Getting less than six hours of shut-eye per day is associated with metabolic syndrome, which can lead to weight gain, raised blood pressure, insulin resistance, and inflammation among other things. However, sleeping for too long, such as more than ten hours per day, can also trigger metabolic syndrome.

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There is still a lot we don’t know about the role of sleep and what exactly goes inside the brain during slumber. But while its functions haven’t been fully mapped out, scientists agree that quality sleep is critical to a healthy lifestyle. For instance, previous studies have found that neurons are remodeled during sleep and that animals and humans deprived of sleep do not perform well on memory tasks. According to National Heart, Lung and Blood Institute, prolonged sleep deficiency is also linked to an increased risk of heart disease, kidney disease, high blood pressure, diabetes, and stroke.

A new study performed by researchers at the Seoul National University College of Medicine adds new depth to the importance of sleep in human health. The team crunched the numbers on a huge dataset from the HEXA study, a large-scale community-based study conducted in Korea during the years 2004-2013, involving 133,608 Korean men and women aged 40 to 69 years. For each HEXA participant samples of plasma, serum, buffy coat, blood cells, genomic DNA, and urine were collected. 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?”

The authors found that nearly 11% of men and 13% of women slept less than six hours each day, while 1.5% of men and 1.7% of women slept more than ten hours.

Compared to individuals who slept six to seven hours per day, men who slept fewer than six hours were likelier to have metabolic syndrome and a higher waist circumference. Women who slept less than six hours were more likely to have a higher waist circumference.

Metabolic syndrome is a cluster of conditions — increased blood pressure, high blood sugar, excess body fat around the waist, and abnormal cholesterol or triglyceride levels — that occur together, increasing your risk of heart disease, stroke, and diabetes.

Both men and women who slept for more than ten hours a day were at a higher risk of developing metabolic syndrome and increased levels of triglycerides than people who slept under ten hours. Additionally, women who slept more than ten hours per day also showed signs of high blood sugar and low levels of ‘good’ cholesterol (HDL-C).

“This is the largest study examining a dose-response association between sleep duration and metabolic syndrome and its components separately for men and women. 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. 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,” said Claire E. Kim, lead author of the new study published in the journal BMC Public Health. 

The prevalence of metabolic syndrome was just over 29% in men and 24.5% in women. However, the biological mechanisms that underlie the association between sleep duration and metabolic syndrome are not clear yet. One possible explanation is that elevated levels of hormones increase appetite and caloric intake or reduce energy expenditure in people who are sleep deprived.

Whatever may be the case, this is yet another study that underscores the importance of sleep for living a healthy life.