sleep supplement is the culprit behind the weight, rather than the other way around.

Partly it also comes from extrapolating from a completely different context — some medications that affect sleep and mood (certain antipsychotics or antidepressants, for example) genuinely are linked to weight gain, which may have been mistakenly generalized to melatonin despite its entirely different mechanism of action. A 2021 randomized clinical trial lets us test this claim directly instead of relying on guesswork.

An important contextual caveat

The study discussed in this article evaluated melatonin given alongside a weight-loss diet, not as a standalone weight-loss intervention. This is an important interpretive limitation we'll return to later in the article.

What the 2021 study found

The work by Mohammadi and colleagues, published in BioMed Research International, is a randomized, double-blind, controlled trial designed specifically to test melatonin's effect on anthropometric indices in overweight and class-I obese adults.

Melatonin Supplementation and Anthropometric Indices: A Randomized Double-Blind Controlled Clinical Trial

Moderate evidence

Mohammadi S, Rastmanesh R, Jahangir F, et al. · BioMed Research International · 2021

The study included 38 overweight or class-I obese adults who took 3 mg of melatonin daily or placebo for 12 weeks while following a weight-loss-oriented diet. Both groups showed a significant drop in body weight, BMI, and waist circumference (P=0.001) over the full study. During weeks 7-12, only the melatonin group continued to significantly improve on these measures (P=0.01). Body fat percentage dropped significantly only in the melatonin group by study end (P=0.008), with no significant change in the placebo group.

View study

The key detail: what happened in the second half of the trial

Moderate evidence

The most informative part of this study isn't the final result on its own, but its trajectory over time. Both groups lost weight similarly in the first six weeks — suggesting the weight-loss diet drove most of the early effect. The divergence appeared later: in weeks 7-12, the placebo group stopped significantly improving, while the melatonin group kept losing weight, waist circumference, and body fat. That pattern fits "melatonin helps sustain the diet's effect" better than a one-off placebo effect.

Why this doesn't prove melatonin causes weight loss on its own

Myth

Melatonin causes weight gain, so anyone watching their weight should avoid it.

Fact

The only available randomized clinical trial testing this claim directly found the opposite — melatonin was linked to further, significant fat-mass reduction in the second half of a 12-week intervention, while the placebo group stopped improving during that same period.

It's important to be precise about what this study actually proves, though. Melatonin was given alongside a weight-loss diet in both groups — the trial didn't test melatonin as a standalone weight-loss intervention in people following no diet at all. The conclusion this data supports is: melatonin doesn't cause weight gain, and in the context of a parallel weight-loss diet it may be linked to an additional benefit in sustaining the effect over time — not that melatonin itself is a weight-loss agent.

A mechanistically plausible explanation for this observation involves melatonin's role in regulating the circadian rhythm of metabolism and brown-fat thermogenesis, described in earlier animal-model research — but this remains a hypothesis to be treated as supplementary reasoning, not a separately confirmed clinical fact within this specific study.

What this means in practice

Practical takeaways from this study

  • The available evidence doesn't support the claim that melatonin supplementation causes weight gain — the only direct study shows the opposite effect
  • Melatonin shouldn't be treated as a standalone weight-loss agent — in the cited study it accompanied a weight-loss diet, it didn't replace one
  • If fear of weight gain has been a reason to avoid melatonin for sleep problems, it's worth revisiting that concern with a doctor in light of the available data
  • The effect observed applied to 12 weeks and a 3 mg daily dose — it shouldn't be extrapolated to other doses or longer periods of use without additional data
  • Melatonin's core use remains tied to sleep and circadian rhythm, described in more depth in our melatonin entry

Limitations of this study

What this data doesn't prove

The trial included only 38 participants, which limits the precision of estimates and the ability to detect rarer side effects. It lasted 12 weeks — it's unknown whether the observed pattern holds over a longer period. Crucially, melatonin was tested only in combination with a weight-loss diet, so the results say nothing about its effect on weight in people following no dietary intervention at all. The study population (overweight or class-I obesity) also limits generalizing the results to people with normal body weight.

Practical summary

QuestionShort answer
Does melatonin cause weight gain?No evidence for this — the only direct RCT shows the opposite effect
Does melatonin cause weight loss on its own?Not tested standalone — the study combined it with a weight-loss diet
What exactly did the study show?In weeks 7-12, the melatonin group kept losing body fat while the placebo group plateaued
What dose and for how long?3 mg daily for 12 weeks, alongside a weight-loss diet
How strong is the evidence?Moderate — a single randomized trial in a small sample, in need of replication

Melatonin and body weight, in brief

Our editorial recommendation

It's rare for a single study to overturn a widespread, though never verified, belief so clearly. "Melatonin makes you gain weight" was a claim that circulated with no direct empirical support at all — and the one study that actually tested it points the other way.

That doesn't mean melatonin is worth taking for weight loss — more and larger studies are needed for that. It does mean that fear of weight gain shouldn't be a reason to rule out melatonin if someone is considering it for a legitimate sleep-support reason.

This topic shows how easily a popular belief takes on a life of its own even though no one ever tested it — and when someone finally did, reality turned out to be exactly the opposite.

Julia Wiśniewska, VitMode editorial team

Frequently asked questions

Mohammadi et al.'s study didn't test melatonin in isolation from diet — both groups followed a weight-loss diet. So it can't be concluded that melatonin alone directly burns fat, only that combined with diet it was linked to continued fat loss when the placebo effect flattened out.

3 mg daily for 12 weeks. This is a higher dose than typically used just for falling asleep — don't assume lower doses produce an identical effect on weight.

No. The trial included 38 people and lasted 12 weeks, too small and too short an evidence base to support weight-loss recommendations. The result is promising and worth further research, but it isn't grounds for a clinical recommendation.

Unknown — the study only included overweight or class-I obese adults, so the results can't automatically be applied to people with normal body weight.

Sources

JW

Julia Wiśniewska

MSc in Cognitive Neuroscience, host of a sleep-optimization podcast

Julia writes about nootropics, chronobiology and recovery protocols.

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Related knowledge base entries

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The hormone that governs your circadian rhythm — as a supplement it works better at resetting the body clock than as a classic 'sleeping pill.'

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Sleep

Sleep isn't a passive shutdown of the body — it's an active, highly organized biological process. Its shortfall (and, counterintuitively, its excess too) is linked to a measurably higher risk of death from any cause.

SenStrong evidence
Kapsułki i tabletki suplementów na marmurowym blacie4.7

Magnesium

A cofactor for more than 300 enzymatic reactions — essential for neuromuscular function, sleep and energy metabolism.

SuplementyStrong evidence
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Sleep and Memory

Learning during the day is only half of the memorization process — the other half happens at night, as the brain actively consolidates and organizes newly acquired information during sleep.

MózgStrong evidence
Kobieta leżąca w łóżku w nocy, niemogąca zasnąć4.7

Insomnia

Chronic difficulty falling or staying asleep isn't just a matter of 'sleep hygiene' — the best-studied intervention, recommended as first-line treatment, is cognitive behavioral therapy for insomnia (CBT-I), not sleeping pills.

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Chronotype

An individual, largely genetically determined preference for sleep and activity timing — ignoring it leads to a phenomenon known as 'social jet lag,' linked to worse metabolic health.

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Comments (2)

  • KW

    Kasia W. 2 weeks ago

    Very clearly explained, especially the interactions section — I hadn't seen it laid out this well anywhere else.

  • MT

    Marek T. a month ago

    Are you planning to update this with the newest study from this year? I saw an interesting meta-analysis.