Getting enough sleep is important… but getting too much could be risky, new research shows

A study of more than 116,000 people in seven regions of the world, published in the European Heart Journal, has linked the amount of time people sleep – daytime naps included – to their risk of developing cardiovascular disease and death.

Researchers found that people who slept for longer 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.

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.

Before adjusting for factors that might affect the results, the researchers found that for every 1000 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, Chinese Academy of Medical Sciences, China, working at the Population Health Research Institute at McMaster.

“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.”

The results of previous studies examining associations between sleep and death or cardiovascular disease have been contradictory, the researchers say.

Importantly, they tended to focus on specific populations, and not take into account cultures where daytime napping is common and considered to be healthy.

Regular daytime naps were more common in the Middle East, China, Southeast Asia and South America. These varied mainly between 30 and 60 minutes.

The new study looked at 116,632 adults, aged 35 to 70, in 21 countries with different income levels, in 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.

“After a median follow-up of 7.8 years, we recorded 4381 deaths and 4365 major cardiovascular events,” the authors wrote.

“It showed both shorter (≤6 h/day) and longer (>8 h/day) estimated total sleep durations were associated with an increased risk of the composite outcome when adjusted for age and sex.

“After adjustment for demographic characteristics, lifestyle behaviours and health status, a J-shaped association was observed.

“Compared with sleeping 6–8 h/day, those who slept ≤6 h/day had a non-significant trend for increased risk of the composite outcome [hazard ratio (HR), 1.09; 95% confidence interval, 0.99–1.20]. As estimated sleep duration increased, we also noticed a significant trend for a greater risk of the composite outcome [HR of 1.05 (0.99–1.12), 1.17 (1.09–1.25), and 1.41 (1.30–1.53) for 8–9 h/day, 9–10 h/day, and >10 h/day, Ptrend < 0.0001, respectively].

“The results were similar for each of all-cause mortality and major cardiovascular events. Daytime nap duration was associated with an increased risk of the composite events in those with over 6 h of nocturnal sleep duration, but not in shorter nocturnal sleepers (≤6 h).”

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,” Ms Wang added.

Professor Salim Yusuf, the Principal Investigator of the PURE study, Distinguished Professor of Medicine and Executive Director of the Population Health Research Institute at McMaster University and Hamilton Health Sciences, concluded: “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 Dr Dominik Linz, a cardiologist at the Royal Adelaide Hospital and Associate Professor at University of Adelaide, Australia, and colleagues write: “This study provides important epidemiological information, but causative factors explaining the described associations with increased CV [cardiovascular] risk remain speculative.”

They agreed with Ms Wang that too much sleep might be an indicator of an underlying, undiagnosed health condition, and asked: “once a ‘pathological sleeping/napping pattern’ has been identified, what interventions (if any) should be applied?

“We need to be aware and communicate to our patients, that sleeping a lot and having daytime naps may not always be harmless.”