Seoul National College of Medicine conducted a study involving 133,608 women and men between the ages of 40-69 years old using data from the HEXA large scale community based study. Data collected included family history, medical history, diet, medication use, reproductive factors, socio demographic factors, lifestyle factors, and physical activity levels, as well as samples of serum, urine, DNA, plasma, blood cells, buffy coat, questionnaires, and full physical examinations. Men who slept less than 6 hours a day were found to be more likely to have metabolic syndrome and higher waist circumference, and women sleeping less than 6 hours were found to be more likely to have higher waist circumference. 10 hours of sleep per day or more was associated with metabolic syndrome, increased levels of triglycerides in men, higher waist circumference, higher levels of blood sugar, and lower levels of HDL-C in women. 11% of men and 13% of women were found to sleep less than 6 hours, 1.5% of men and 1.7% of women were found to sleep more than 10 hours.
Participants were classed as having metabolic syndrome if they displayed at least three symptoms which included: high triglyceride levels, elevated waist circumference, hypertension, high fasting blood sugar, and low levels of HDL-C. Prevalence of metabolic syndrome was 24.5% in women and 29% in men.
Biological mechanisms underlying associations between metabolic syndrome and sleep duration are unclear, but may include elevated levels of hormones which increase appetite or reduce energy expenditure which may lead to increased waist circumference and obesity. Cause and effect can not be determined due to the cross sectional observational nature of the study. Sleep duration estimates were based off self reported data and may reflect in bed time rather than time spent sleeping. Naps and sleep were not distinguished between, impacts on health could not be assessed separately.
