Sleeping for longer than eight hours a day can double your risk of a stroke, scientists have discovered.
Experts at the University of Cambridge suggested the findings from their 10-year study of 10,000 people could save lives and slash healthcare costs.
Every year there are more than 150,000 strokes in the UK.
It is the equivalent of one every three and a half minutes and around one in every four people who suffer a stroke will die.
A stroke is a serious, life-threatening medical condition which happens when the blood supply to part of the brain is cut off.
It is Britain’s third biggest killer, behind heart disease and cancer.
Researchers began collecting data in 1998 by asking people aged between 42 and 81 how long they slept on an average day.
They checked their sleeping patterns four years later and then six years after that.
The study found that those who said they slept more than eight hours a night at both points of the study were twice as likely to suffer a stroke.
And people who went from sleeping less than six hours a night to more than eight hours were four times as likely to have a stroke.
It is not known why there is an increased risk of stroke if you you sleep too long.
Importantly, the study only found an association between sleep length and risk of stroke. It did not find that sleeping for too long actually causes stroke.
Yue Leng, of the University of Cambridge, 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.
“In this study we are implying that a long sleep is an indicator that we could be at risk of stroke. It is not suggesting longer sleep itself is a risk factor.
“More research is definitely needed. This is the first one in the UK to find this association and we found sleeping duration is an indicator.
“It can have a positive impact on public health as research like this could end up saving the NHS money in the long term.”
Previous studies have suggested a link between sleep and the risk of stroke.
But the new research published last night (WED) in journal Neurology is the first to investigate in detail the link between sleep duration and stroke risk.
Researchers say it is unclear why the link between sleep and stroke risk exists.
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.
But the findings suggest that the link was independent of normal risk factors for cardiovascular disease.
The study followed 10,000 people aged 42 to 81 and found those who slept longer than eight hours on average at any point faced a 46% greater risk of stroke.
The study found that those who said they slept more than eight hours a night at both points of the study were twice as likely to suffer a stroke as those sleeping between six and eight hours.
Most of them reported sleeping between six and eight hours a day with just one in 10 reporting sleeping for more than eight hours a day.
People who slept less than six hours a day were at an 18% increased risk than those who slept between six and eight hours.
Once they had collected their data the scientists combined their findings with 11 other global studies.
Their final analysis included more than 500,000 people from seven countries and supported the findings of their own research.
Prof Kay-Tee Khaw, senior author on the study, said: “We need to understand the reasons behind the link between sleep and stroke risk.
“With further research, we may find that excessive sleep proves to be an early indicator of increased stroke risk, particularly among older people.”
The study was supported by the Medical Research Council and Cancer Research UK.
Tonight the Stroke Association said the study on sleeping longer “does not provide sufficient evidence to prove that this is of concern”.
PUBLISHED: 21:00, 25 February 2015