"Restful night 'cuts risk' of heart attack," reports The Times.
Doctors have long suspected a link between unhealthy sleep behaviours and cardiovascular diseases such as heart disease and stroke. Researchers created a 'healthy sleep score', which combined 5 factors:
- being a morning person rather than a 'night owl'
- sleeping 7 to 8 hours most nights
- never or rarely having difficulty getting to sleep or waking in the night
- not snoring
- not dozing off during the daytime without meaning to
They applied the score to more than 385,000 people from the UK without cardiovascular disease, who were followed up for an average of 8.5 years. Researchers were interested to see how their sleep score affected their chance of developing heart disease or stroke.
People who had all the healthy sleep factors (a score of 5) were compared to people with the lowest score (0 to 1). Those with the highest score were 35% less likely to develop heart disease or have a stroke than those with the lowest score.
Where did the story come from?
The researchers who carried out the study were from Tulane University in the US and Peking University Health Centre in China, although they used information gathered in the UK.
The study was funded by the US National Heart, Lung and Blood Institute and the National Institute of Diabetes and Digestive and Kidney Diseases. It was published in the peer-reviewed European Heart Journal on an open access basis so it's free to read online.
The reports in the UK media were mostly balanced and accurate, although they did not all make the point that the study did not prove cause and effect. It was also not made clear that the widely reported 34% difference in cardiovascular risk was based on a comparison between the best and worst sleep scores, rather than the best compared to the average scores.
What kind of research was this?
This was a cohort study. Cohort studies are useful ways to examine links between risk factors such as poor sleep and outcomes such as heart attacks and strokes. But they cannot tell us whether risk factors directly cause outcomes. Other factors may be involved.
What did the research involve?
Researchers used information from the UK Biobank cohort study, which is an ongoing study of over 500,000 UK adults aged 37 to 73, recruited from 2006 to 2010. At recruitment, people were asked about a wide range of health conditions and lifestyle behaviours, and gave genetic samples.
The researchers used data from people who did not have cardiovascular disease at the start of the study and who answered all the questions about their sleep habits. They followed them up until February 2016 to see if they had been admitted to hospital with heart disease or stroke.
People were asked about their sleep and the researchers used their answers to give them a score based on the 5 sleep behaviours. They looked at how likely they were to have had heart disease or stroke, depending on their sleep score.
The researchers adjusted their figures to take into account a number of potential confounders that could affect both cardiovascular risk and sleep quality. These were:
- age, sex and ethnic background
- deprivation level
- physical activity level
- smoking and alcohol consumption
- family history of heart disease and stroke
- body mass index (BMI)
- whether they had high blood pressure or diabetes
They also investigated whether genetic risk of cardiovascular disease affected their results. They used people's genetic samples to put them into high, medium or low genetic risk categories, then looked to see whether sleep patterns still affected people's chance of getting cardiovascular disease when combined with genetic risk.
What were the basic results?
Of the 385,292 people in the study, 21.8% had the healthiest sleep score of 5, and 2.3% had the worst sleep score of 0 or 1. During the 8.5 year follow-up, 7,280 people developed heart disease or stroke.
Compared to those with the worst sleep score, those with the healthiest sleep score were 35% less likely to have had heart disease or stroke (hazard ratio (HR) 0.65, 95% confidence interval (CI) 0.52 to 0.81). The researchers calculated that each additional healthy sleep score point lowered the risk of heart disease or stroke by 8% (HR 0.92, 95% CI 0.89 to 0.94).
We do not know whether healthy sleep directly causes the lower risk of cardiovascular disease. But the researchers calculated that, if it did, 11.5% of the total number of cases of heart disease and stroke in the study would have been avoided.
When looking at genetic risk, the researchers found that better sleep patterns were still linked to lower risk of cardiovascular disease. People who had low genetic risk saw their risk rise if they had poor sleep patterns, while people at high genetic risk saw their risk fall if they had good sleep patterns.
How did the researchers interpret the results?
The researchers said: "We found that a healthy sleep pattern was related to a reduced risk of CVD events. Our findings underline the importance of considering sleep behaviours in cardiovascular practice."
They added: "We found that a high genetic risk could be partly offset by a healthy sleep pattern, but in addition, individuals with low genetic risk could lose their inherent protection if they had a poor sleep pattern."
The study confirms that good sleep is part of a healthy lifestyle – including plenty of physical activity and a balanced diet – which can reduce the risk of cardiovascular disease. The study has some limitations:
- Observational studies cannot prove cause and effect, because they could be affected by unmeasured factors.
- People reported their own sleep patterns, which might not be completely accurate.
- The healthy sleep score might not have included every important sleep measure – for example, it did not include whether people had conditions such as restless legs syndrome.
The study also only measured sleep patterns once, at the start of the study, so we do not know how changing sleep patterns might affect cardiovascular risk.
Having said that, it seems likely that good sleep, as part of an overall healthy lifestyle, can contribute to a lowered chance of getting conditions such as cardiovascular disease.
Analysis by Bazian
Edited by NHS Website
Links to the headlines
Metro, 18 December 2019
Mail Online, 18 December 2019
The Times, 18 December 2019
Links to the science
European Heart Journal, Published online 18 December 2019