It has long been known that not getting enough sleep might get your heart into trouble literally. Now University of Chicago researchers support the same idea saying that lack of sleep might lead to hardening of the arteries, an early step to heart disease.

Their study was published in the Journal of the American Medical Association. It followed new cases of coronary artery calcification among nearly 500 middle-aged US adults over a five-year period and found that people who slept five to seven hours nightly were almost twice as likely as those who slept longer to develop early blood-vessel damage that may lead to heart problems.

“There’s mounting evidence that really short sleep duration, as in less than five hours a night, may well have health consequences. It’s pretty safe to say that it’s a good idea to get more than five hours of sleep a night,” said Diane Lauderdale, the University of Chicago epidemiologist who led the research team.

In order to determine volunteers’ underlying heart health, researchers did computerized tomography (CT), scans of their coronary arteries. Participants slept about six hours a night; only few of them got more than eight hours of nightly sleep.

At the beginning, none of participants, aged 35 to 47, had evidence of coronary artery calcification. However, five years later, 27 percent of those who were sleeping less than five hours a night on average had developed coronary artery calcification for the first time, while only 6 percent of those who were sleeping seven hours or more developed it. Furthermore, among those who were sleeping between five and seven hours a night, 11 percent had developed coronary artery calcification.

There are some explanations for these findings. Firstly, there may be some factor not yet identified that can both reduce sleep duration and increase calcification. Or, it’s a known fact that less sleep leads to high blood pressure which increases the likelihood of calcification and blood pressure goes down during sleep.

Also, stress or a stress hormone like cortisol, which has been tied to decreased sleep and increased calcification, may play a role.