Taking the attitude that you should try and get as much sleep as possible may not be a good idea.
Sleeping over eight hours a night significantly increases your risk of having a stroke, according to research from the University of Cambridge, but the puzzle is that no one knows why.
Over a 10 year period, the scientists followed the sleeping habits of just under 10,000 people in the UK aged 42 to 81, and studied how these related to the likelihood of suffering a stroke.
The results, published in the journal Neurobiology, show people who regularly get more than eight hours of sleep a day are 46 per cent more likely to suffer a stroke than the average. This compares to an 18 per cent higher risk for those getting less than six hours.
The link is not entirely surprising – previous studies have shown a similar connection, but this is the first time a detailed study on the British population has been published.
The danger is even greater for older people, whose chance of a stroke doubles if they persistently sleep for longer than the recommended six to eight hours.
Continuity is key
Over the long term, the most dangerous thing you can do is to go from getting continually little sleep to getting large amounts of sleep. Making such a drastic change increases the risk of stroke by four-fold compared to those who are consistent in their sleeping habits.
Cause or symptom?
The connection is clear, but the scientists say they are unaware of why it exists – there are lots of different factors at play, all of which could be influencing the results. These include disrupted metabolism due to sleep pattern, increased levels of stress hormones and higher blood pressure.
In fact, the study's authors even suggest that the cause of increased stroke risk could be something entirely different, and that longer sleeping time might just be another symptom of this.
Professor Kay-Tee Khaw, who led the research, said it was important to continue unravelling the mystery:
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.