The recommended amount of sleep for adults is six to eight hours a night. Sleeping more than those hours is associated with an increased risk of death and cardiovascular diseases, says a global study published Wednesday, December 5 in the European Heart Journal, CNN reports.
Looking at data from 21 countries, across seven regions, the research team found that people sleeping more than the recommended upper limit of eight hours increased their risk of major cardiovascular events, like stroke or heart failure, as well as death by up to 41%.
But a possible reason for this could be that people have underlying conditions causing them to sleep longer, which in turn could raise the risk of cardiovascular disease or mortality, explain the authors of the study.
The team, led by Chuangshi Wang, a Ph.D. student at McMaster and Peking Union Medical College in China, also identified a rising risk among daytime nappers.
"Daytime napping was associated with increased risks of major cardiovascular events and deaths in those with [more than] six hours of nighttime sleep but not in those sleeping [less than] 6 hours a night," Wang said.
In those who underslept, "a daytime nap seemed to compensate for the lack of sleep at night and to mitigate the risks," Wang explained.
Previous studies into this topic were mainly carried out in North America, Europe and Japan. The new study brings a global picture.
But the findings are observational, meaning the cause of this association remains unknown.
"Even though the findings were very interesting they don't prove cause and effect," said Julie Ward, a senior cardiac nurse at the British Heart Foundation, who was not involved in the study.
Having less sleep -- under six hours -- was also shown to increase these risks by 9%, compared with people who slept for the recommended six to eight hours, but this finding was not considered to be statistically significant by the team.
In 2014, 35.2% of American adults reported not getting enough sleep with less than seven hours per night, according to the CDC.