How much you sleep is linked to your risk of developing cardiovascular disease and death, according to a McMaster University-led study of more than 116,000 people in seven regions of the world, published in the European Heart Journal this week.
The researchers found people who slept for more than the recommended six to eight hours a day had an increased risk of dying or developing diseases of the heart or blood vessels in the brain. The study, of more than 116,000 people worldwide, was published in the European Heart Journal this week.
Compared to people who slept for the recommended time, those who slept a total of eight to nine hours a day had a 5% increased risk; people sleeping between nine and ten hours a day had an increased risk of 17% and those sleeping more than ten hours a day had a 41% increased risk. They also found a 9% increased risk for people who slept a total of six or fewer hours, but this finding was not statistically significant. Naps were included in the time of sleep.
Before adjusting for factors that might affect the results, the researchers found that for every 1,000 people sleeping six or fewer hours a night, 9.4 developed cardiovascular disease (CVD) or died per year; this occurred in 7.8 of those sleeping six to eight hours, 8.4 of those sleeping eight to nine hours, 10.4 of those sleeping nine to ten hours and 14.8 of those sleeping more than ten hours.
"Our study shows that the optimal duration of estimated sleep is six to eight hours per day for adults," said lead author Chuangshi Wang, a PhD student at McMaster and Peking Union Medical College in China.
"Given that this is an observational study that can only show an association rather than proving a causal relationship, we cannot say that too much sleep per se causes cardiovascular diseases. However, too little sleep could be an underlying contributor to death and cases of cardiovascular disease, and too much sleep may indicate underlying conditions that increase risk."
She is working with the Population Health Research Institute (PHRI) of McMaster and Hamilton Health Sciences. Associations between sleep and death or cardiovascular and other diseases have been suggested by other studies, but results have been contradictory. In addition, they tended to look at particular populations and did not necessarily take account of the fact that in some countries daytime napping can be common and considered healthy.
The study looked at adults aged between 35 and 70 years in 21 countries with different income levels in seven geographic regions (North America and Europe, South America, the Middle East, South Asia, Southeast Asia, China and Africa). They are part of the Prospective Urban Rural Epidemiology (PURE) study that started in 2003.
During an average (median) follow-up time of nearly eight years, 4,381 people died and 4,365 suffered a major cardiovascular problem such as a heart attack or stroke. The researchers adjusted the results to take account of factors that could affect outcomes, such as age, sex, education, smoking, alcohol consumption, whether the participants lived in urban or rural areas, had a family history of cardiovascular disease, or had a history of diabetes, raised blood pressure, chronic obstructive pulmonary disease or depression.
They found that regular daytime naps were more common in the Middle East, China, Southeast Asia and South America. The duration of daytime naps varied mainly from 30 to 60 minutes.
"Although daytime napping was associated with higher risks of death or cardiovascular problems in those with sufficient or longer sleep at night, this was not the case in people who slept under six hours at night. In these individuals, a daytime nap seemed to compensate for the lack of sleep at night and to mitigate the risks," Wang added.
Salim Yusuf, professor of medicine at McMaster and executive director of the PHRI, added: "The general public should ensure that they get about six to eight hours of sleep a day.
"On the other hand, if you sleep too much regularly, say more than nine hours a day, then you may want to visit a doctor to check your overall health. For doctors, including questions about the duration of sleep and daytime naps in the clinical histories of your patients may be helpful in identifying people at high risk of heart and blood vessel problems or death."
In an accompanying editorial, Dominik Linz, a cardiologist and associate professor at University of Adelaide, Australia, and colleagues wrote: "This study provides important epidemiological information, but causative factors explaining the described associations with increased CV [cardiovascular] risk remain speculative."
They conclude: "We need to be aware and communicate to our patients, that sleeping a lot and having daytime naps may not always be harmless. Perhaps the ancient Greek poet Homer, author of the Iliad and the Odyssey, summed it up millennia of years ago when he said: 'Even where sleep is concerned, too much is a bad thing'."