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Doctors have long known that too little or interrupted sleep raises the risk of heart disease. But heart experts say this kind of so-called observational study can't prove that melatonin use plays any role — instead of the insomnia patients were trying to treat.

“We should not raise the alarm and tell patients to stop taking all their melatonin,” said Dr. Pratik Sandesara, an interventional cardiologist at Emory Healthcare who wasn’t involved with the research.

Our bodies naturally produce melatonin, a hormone that regulates our sleep cycles. Levels normally increase as it gets darker in the evening, triggering drowsiness.

People may take lab-produced melatonin to help them fall asleep or to adjust for jet lag or time changes.

The new study used international electronic health records, tracking adults diagnosed with insomnia who had a melatonin prescription that suggested they used the supplement for at least a year.

Over five years, 4.6% of the chronic melatonin users developed heart failure compared to 2.7% of insomnia patients whose charts showed no melatonin use, the researchers found. The study is being presented at an American Heart Association meeting but hasn’t undergone peer review.

But only certain countries require a melatonin prescription. It’s over-the-counter in the U.S., meaning Americans in the study might have used the supplements without it being recorded, said Northwestern University cardiology chief Dr. Clyde Yancy, who wasn't involved in the study. The study also did not show dosages.

Also, U.S. supplements don’t require government approval, meaning brands can vary in their ingredients. The researchers, from SUNY Downstate Health Sciences University, characterized the findings as a call for more research.

Meanwhile, patients wondering about melatonin should talk it over with their doctors, said Emory’s Sandesara. Generally doctors recommend it for short-term use, like for jet lag.

Yancy noted that while the study doesn't prove there's a danger from long term melatonin use, there's also no evidence that people should use melatonin indefinitely.

And one key to better shut-eye is to , like making sure your room is dark.

“When we expose ourselves to blue light in particular at night, we are diminishing our melatonin levels. That’s science,” he said. Sleep problems aren’t about “just being sleepy and tired — they’re putting yourself at risk.”

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This story has been corrected to show that the Northwestern University cardiology chief is Dr. Clyde Yancy, not Yancey.

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The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Department of Science Education and the Robert Wood Johnson Foundation. The AP is solely responsible for all content.