Too Much of A Good Thing?

By Jane Farrell

When it comes to sleep, more isn’t necessarily better, according to a global study.

Researchers who studied more than 116,000 people in seven regions of the world found that who slept for longer than the recommended duration of six to eight hours a day had an increased risk of dying or developing diseases of the heart or blood vessels in the brain. 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.

The investigators also found a 9% increased risk for people who slept a total of six or fewer hours.

The results were published in the European Heart Journal.

The lead author, Chuangshi Wang, a PhD student at McMaster University, Canada, and Peking Union Medical College, Chinese Academy of Medical Sciences, China, working at the Population Health Research Institute at McMaster, said: “Our study shows that the optimal duration of estimated sleep is six to eight hours per day for adults. 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.”

People who slept more than ten hours a day had a 41% increased risk of developing diseases of the heart or blood vessels in the brain.

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.

This study looked at a total of 116,632 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 were part of the Prospective Urban Rural Epidemiology (PURE) study that started in 2003.

During an average (median) follow-up time of nearly eight years, 4381 people died and 4365 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. People who slept six or fewer hours at night, but took a daytime nap, and so slept an average of 6.4 hours a day in total, had a slightly increased risk compared to those who slept between six and eight hours at night without a daytime nap, but this finding was not statistically significant.

“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 said.