According to an unpublished epidemiological study, presented in October 2025 at a conference of the American Heart Association, melatonin supplementation may increase the risk of heart failure. [sciencedaily.com November 4, 2025] We doubt whether that's actually true. Trials have shown that melatonin supplementation actually has a positive effect on heart failure.

Study
Researchers from Alborz University of Medical Sciences in Iran tracked four clinical trials in which one group of people with heart failure was treated with melatonin and another group was given a placebo.

The participants took their supplements 30 to 60 minutes before bed. Doses ranged from 3 to 20 milligrams per day, and the trials lasted 8 to 24 weeks.

The participants were diagnosed with a form of heart failure in which they noticed their heart function was impaired during exercise, but not yet during rest. Their heart failure had a score of NYHA-2 or NYHA-3. NYHA stands for New York Heart Association [NYHA] Functional Class.

All four trials came from Iran. [Am J Lifestyle Med. 2019 Sep 14;16(1):130-140.] [Clin Cardiol. 2021 Sep;44(9):1263-71.] [Clin Cardiol. 2022 Apr;45(4):417-26.] [J Res Med Sci. 2012 Mar;17(Spec 1):S13-6.]

The researchers pooled the results from the trials and reanalyzed them.

Results
Two trials assessed the effect of melatonin supplementation on quality of life (QoL). When participants had to rate their QoL, it increased from a 7.2 to a 7.8. The effect was statistically and clinically significant.

This was partly because the supplementation improved sleep, reduced daytime fatigue, and increased appetite.

According to statistical analyses, the effect on the severity of heart failure was right on the borderline between significance and non-significance. In the placebo group, the NYHA class improved in 3 percent of participants. The p-value was exactly 0.05.

Click on the figures and the table below for a larger version.

Mechanism
Melatonin made blood vessels more flexible and increased their ability to transport oxygen and nutrients to cells. Flow-mediated dilation, or FMD, increased from 5.6 percent to 7.5 percent

Conclusion
"Melatonin, by increasing psychologic parameters and cardiac potency, could be advised as a novel drug for treatment and palliating heart failure patients," the researchers write.