“Melatonin supplements are widely thought of as a safe and ‘natural’ option to support better sleep, so it was striking to see such consistent and significant increases in serious health outcomes, even after balancing for many other risk factors,” Nnadi said in a statement.

In a new, not yet peer-reviewed report published by the American Heart Association, researchers looked at five years of medical records from 130,828 people and found that those who were prescribed an...