Sleeping for over ten hours or less than six hours a day is likely to cause metabolic syndrome - a cluster of conditions that increase the risk of heart disease, stroke and diabetes, a study has found.
Researchers at Seoul National University in South Korea found that compared to individuals who slept six to seven hours per day, men who slept fewer than six hours were more likely to have metabolic syndrome and higher waist circumference.
Women who slept fewer than six hours were more likely to have higher waist circumference.
Sleeping more than ten hours per day was associated with metabolic syndrome and increased levels of triglycerides in men, and with metabolic syndrome, higher waist circumference, higher levels of triglycerides and blood sugar, as well as low levels of 'good' cholesterol (HDL-C) in women.
Researchers found that nearly 11 per cent of men and 13 per cent of women slept less than six hours, while 1.5 per cent of men and 1.7 per cent of women slept more than ten hours.
"This is the largest study examining a dose-response association between sleep duration and metabolic syndrome and its components separately for men and women," said Claire E Kim, lead author of the study published in the journal BMC Public Health.
"Because we were able to expand the sample of our previous study, we were able to detect associations between sleep and metabolic syndrome that were unnoticed before," said Kim.
"We observed a potential gender difference between sleep duration and metabolic syndrome, with an association between metabolic syndrome and long sleep in women and metabolic syndrome and short sleep in men," she said.
Based on common definitions, participants were considered to have metabolic syndrome if they showed at least three of the following: elevated waist circumference, high triglyceride levels, low levels of 'good' cholesterol, hypertension, and high fasting blood sugar.
The prevalence of metabolic syndrome was just over 29 per cent in men and 24.5 per cent in women. Researchers suggest that as the prevalence of metabolic syndrome in Korea is high, it is critical to identify modifiable risk factors such as sleep duration.
They used data from the HEXA study, a large-scale community-based study conducted in Korea during the years 2004-2013, which included information on socio-demographic characteristics, medical history, medication use, family history, lifestyle factors, diet, physical activity, and reproductive factors for women.
As part of the HEXA study, samples of plasma, serum, buffy coat, blood cells, genomic DNA, and urine were collected, and participants underwent physical examinations by medical professionals.
Sleep duration was assessed by asking the question: "In the past year, on average, how many hours/minutes of sleep (including daytime naps) did you take per day?"
Although the biological mechanisms that underlie the association between sleep duration and metabolic syndrome remain unclear, several potential processes have been reported.
These include elevated levels of hormones which increase appetite and caloric intake or reduce energy expenditure in people who sleep less than seven hours per day, which may lead to increased waist circumference and development of obesity.
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