Melatonin is widely considered a safe, natural sleep aid, but emerging research reveals a complex picture: long-term supplementation may increase heart failure risk in some people, while low natural melatonin levels predict cognitive decline in others. A major 2025 study presented by the American Heart Association found that adults with insomnia who used melatonin for 12 or more months had an 89% increased risk of heart failure diagnosis over five years compared to those who never used it . Yet paradoxically, the same research shows that naturally low melatonin levels are consistently linked to Alzheimer's and Parkinson's disease, suggesting the hormone may protect the brain when produced by your body .

Why Does Long-Term Melatonin Use Increase Heart Failure Risk?

The American Heart Association study analyzed electronic health records from 130,828 adults with insomnia and found striking differences between melatonin users and non-users . Among melatonin users, 4.6% developed heart failure over five years, compared to 2.7% of non-users. More concerning, melatonin users experienced 3.4 times more heart failure-related hospitalizations and 2.1 times higher all-cause mortality .

However, the research team emphasized an important distinction: the study shows association, not direct causation. Researchers hypothesized that melatonin may be masking underlying cardiac symptoms rather than directly causing heart disease . Insomnia itself is a known risk factor for cardiovascular disease, increasing blood pressure, stress hormones, and systemic inflammation. A supplement that improves sleep quality without addressing the underlying cardiac risk could delay diagnosis of developing heart failure until the condition becomes severe, creating a form of medical harm even if melatonin itself doesn't directly cause the disease .

Can Melatonin Prevent Cognitive Decline and Dementia?

The story shifts dramatically when examining melatonin's effects on the brain. Reduced melatonin levels are consistently documented across the Alzheimer's disease continuum, with deficiencies appearing before diagnosis . Researchers have proposed melatonin as a potential biomarker for early neurodegeneration, suggesting that low melatonin may contribute to disease progression rather than result from it .

In both Alzheimer's disease and Parkinson's disease, researchers identified melatonin deficiency at the pineal gland level, a consequence of age-related calcification that typically begins in the third decade of life . Sleep disturbances often precede cognitive decline by years, and melatonin's known antioxidant and anti-inflammatory properties suggest mechanisms by which supplementation could theoretically slow neurodegeneration .

Clinical evidence supports modest but consistent benefits. A 2021 meta-analysis of randomized controlled trials in Alzheimer's disease found that melatonin supplementation ranging from 0.15 to 6 milligrams before bed improved cognitive function as measured by the Mini-Mental State Examination, with a mean improvement of 1.82 points . Melatonin also improved sleep quality and reduced "sundowning," the agitation and confusion that emerges in late afternoon in dementia patients .

How Does Dose and Duration Change Melatonin's Effects?

The critical factor distinguishing melatonin's risks from its benefits appears to be dose and duration of use. Disease associations are strongest with long-term use of 12 or more months and dosing patterns that differ from natural circadian melatonin production . Short-term, protocol-based use shows different risk profiles .

For cognitive protection, effective doses appear modest. The research showing cognitive benefits used doses between 0.15 and 6 milligrams, with most studies using lower end dosing . In contrast, some human studies examining metabolic effects used high-dose melatonin ranging from 40 to 200 milligrams daily, far exceeding typical supplementation .

The current research consensus suggests melatonin may be most effective as a preventive agent early in the disease process, potentially slowing neurodegeneration before irreversible cognitive loss occurs . Once significant neurodegeneration has occurred, melatonin appears unable to reverse damage, only potentially slow progression .

What About Metabolic Health and Blood Sugar Control?

Metabolic syndrome, the clustering of obesity, insulin resistance, high blood pressure, and abnormal cholesterol levels, affects more than 25% of the population in developed countries and increases cardiovascular disease and diabetes risk . Research consistently shows that adults with metabolic syndrome have lower circulating melatonin levels than healthy controls, and this deficiency appears causally linked to metabolic dysfunction .

In animal models, the evidence is compelling. Melatonin supplementation reversed fructose-induced metabolic syndrome, reducing systolic blood pressure, total cholesterol, LDL cholesterol (the harmful type), triglycerides, and body weight . These effects reflect melatonin's roles in mitochondrial function, antioxidant defense, and immune regulation, all dysregulated in metabolic disease .

In humans, the picture is murkier. A 2024 study of 81 elderly patients treated with high-dose melatonin for more than four years found significant improvement in arterial hypertension, ischemic heart disease, and diabetes mellitus compared to untreated controls . However, the dose range far exceeds typical supplementation, and the study was retrospective without randomization, limiting causal inference .

Steps to Consider Before Using Melatonin Long-Term

• Medical Evaluation First: Before starting long-term melatonin supplementation, especially if you have a history of heart disease, high blood pressure, or family history of cardiovascular disease, consult with a healthcare provider who can assess your individual risk profile.
• Short-Term Use for Acute Sleep Issues: Melatonin appears safer for short-term use addressing temporary sleep disruption from jet lag, shift work, or acute stress rather than chronic daily supplementation lasting 12 or more months.
• Lower Doses for Cognitive Protection: If using melatonin for potential cognitive benefits in early-stage cognitive decline, research supports doses between 0.15 and 6 milligrams, substantially lower than many over-the-counter supplements.
• Address Underlying Sleep Disorders: If insomnia is your primary concern, work with a sleep specialist to identify underlying causes like sleep apnea, circadian rhythm misalignment, or hormonal imbalances rather than relying solely on melatonin supplementation.
• Monitor for Cardiac Symptoms: If you use melatonin long-term, remain alert to symptoms like shortness of breath, chest discomfort, unusual fatigue, or swelling in your legs, which could indicate developing heart problems.

What Does the Research Actually Tell Us About Melatonin Safety?

The emerging evidence suggests melatonin is neither universally safe nor universally harmful. Instead, its effects depend on context: who is taking it, how much they're taking, how long they're taking it, and what underlying health conditions they have . The distinction between naturally low melatonin (which predicts disease risk) and supplemental melatonin (which may increase certain risks) is crucial and often overlooked in popular discussions of the hormone .

For people with early cognitive decline or family history of dementia, modest melatonin supplementation may offer protective benefits. For people with insomnia and underlying cardiovascular risk factors, long-term melatonin use may delay diagnosis of developing heart disease rather than address the root cause of sleep disruption .

The takeaway is not that melatonin is dangerous, but that it requires the same thoughtful, individualized approach as any other supplement or medication. One-size-fits-all recommendations miss the nuance that research now reveals. If you're considering melatonin supplementation, particularly for long-term use, a conversation with your healthcare provider about your specific health profile, risk factors, and sleep goals is essential.