Irregular sleep, such as sleeping for an inconsistent number of hours each night or falling asleep at different times, may increase the risk of developing atherosclerosis, or the hardening of the arteries, among adults over age 45, a new study suggests.

In particular, a variation in sleep duration of more than 2 hours per night in the same week was tied to the development of hardened arteries. “Poor sleep is linked with several cardiovascular conditions, including heart disease, hypertension, and type 2 diabetes,” says study author Kelsie Full, PhD, assistant professor of medicine at Vanderbilt University Medical Center.

“Overall, we found that participants who slept varying amounts of hours throughout the week (meaning that one night they slept less, one night they slept more) were more likely to have atherosclerosis than participants who slept about the same amount of time each night,” she says. The findings were published in the Journal of the American Heart Association.

Atherosclerosis is the buildup of fatty deposits, known as plaque, on artery walls. This can lead to narrowed arteries and reduced blood flow and oxygen in the body. The plaque can also burst and create a blood clot that blocks the artery, which can lead to a heart attack or stroke.

To examine the links to sleep, Full and colleagues examined observational data from more than 2,000 participants in the Multi-Ethnic Study of Atherosclerosis (MESA) Sleep Ancillary Study. The study included adults between ages 45 to 84 in six U.S. communities: St. Paul, MN; Baltimore City and Baltimore County, MD; Chicago; Forsyth County, NC; Los Angeles County, CA; and Northern Manhattan and the Bronx, NY.

Between 2010 to 2013, the people in the study wore a wrist device that detected when they were awake and asleep for 7 days in a row, and they completed a 7-day sleep diary. They also completed a one-night sleep study to measure sleep disorders that involve breathing, sleep stages, and heart rate. The research team looked at sleep duration, or the total amount of time spent in bed fully asleep in a night, and sleep timing, which was defined as the time the person fell asleep each night. They measured the presence of plaque in the arteries by assessing for coronary artery calcium (or calcified plaque buildup in arteries), carotid plaque (or fatty plaque buildup in neck arteries), carotid intima-media thickness (or thickness of the inner two layers of the neck arteries), and the ankle-brachial index (or narrow peripheral arteries), all of which indicate the presence of atherosclerosis.