People whose sleep is split between work week hours — early to bed and early to rise — followed by a weekend preference for late nights and late mornings could be increasing their risk of metabolic problems like heart disease and diabetes, a study suggests.

A significant difference between sleep schedules we naturally prefer and those imposed by society — what researchers term "social jetlag" — has been linked with elevated cardiometabolic risks that can contribute to diabetes, obesity and cardiovascular disease, researchers say.

It has long been known that people who work odd hours or late shifts have increased health risks, the researchers note.

"However, this is the first study to extend upon that work and show that even among healthy, working adults who experience a less extreme range of mismatches in their sleep schedule, social jetlag can contribute to metabolic problems," says research leader Patricia Wong, a doctoral candidate at the University of Pittsburgh.

In the study reported in the Journal of Clinical Endocrinology & Metabolism, 447 men and women aged 30 to 54, who worked at least 25 hours a week outside the home, wore wristbands that recorded sleep hours and movement around the clock for a week.

Researchers also took blood samples and used questionnaires to gather data on the participants' exercise and diet habits.

Almost 85 percent of them slept longer on their days off than on the days they worked, the researchers found. The researchers defined the difference in the two schedules, measured in minutes, as social jetlag.

Those who had significant differences in the sleep schedules between workdays and days off, more than 60 minutes, had a tendency to display unhealthy cholesterol and fasting insulin levels, the researchers found.

They also tended to have larger waist sizes and higher body mass index (BMI) numbers, they discovered.

The findings remained even after the researchers adjusted for other factors, such as sleep quality, physical activity during waking hours, alcohol consumption and caloric intake.

The researchers point out that an association between inconsistent sleep patterns and disease is not the same as a proven cause-and-effect.

However, Wong suggests, it is a strong enough association to warrant more investigation.

"If future studies replicate what we found here, then we may need to consider as a society how modern work and social obligations are affecting our sleep and health," she says.

Social jetlag even among healthy people may raise health risks because it has them "working 'against' their biological clock," Wong says.

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