Does sleeping too little or too much raise your risk of heart disease?

Sleeping more than seven or less than six hours a night has been linked to a higher risk of a heart attack or stroke, according to new research. But how concerned should we be with clocking the ‘right’ numbers of hours’ sleep? We look behind the headlines and give the BHF’s view.

It’s long been known that there is an association between lack of sleep and your risk of heart disease or stroke. But according to recent research, too much sleep can affect your risk too.

The study, presented at the American College of Cardiology’s 70th Annual Scientific Session and led by Dr Kartik Gupta of the Henry Ford Hospital in Detroit, looked at data from over 14,000 Americans who took part in the 2005-2010 National Health and Nutrition Examination Survey.

Participants were divided into three groups: those who slept less than six hours a night, those who slept six to seven and those who slept more than seven. Researchers followed them over several years to see if they died due to heart attack, heart failure or stroke. Although the full research has not been published, the authors said that people who slept six to seven hours the lowest risk of a heart or circulatory problem (such as a heart attack or stroke).

The researchers assessed participants’ cardiovascular risk score, a score which is used in the United States to predict how likely someone is to have a heart or circulatory problem, such as a heart attack or stroke, in the next 10 years – similar to the risk score you might get if you have an NHS Health Check. They found that the median (average) 10-year risk among people with less than six hours sleep a day was 4.6 percent; for those sleeping six to seven or more than seven hours a day it was the same: 3.3 percent. (A risk score less than 5 percent is classed as low risk by the American College of Cardiology.) These findings, as published, don’t obviously support the conclusions that the authors have come to that sleeping six to seven hours is linked to the lowest risk of heart disease. Understanding the detail of the findings and how significant they may be is always a challenge when headline data appear in press releases before a full study has been published, and means it is hard for others to accurately interpret the research.

The study also looked at participants’ levels of C-reactive protein (CRP) in the blood. CRP is made in the liver and increases when there is inflammation in the body. It is known to be higher in people who have heart disease. Researchers found that levels of CRP were lowest in people who slept for six to seven hours, although it’s not clear whether this effect was caused by differences in sleep, or was caused by some other reason (which could have also caused the differences in sleep).

The researchers said that the study shows that, “sleep, similar to diet, smoking and exercise, may play a defining role” in the risk of heart and circulatory disease.

How good was the research?

The full paper hasn’t yet been published so it’s hard to fully understand how good this research is.

One strength of the study is that it sampled a large number of people, of which half were women, and included people from different ethnic backgrounds. The reported findings have similarities to findings from a similar report presented to the European Society of Cardiology Congress in 2018, which looked at 11 studies, with over a million participants in total. That study found that people who slept for less than six hours per night or more than eight hours were at an increased risk of developing or dying from coronary heart disease or a stroke.

A weakness of this more recent study is that it relied on the answers of a self-completed survey about people’s average length of sleep. It is possible that people don’t always estimate how long they sleep correctly.

This kind of study is observational – it doesn’t prove cause and effect, i.e. that too little or too much sleep is actually responsible for increased cardiovascular risk.

The researchers themselves also highlighted that more research is needed not just on the quantity of sleep but on the quality or how well or deeply someone sleeps.

The BHF view

Getting a good night’s sleep is important for good health. When it comes to our heart and circulatory health, this study suggests there might be a sweet spot between getting too much and getting too little sleep. But it’s difficult to say this for sure based on one study, which hasn’t even been published in full, so may not yet have been peer reviewed by other scientists. What’s more, this type of research can’t prove cause and effect, and we wouldn’t suggest you change your habits based on this research.

The research shouldn’t trigger alarm bells for those of us who might have the occasional bad night’s sleep or long lie-in. However, if you are really struggling with your sleep, it’s important to talk to your GP.

How accurate was the media coverage?

The story was covered in the Daily Mail and The Sun.

The Sun headline was: “The ‘sleep formula’ that can slash risk of heart attack and stroke revealed”, followed by “Sleeping for six to seven hours a night can slash your risk of a heart attack and stroke.” Firstly, the research did not find a “formula” for the optimum amount of sleep, and in any case is not the kind of study that can prove cause and effect. It found that sleeping more was linked to a lower risk of heart attack and stroke than sleeping six hours. The authors suggest that sleeping six to seven hours was linked to a lower risk than sleeping more than seven hours, but these differences in risk have not been clearly presented in the research. “Slash” would suggest dramatically cutting down but as far as we can tell from the research, the differences found were not dramatic.

The Mail article is fairly similar to the press release issued about the research.

It does helpfully highlight that there are limitations to this research, saying: “The researchers cautioned that further research will be needed to validate these findings and that the study was limited by only looking at sleep quantity, rather than factoring in how well each participant slept as well.”