You know a lack of sleep can make you feel drained and foggy the next day. But US researchers believe poor sleep may also increase your risk of a stroke, and they can explain why.

In the new study, published in the open-access journal PLOS Biology, the researchers found that people with fragmented sleep tend to have higher numbers of inflammatory white blood cells called neutrophils, which then leads to them having higher coronary artery calcium, a measure of blocked blood vessels. The researchers say this suggests that getting a good night’s sleep may be one way to reduce inflammation and reduce the risk of atherosclerosis (blocked blood vessels), which can cause heart attacks and strokes.

In order to test their theory, the researchers measured sleep disruption through both sleep lab-based polysomnography and a simple movement detector worn on the wrist over multiple nights (actigraphy). They used standard blood cell counts to measure levels of neutrophils and monocytes, two types of white blood cells responsible for driving inflammatory pathways. They found that poor sleep led to an increase in neutrophils, which in turn led to an increase in atherosclerosis.

“Improving sleep may offer a novel way to reduce inflammation and thus reduce the risk of atherosclerosis,” lead author Matthew Walker 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.”

A lack of sleep can contribute to a long list of health problems such as heart disease, high blood pressure, diabetes, obesity, early ageing and even Alzheimer’s disease. A study published in the medical journal Neurology found one night of bad sleep may raise Alzheimer’s risk. In the short-term, lack of sleep can cause memory loss, anxiety and reduced concentration.

So, how many hours of sleep do we really need? We often hear eight hours as being the magical number to strive for, but in fact, the amount of sleep you need depends on your age. According to a study that ran over two years at the US National Sleep Foundation, the amount of sleep needed can vary from between seven to 17 hours.

Most adults aged 26 to 64 years need about seven to nine hours of sleep per night, while those aged 65 or older need seven to eight hours of sleep per night to feel rested and alert.