People who sleep for more than 8 hours a day have an increased risk of stroke, according to a study by the University of Cambridge—and this risk doubles for older people who persistently sleep longer than average. However, the researchers say it is unclear why this association exists and call for further research to explore the link.

Previous studies have already suggested a possible association between sleep and risk of stroke, but this study, published yesterday in the journal Neurology, provides detailed information about the British population and examines the relationship between a change in sleep duration over time and subsequent stroke risk.

Researchers from the Department of Public Health and Primary Care at the University of Cambridge followed just under 10,000 people aged 42-81 years of age from the European Prospective Investigation into Cancer (EPIC)-Norfolk cohort over 9.5 years. During 1998-2000 and then again 4 years later, they asked the cohort how many hours on average they slept in a day and whether they generally slept well.

Almost seven out of 20 participants reported sleeping between 6 and 8 hours a day, whilst one in 10 reported sleeping for over 8 hours a day. Participants who slept for less than 6 hours or more than 8 hours were more likely to be older, women, and less active.

Over the almost 10-year period of the study, 346 participants suffered a stroke, either nonfatal or fatal. After adjusting for various factors including age and sex, the researchers found that people who slept longer than 8 hours a day were at a 46% greater risk of stroke than average. People who slept less than 6 hours a day were at an 18% increased risk, but the small number of people falling in this category meant the association was not statistically significant.

Participants who reported persistently long sleep—in other words, they reported sleeping over 8 hours when asked at both points of the study—were at double the risk of stroke compared to those with persistently average sleep duration (between 6 and 8 hours a day). This risk was even greater for those whose reported sleep increased from short to long over the 4 years—their risk was close to four times that of people who maintained an average sleep duration.

In addition to studying the EPIC-Norfolk cohort, the researchers carried out a study of combined data from 11 other studies related to identifying the association between sleep duration and patterns of stroke risk. Their final analysis, including 560,000 participants from seven countries, supported the findings from the EPIC-Norfolk cohort study.

Yue Leng, PhD candidate at the University of Cambridge, says in a release: “It’s apparent both from our own participants and from the wealth of international data that there’s a link between sleeping longer than average and a greater risk of stroke. What is far less clear, however, is the direction of this link, whether longer sleep is a symptom, an early marker, or a cause of cardiovascular problems.”

While older people have less work and fewer social demands and therefore often have the option of sleeping longer, previous research has shown that, in fact, they tend to sleep on average for shorter periods.

The researchers say it is unclear yet why the link between sleep and stroke risk should exist. Lack of sleep has been linked with factors such as disrupted metabolism and raised levels of cortisol, all of which may lead to higher blood pressure and increased stroke risk. However, the current study suggests that the association between longer sleep duration and higher risk of stroke was independent of normal risk factors for cardiovascular disease.

Professor Kay-Tee Khaw, senior author on the study, adds: “We need to understand the reasons behind the link between sleep and stroke risk. What is happening in the body that causes this link? With further research, we may find that excessive sleep proves to be an early indicator of increased stroke risk, particularly among older people.”

Alberto Ramos, MD, MSPH, of the University of Miami Miller School of Medicine and a member of the American Academy of Neurology, who wrote a corresponding editorial, said, “Since people whose sleep patterns changed from short to long were nearly four times as likely to have a stroke, it’s possible that this could serve as an early warning sign, suggesting the need for additional tests or for people to take steps known to reduce stroke risk, such as lowering blood pressure and cholesterol.”