SLEEPING TOO MUCH or not enough may have a negative impact on a person’s health, according to research.

Fewer than six and more than 10 hours of sleep per day are associated with metabolic syndrome – a group of conditions including increased blood pressure, high blood sugar, excess body fat around the waist and abnormal cholesterol levels – a new study has found.

The research, which involved 133,608 Korean men and women aged 40-69 years, has been published in the journal BMC Public Health.

Researchers at Seoul National University College of Medicine 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 a larger waist circumference.

Women who slept fewer than six hours were also more likely to have a larger waist circumference.

Sleeping more than 10 hours per day was associated with metabolic syndrome and increased levels of triglycerides (a type of fat that the body converts excess calories into), in men.

It was associated with metabolic syndrome, larger waist circumference, higher levels of triglycerides and blood sugar as well as low levels of ‘good’ cholesterol (HDL-C) in women.

The researchers found that nearly 11% of men and 13% of women slept less than six hours, while 1.5% of men and 1.7% of women slept more than 10 hours.

Potential gender difference

Claire E Kim, lead author of the study, said: “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.

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% in men and 24.5% in women. The authors suggest that as the prevalence of metabolic syndrome in Korea is high, it is critical to identify modifiable risk factors such as sleep duration.

Previous study 

The authors used data from the Health Examinees (HEXA) study, a large-scale community-based study conducted in Korea between 2004 and 2013, which included information on sociodemographic 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, 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.

The authors caution 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-reporting data rather than objective measures and may reflect time spent 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.