June 4 (UPI) -- Sleeping poorly on a regular basis may increase a person's risk for heart disease, a study published Thursday by the journal PLOS Biology has found.
The connection may be that fragmented sleep increases inflammation in the body, which in turn raises a person's risk for atherosclerosis and stroke, according to researchers from the University of California at Berkeley.
"Improving sleep may offer a novel way to reduce inflammation and thus reduce the risk of atherosclerosis," study co-author Matthew Walker, a professor of neuroscience and psychology at the school, said in a press release.
In all, 35 percent of Americans report "poor" or "fair" sleep quality, and one in five say they don't wake up feeling refreshed, according to the Sleep Foundation.
Atherosclerosis, or hardening of the arteries, is caused by chronic inflammation, Walker and his colleagues said. It is believed to be the underlying cause of roughly half of all cases of heart disease -- and all heart disease-related deaths -- worldwide.
Sleep loss has been linked with chronic inflammation in multiple studies.
For their research, Walker and his colleagues assessed the sleep habits and quality of more than 1,600 adults using sleep lab-based polysomnography -- a commonly used sleep evaluation tool -- and actigraphy -- a movement detector worn on the wrist over multiple nights.
They also used standard blood cell counts to measure levels of neutrophils and monocytes, two types of white blood cells responsible for driving inflammation in the body.
In general, study participants with "fragmented sleep" based on actigraphy had higher neutrophil counts and more evidence of coronary artery calcification -- or hardening -- than those who had better-quality sleep.
Using a statistical method known as mediation analysis, the researchers showed that poor sleep led to an increase in neutrophils, which in turn led to an increase in atherosclerosis.
The influence of sleep disruption on neutrophils and atherosclerosis remained significant even after researchers accounted for multiple known contributors to coronary artery disease, including age, gender, ethnicity, body mass index, smoking and blood pressure.
The results help explain the long-standing observation that poor sleep increases the risk for heart disease and stroke and suggest simple and direct ways to reduce such risk -- by improving sleep, among other steps -- the researchers said.
"These findings may help inform public health guidelines that seek to increase the continuity of sleep as a way to improve health and decrease the burden of heart disease on society," Walker said.