A new study that included nearly half a million participants strikingly found that sleep duration plays a crucial role in whether or not a person is at risk of a heart attack — even if a person exercises, doesn’t smoke, or has no genetic predisposition to cardiovascular disease. In particular, the researchers found that too much sleep, as well as too little sleep, boosts the risk of having a heart attack.

The researchers at the Massachusetts General Hospital and the University of Manchester analyzed a dataset of 461,000 individuals aged 40 to 69, part of UK biobank, who had no prior history of heart attack or heart disease. The participants’ health was tracked over a period of seven years.

Those who slept less than six hours a night were 20% more likely to have a heart attack than those sleeping 6 to 9 hours.

The fact that sleep deprivation and low quality sleep are linked to negative health outcomes is, by now, no surprise. Study after study has linked poor sleep to anxiety, depression, obesity, diabetes, and heart disease.

But what was surprising about this study was that people who slept more than nine hours per night were 34% more likely to suffer a heart attack than the participants who slept 6 to 9 hours — that’s a significantly higher risk than the sleep-deprived.

The farther people fell outside the 6 to 9-hour range, the higher the recorded risk of having a heart attack. People who slept for only 5 hours had a 52% risk of heart attack, whereas those who slept 10 hours nightly were twice as likely to have one than normal sleepers.

Shockingly, these effects held true even when taking into account more than 30 other factors, including physical activity, mental health, and socioeconomic status. In other words, sleep duration influenced the risk of having a heart attack independently of other lifestyle factors — and genes were no exception either.

“This gives us even more confidence that there is a causal relationship here—that it is sleep duration, not something else, influencing heart health,” said Celine Vetter, co-author of the new study and an assistant professor of physiology at the University of Colorado Boulder.

It’s not clear why sleep duration is such a big contributing factor to heart disease. Some explanations suggest that sleeping too little can affect arteries, bone marrow, as well as lead to poor lifestyle choices such as eating low-quality food. In contrast, sleeping too much may boost inflammation in the body.

There’s some good news, though. The study suggests that sleeping for a reasonable number of hours can be one of the most impactful things you can do to avoid heart disease.

“It’s kind of a hopeful message, that regardless of what your inherited risk for heart attack is, sleeping a healthy amount may cut that risk just like eating a healthy diet, not smoking, and other lifestyle approaches can,” said lead author Iyas Daghlas, who is a medical student at Harvard.

“Just as working out and eating healthy can reduce your risk of heart disease, sleep can too,” said Vetter.

The study was published in the Journal of the American College of Cardiology.