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Like good diet and exercise, sleep is a critical component to overall health. Current sleep guidelines recommend that adults between the ages of 26 and 64 sleep 7-8 hours per night and practice healthy sleep hygiene which includes limiting daytime naps to 30 minutes, avoiding stimulants such as caffeine and energy drinks before bedtime and avoiding heavy or rich foods before bedtime.

Previous research has shown that insufficient as well as excessive sleep are associated with health-related conditions such as hypertension, obesity, diabetes, cardiovascular events or mortality and stroke.

Metabolic syndrome, which is highly prevalent in Korea, is defined as a metabolic disorder consisting of at least three of the following: elevated waist circumference, high triglyceride levels, low high-density ‘good’ cholesterol levels (HDL-C), hypertension and high fasting glucose.

Researchers at Seoul National University College of Medicine have carried out the largest study examining a dose-response relationship between sleep duration and metabolic syndrome and its components in 133,608 Korean men and women aged 40-69 years. To do this, 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.

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 study, published today in BMC Public Health, finds that compared to individuals who sleep six to seven hours per day, men who sleep fewer than six hours are more likely to have metabolic syndrome and higher waist circumference. Women who sleep fewer than six hours are more likely to have higher waist circumference. Furthermore, 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 HDL-C in women.

The exact mechanisms for the gender differences observed are unclear but a possible explanation is that women experiencing menopausal transition face erratic fluctuations and eventual decline in estrogens as well as ovarian estradiol which may lead to frequent sleep disruptions, a common characteristic of long sleep duration. Another theory is that women may have shorter circadian periods which contributes to higher insomnia.

The biological mechanisms that underlie the association between sleep duration and metabolic syndrome are also unclear but potential factors include 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.

It’s important to note that the cross-sectional, observational nature of this study does not allow for conclusions about cause and effect. Estimates of sleep duration were based on self-report data rather than objective measures and may reflect ‘time in bed’, actual time spent asleep or time people believed they slept. Also, as the study did not distinguish between daytime naps and nighttime sleep, their impact on health could not be assessed separately.