Longer sleep linked to stroke
“Too much sleep could kill you,” is the baseless and needlessly alarmist headline on the front cover of today’s Daily Express.
The study it is reporting on actually showed that people who sleep for more than eight hours a night had a 46% increased risk of stroke over the following 10 years, compared with people sleeping six to eight hours.
While these results certainly warrant further investigation, it does not show that the increased sleep caused strokes, let alone death.
The researchers assessed the usual sleep patterns of nearly 10,000 adults in 1998 and again in 2002, looking for associations between the amount of sleep and the number of people having a stroke over the next 10 years.
They also pooled the results from similar studies. These also showed a 45% increased risk for people who sleep more than eight hours.
When the results were analysed by sex, the link was statistically significant for women, but not men. This wasn’t made clear in the UK media coverage. Women’s risk was 80% higher, which is almost double the 46% risk when the sexes were combined.
The study took into account cardiovascular risk factors such as high blood pressure and cholesterol, but not other illnesses. Without accounting for other illnesses, it is not clear what association the length of sleep has with risk of stroke from these studies. As that widely used, though valid, scientific cliché goes: “further research is needed”.
Where did the story come from?
The study was carried out by researchers from the University of Cambridge and the University of Warwick. It was funded by the Medical Research Council and Cancer Research UK.
The study was published in the peer-reviewed medical journal Neurology. The study was published on an open-access basis, meaning that anyone can read and download it for free online. There is also a related editorial.
The quality of the UK media's reporting of the study was mixed. The Independent and The Daily Telegraph took a measured approach, making clear the uncertainties of the study.
The Daily Mirror somewhat contradicted itself, saying first that: “Shock study reveals sleeping for longer than eight hours 'could cause a stroke'.” Whereas later on, it correctly says: “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.”
The Daily Express and the Metro said that increased sleep causes strokes, when this is not actually what the study found.
At most, the study found that increased sleep is associated with an increased risk of stroke in women, but it did not take illnesses other than diabetes, high blood pressure and previous stroke into account, which could have affected the results.
A lot of the media carried a useful quote from Yue Leng, from the University of Cambridge, saying: “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.”
What kind of research was this?
This was a cohort study, which aimed to see if there was an association between sleep duration and risk of stroke. The researchers also performed a systematic review to find other relevant research, and pooled all the results in a meta-analysis.
A cohort study is the most appropriate type of study when looking at the long-term effect of sleep patterns, as it would not be feasible or ethical to conduct a randomised controlled trial over a long time period. Combining the results with other similar studies in a meta-analysis increases the strength of the evidence. However, due to the nature of the study types, they can only show an association between sleep duration and risk of stroke – they cannot prove that sleep duration causes a stroke.
What did the research involve?
The researchers assessed the regular sleep patterns of nearly 10,000 adults, looking for links between the amount of sleep they got and the number of people who had a stroke over the next 10 years. They systematically searched for similar studies and pooled their own results with these others in a meta-analysis.
The researchers recruited 9,692 participants from a larger longstanding study called the European Prospective Investigation of Cancer-Norfolk cohort, EPIC-Norfolk. They were given a questionnaire in 1998-2000, and again in 2002 to 2004, asking how much sleep they usually have over a 24-hour period, with the following options:
- less than four hours
- four to six hours
- six to eight hours
- eight to 10 hours
- 10 to 12 hours
- more than 12 hours
They were also asked if they sleep well, to which they could respond “yes” or “no”.
Participants were excluded from the study if they had already had a stroke. The researchers then obtained all cases of stroke from the National Health Services district database and the UK Office of National Statistics up until March 2009.
They analysed the results according to the average sleep duration, or the change in sleep duration between the two questionnaires. They also took into account all of the following potential confounding factors:
- age
- sex
- social class
- education
- marital status
- smoking
- alcohol intake
- hypnotic drug use (sedatives and “sleeping tablets”)
- family history of stroke
- physical activity
- major depressive disorder in the previous year
- previous heart attack
- diabetes
- use of blood pressure medication
- body mass index (BMI)
- blood pressure
- cholesterol
Finally, they performed the systematic review and meta-analysis using all available trials up until May 2014.
What were the basic results?
The average age of the participants at the start of the study was 62, and ranged from 42 to 81 years. Most of them slept between six and eight hours per day (69%), with 10% sleeping for more than eight hours. In total, 346 people had a stroke during the 9.5-year follow-up period.
After adjusting for all of the confounding factors listed above, sleep of more than eight hours:
- increased the risk of stroke by 46% (hazard ratio (HR) 1.46, 95% confidence interval (CI) 1.08 to 1.98)
- increased the risk of stroke in women by 80% (HR 1.80, 95% CI 1.13 to 2.85)
- was not associated with stroke in men
There was no statistically significant association between sleep of less than six hours and stroke.
The systematic review identified 11 relevant studies, including 559,252 participants from seven countries. They were followed up for between 7.5 and 35 years. The pooled relative risks for sleep duration and stroke were:
- increased risk of 15% for sleep of less than six hours (relative risk (RR) 1.15, 95% CI 1.07 to 1.24)
- increased risk of 45% for sleep of more than eight hours (RR 1.45, 95% CI 1.30 to 1.62)
How did the researchers interpret the results?
The researchers concluded that this study, “suggested a significant increase in stroke risk among long sleepers and a modest increase among short sleepers”. They say that, “the underlying mechanism needs further investigation”.
Conclusion
This cohort study found that, overall, people who sleep for more than eight hours have a 46% increased risk of stroke. When analysed separately, there was no statistically significant association for men, but a much higher increased risk for women, of 80%.
A major strength of the study is the number of potential confounding factors that the researchers tried to account for, including many cardiovascular risk factors. However, it did not account for other illnesses such as sleep apnoea or cancer, which may have had an effect on the amount of sleep and risk of stroke.
In addition, the study is reliant on the information provided in the questionnaires, which may not be entirely accurate:
- alcohol intake is famously under-reported
- perception of sleep duration and actual duration may be different and could be affected by illness and memory problems
The results of the meta-analysis were in line with the results of this study, though they also found an increased risk for people who have less than six hours sleep.
Professor Kay-Tee Khaw, senior author on the study, said in the Mirror that: “We need to understand the reasons behind the link between sleep and stroke risk”. She added that, “With further research, we may find that excessive sleep proves to be an early indicator of increased stroke risk, particularly among older people.”
In conclusion, without accounting for other illnesses, it is not clear what association the length of sleep has with risk of stroke from these studies. Known modifiable risk factors that can reduce your risk of stroke are to stop smoking, eat healthily, do physical exercise, and keep blood pressure and cholesterol within normal limits through lifestyle and use of medication where required.
If you are concerned that your normal sleep patterns have changed for no apparent reason, visit your GP.
