For the research, led by Chuangshi Wang of China’s McMaster and Peking Union Medical College, Wang and her colleagues examined sleep data on 116,632 adults from 21 countries.
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Over a span of about 7.8 years, the researchers assessed how much each individual slept per night, whether they were getting more than or less than the recommended eight hours per day, their napping habits and overall health.
According to the study, people who slept more than the highest recommended amount (eight hours) had an increased risk of experiencing heart failure and stroke. These individuals also had an increased risk of mortality by up to 41 percent. The risk further rose with more sleep.
For example, folks who slept eight to nine hours per night saw a 5 percent increased risk of death and disease, nine to 10 hours resulted in a 17 percent increased risk and more than 10 hours of sleep meant a whopping 41 percent increased risk of disease or death compared with people who slept the recommended amount.
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Those who slept more than six hours per night and napped during the day also had increased risks of disease and early death compared to those who napped but slept less than six hours per night.
"A daytime nap seemed to compensate for the lack of sleep at night and to mitigate the risks," Wang told CNN.
While previous research has pointed out that not getting enough sleep or getting poor-quality sleep increases one's risk of high blood pressure, obesity, heart disease and diabetes, the new study didn't find such an association to be statistically significant.
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The main finding, according to Wang, is that six to eight hours per day is the optimal duration of sleep.
The American Academy of Sleep Medicine and the Sleep Research Society recommends a sweet spot of at least seven hours for adults aged 18-60 years.
But according to the Atlanta-based Centers for Disease Control and Prevention, more than one-third of Americans still don't get enough sleep.
"Lifestyle changes such as going to bed at the same time each night; rising at the same time each morning; and turning off or removing televisions, computers, mobile devices from the bedroom, can help people get the healthy sleep they need," Wayne Giles, director of CDC's Division of Population Health, said in the 2016 report.
Read the full study at academic.oup.com.