Ah, sleep. You hardly need a doctor to tell you that getting too little of it can make you irritable and lethargic. Now it looks like how many zzz's you get may affect whether fatty plaque deposits build up in your arteries — a precursor to heart attacks and angina, or chest pain.

University of Chicago docs recorded how much sleep 495 middle-aged folks got over three nights using a monitor worn on their wrists, and examined their hearts for coronary artery calcification using computed tomography (CT) scans. Then the scientists checked back in with them five years later, conducting the same tests.

About 12 percent had developed coronary artery calcification, but the condition was more common in people who slept the least. Of those who had it, 27 percent slept fewer than five hours a night, compared to 11 percent among those who slept five to seven hours and 6 percent among those who slept more than seven hours. The results are in today's Journal of the American Medical Association.

"It would be premature to say this is a direct or causal effect," says study co-author Diane Lauderdale, an epidemiologist and associate professor of health studies at the university. But, "It's a strong association."

Exactly why the association exists isn’t understood, Lauderdale says, but she offers three possible explanations:

•    People who sleep less are known to produce more of the stress hormone cortisol. In ways scientists don’t understand, having lots of cortisol affects pathways that lead to heart disease.

•    Blood pressure falls during sleep, and having lower bp is better for your heart. While there wasn’t much difference in the daytime blood pressure between the people who slept more and those who snoozed less, it's possible that the short sleepers experienced less of a dip in bp or a shorter drop in the middle of the night. "It may be that a 24-hour pattern of blood pressure is the link between sleep and coronary artery calcification," Lauderdale says.

•    Unknown factors that may influence how much people sleep may also be associated with coronary artery calcification.
There are no symptoms of coronary artery calcification, and most people wouldn't know they have it unless they get a CT scan, which is uncommon (but growing more popular and controversial) in the doctor's office. But scientists see the condition as a marker, or possible predictor, of heart disease.

For unknown reasons, the association between sleep duration and coronary artery calcification was stronger for women. "We have no idea why," Lauderdale says. "It may be that women need to sleep more, so not sleeping enough may be more harmful."

Lack of sleep has previously been linked to poor glucose metabolism, high blood pressure and obesity, all of which can influence heart disease.

Image by iStockphoto/Lisa Gagne