Not getting enough sleep is known to raise the risk of heart disease; now, a new study may have uncovered why a poor night’s sleep is bad for your heart and blood vessels.

The study, conducted in mice, found that fragmented sleep alters the levels of a certain hormone, which in turn, increases production of inflammatory cells in the bone marrow. This inflammation plays a role in the development of atherosclerosis, or hardening of the arteries due to plaque buildup.

The findings, published in the journal Nature, suggest that proper sleep “protects against atherosclerosis” and, conversely, that disrupted sleep makes the condition worse, the researchers said.

Still, because the study was conducted in mice, the findings need to be confirmed in people, the researchers said.

Sleep and the heart
Numerous studies have linked not getting enough sleep with an increased risk of heart problems, including high blood pressure, heart disease, heart attacks and stroke, according to the Centers for Disease Control and Prevention (CDC). But the underlying biological reasons for this link have been unclear.

In the new study, the researchers looked at mice that were genetically prone to atherosclerosis. Some of the mice were allowed to get a sufficient amount of sleep, while others had their slumber frequently interrupted by a “sweep bar” that automatically moved across the bottom of the cage.

The sleep-deprived mice didn’t experience any changes in weight or cholesterol levels compared with the sleep-sufficient mice. But the sleep-deprived mice did have larger plaques in their arteries and higher levels of inflammation in their blood vessels, compared with the sleep-sufficient mice, the study found.

The sleep-deprived mice also had lower levels of a hormone called hypocretin (also known as orexin) in a part of their brain called the hypothalamus. In humans, hypocretin is thought to promote wakefulness, and levels of the hormone are known to be reduced in people with the sleep disorder narcolepsy. Interestingly, some studies suggest that people with narcolepsy also have a higher risk of heart disease than people who don’t have narcolepsy, the researchers noted.

The researchers found that the drop in hypocretin levels led to an increase in levels of a signalling protein called CSF1, which in turn increased production of inflammatory white blood cells in the bone marrow and accelerated atherosclerosis. What’s more, restoring hypocretin levels in the mice reduced atherosclerosis.

“We have discovered that sleep helps to regulate the production … of inflammatory cells and the health of blood vessels and that, conversely, sleep disruption breaks down control of inflammatory cell production, leading to more inflammation and more heart disease,” study senior author Filip Swirski, of the Massachusetts General Hospital Center for Systems Biology, said in a statement. “We also have identified how a hormone in the brain known to control wakefulness … protects against cardiovascular disease.”

“We now need to study this pathway in humans” and explore other ways that sleep may affect heart health, Swirski said.