Toronto: Too much or too little sleep can increase the risk of cardiovascular disease and early death, according to a study of over 116,000 people from across the world published Wednesday.
The researchers found that people who slept for longer than the recommended duration of six to eight hours a day had an increased risk of early death 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 per cent increased risk; people sleeping between nine and ten hours a day had an increased risk of 17 per cent and those sleeping more than ten hours a day had a 41 per cent increased risk.
They also found a nine per cent increased risk for people who slept a total of six or fewer hours, but this finding was not statistically significant.
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 Chuangshi Wang, a PhD student at McMaster University in Canada.
"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," Wang said, lead author of the study published in the European Heart Journal.
"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 said.
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 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).
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.
Researchers 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," Wang said.
"In these individuals, a daytime nap seemed to compensate for the lack of sleep at night and to mitigate the risks," she said.