Melatonin is one of the most widely used supplements in the United States, taken by millions of people who assume it is essentially harmless. So a headline reporting a 90% higher risk of heart failure travelled fast.
The finding is real and worth knowing about. What it means is considerably narrower than the coverage suggested, and the gap between the two is instructive.
What the study found
Researchers used the TriNetX Global Research Network, an international database of de-identified electronic health records, to examine 130,828 adults with insomnia — average age 55.7, 61.4% women. Half had used melatonin for at least a year. They were followed for five years.
- Heart failure occurred in 4.6% of melatonin users versus 2.7% of non-users — the roughly 90% relative increase
- Users were 3.5 times more likely to be hospitalised for heart failure
- Users were nearly twice as likely to die from any cause
It was presented at the American Heart Association Scientific Sessions in November 2025.
Four reasons to hold this loosely
It was never peer-reviewed. The AHA states this explicitly: abstracts presented at its meetings are not peer-reviewed, and findings are preliminary until published as a full manuscript. This one still has not been.
It is observational, and the confounding here is severe. Everyone in the study had insomnia. But people who take melatonin for over a year likely have worse insomnia than those who do not — and chronic insomnia is itself associated with cardiovascular disease. The study lacked data on insomnia severity and psychiatric conditions, which is precisely the information needed to separate the supplement from the illness.
The data are built on prescription records. In the US melatonin is sold over the counter. To appear in these records, a patient generally had to be in a country where melatonin is prescribed, or to have it recorded for a specific reason. The AHA notes this creates potential misclassification — and it means many people counted as “non-users” may have been buying melatonin off a shelf.
Reverse causation is entirely plausible. Early heart failure causes breathlessness when lying flat, nocturnal waking and disrupted sleep. Someone in the early stages might reach for melatonin because of symptoms their undiagnosed heart failure was already producing.
What can reasonably be concluded
That long-term melatonin use in people with chronic insomnia deserves proper study. That is genuinely the correct takeaway, and it is not nothing — melatonin has been treated as consequence-free for years on remarkably thin long-term safety data.
What cannot be concluded is that melatonin causes heart failure. This design cannot establish that, and the researchers did not claim it.
What was already known about melatonin
Setting this study aside, the honest picture is less impressive than the marketing (the full evidence).
Melatonin is a circadian signal, not a sedative. It tells your brain that darkness has arrived. It works well for jet lag and for delayed sleep phase — genuine circadian problems. For ordinary insomnia, meta-analyses find it reduces time to fall asleep by only around 7 to 12 minutes.
Typical doses are also far too high. Physiological effect is achieved around 0.5 to 1 mg; products commonly sell 5, 10 or 20 mg. And US supplement testing has repeatedly found actual melatonin content varying dramatically from the label — in one analysis, from a small fraction to several times the stated amount.
What to do instead
For chronic insomnia, cognitive behavioural therapy for insomnia (CBT-I) is first-line treatment and outperforms medication over the long term. That is a strong, consistent evidence base, and it remains badly underused (what actually causes insomnia).
The levers with the clearest physiology are morning light exposure, which is a far stronger circadian signal than any tablet, a cool bedroom, and a consistent schedule.
And if you are exhausted despite adequate hours, undiagnosed sleep apnea is a more likely explanation than a melatonin deficiency.
When to see a doctor
See a doctor for insomnia lasting more than three months, and ask specifically about CBT-I. Discuss melatonin use with your doctor if you take anticoagulants, immunosuppressants, blood pressure medication or antiseizure drugs, and before giving it to children, where long-term data are especially limited.
Seek prompt evaluation for breathlessness when lying flat, waking at night gasping, swelling in the ankles or legs, or reduced exercise tolerance — these can indicate heart failure and should not be treated as a sleep problem.
The honest bottom line
A large observational analysis found more heart failure among long-term melatonin users with insomnia. It was not peer-reviewed, it could not measure how severe anyone’s insomnia was, it relied on prescription records for a supplement most Americans buy over the counter, and early heart failure disrupts sleep in exactly the way that would cause someone to start taking melatonin. Treat it as a reason for proper research, not as evidence of harm. Separately, melatonin buys most people about ten minutes of sleep onset — which is the more useful thing to know.
Frequently asked questions
Does melatonin cause heart failure?
The study behind that headline cannot establish causation. It was an observational analysis of health records, was never peer-reviewed, could not measure insomnia severity, and relied on prescription records for a supplement most Americans buy over the counter. Since chronic insomnia itself raises cardiovascular risk, and early heart failure disrupts sleep in ways that would prompt someone to start melatonin, the association may reflect the underlying condition rather than the supplement. It is a reason for proper research, not evidence of harm.
How much melatonin should I take?
Physiological effect is achieved at roughly 0.5 to 1 mg, yet products are commonly sold at 5, 10 or 20 mg. Higher doses do not work better and may worsen next-day grogginess. Timing matters more than dose for circadian problems — taken several hours before bedtime rather than at bedtime for delayed sleep phase. Independent testing of US supplements has repeatedly found actual content varying widely from what the label states.
Does melatonin actually work for insomnia?
Less than most people expect. Melatonin is a circadian signal telling the brain that darkness has arrived, not a sedative. It works genuinely well for jet lag and delayed sleep phase syndrome, which are circadian problems. For ordinary insomnia, meta-analyses find it reduces time to fall asleep by only about 7 to 12 minutes. Cognitive behavioural therapy for insomnia is first-line treatment and outperforms medication over the long term.