"The study has several limitations. First, the database includes countries that require a prescription for melatonin (such as the United Kingdom) and countries that don’t (such as the United States), and patient locations were not part of the de-identified data available to the researchers. Since melatonin use in the study was based only on those identified from medication entries in the electronic health record, everyone taking it as an over-the-counter supplement in the U.S. or other countries that don’t require a prescription would have been in the non-melatonin group; therefore, the analyses may not accurately reflect this. Hospitalization figures were also higher than those for initial diagnosis of heart failure because a range of related diagnostic codes may be entered for the hospitalization, and they may not always include the code for a new diagnosis of heart failure. The researchers also lacked information on the severity of insomnia and the presence of other psychiatric disorders.

“Worse insomnia, depression/anxiety or the use of other sleep-enhancing medicines might be linked to both melatonin use and heart risk,” Nnadi said. “Also, while the association we found raises safety concerns about the widely used supplement, our study cannot prove a direct cause-and-effect relationship. This means more research is needed to test melatonin’s safety for the heart.”"

Multidimensional Sleep Health: Definitions and Implications for Cardiometabolic Health: A Scientific Statement From the American Heart Association https://www.ahajournals.org/doi/epub/10.1161/HCQ.0000000000000139

I seem to be missing something. When I keyword search “melatonin” on the above paper, the word melatonin does not appear in the paper. It is a lengthy piece on many aspects of sleep on cardiovascular health, but does not appear to mention melatonin. I gather that this paper is still in peer review, and not to be presented until Nov. 7-10.