There’s a sweet spot for how much sleep we need for a healthy heart, based on a recent study that found getting fewer than six or more than nine hours of sleep a night increases risk for heart attack.

Published in the Journal of the American College of Cardiology, this study explored the association between sleep duration and heart risks. It included more than 461,000 adults in a UK health registry who reported their sleep habits and were followed for about seven years.

Participants were between 40 and 69 years old and enrolled in the study between 2006 and 2010. In addition to completing a medical exam upon enrollment, participants answered the question “About how many hours sleep do you get every 24 hours?” Researchers then grouped participants based on their sleep duration and compared heart attack rates by the end of the study.

Compared to adults sleeping 6–9 hours a night, short sleepers had 20% higher risk for heart attack, while long sleepers had 34% higher risk after seven years. Short sleep was defined as less than 6 hours a night and long sleep included sleep greater than 9 hours a night.

Based on genetic testing, researchers also found that getting 6–9 hours of sleep helped reduce heart attack risk by 18% in participants with high cardiovascular risk.

Authors note that associations remained significant after adjusting for factors like age, income and health, which can impact heart risks.

Findings confirm that getting too little and too much sleep can increase risk for heart attack, as other studies have shown. They also suggest that a healthy sleep duration could help prevent heart events in high-risk individuals.

Heart attack is a leading cause of death in the United States and affects an estimated 735,000 Americans each year. There are many ways to help reduce risk for heart attack, such as eating healthy, staying active, and managing weight, blood pressure and cholesterol. Findings suggest that getting 6 to 9 hours of sleep a night may offer another way to further reduce risk for heart attack.

The next step, according to authors, is conducting clinical trials to confirm the association between sleep and cardiovascular risk. Experts hope that in addition to current prevention strategies, encouraging sleep could offer another way for patients—especially those that are high risk—to help prevent heart events.