IF you sleep for more than eight hours per day, you may be at increased risk of stroke – and this doubles for older people who persistently sleep longer than average.

This is according to a study by the University of Cambridge, however, the researchers say it is unclear why this association exists and call for further research to explore the link.

Although previous studies have suggested a possible association between sleep and risk of stroke, the university’s study is the first to detail information about the British population and to examine the relationship between a change in sleep duration over time and subsequent stroke risk.

A stroke occurs when there is inadequate blood supply to the brain, which deprives the brain tissue of oxygen and nutrients.

Published in the journal Neurology, the near 10-year study of just under 10,000 people between 42-81 years of age looked at how many hours on average they slept in a day and whether they generally slept well.

“Almost seven out of 10 participants reported sleeping between six and eight hours a day, whilst one in 10 reported sleeping for over eight hours a day,” the university’s website stated. “Participants who slept for less than six hours or more than eight hours were more likely to be older, women and less active.”

Experts at the University of Cambridge report that over the period, 346 participants suffered a stroke, either non-fatal or fatal stroke.

“After adjusting for various factors including age and sex, the researchers found that people who slept longer than eight hours a day were at a 46 per cent greater risk of stroke than average,” the website stated. “People who slept less than six hours a day were at an 18 per cent 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 were at double the risk of stroke when compared to those who had average sleep, that is between six and eight hours per day.

“This risk was even greater for those whose reported sleep increased from short to long over the four years – their risk was close to four times that of people who maintained an average sleep duration,” the website continued.

PhD candidate at the University of Cambridge, Yue Leng, said: “It’s apparent both from our own participants and 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.”

The university’s website said lack of sleep has been linked with factors such as disrupted metabolism and raised levels of the ‘stress hormone’ 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.

The study was supported by the Medical Research Council and Cancer Research UK.