By Amanda Gardner
Do you wake up to the sound of an alarm each weekday? That may be a sign that your body clock doesn't agree with your daily schedule, a situation that over time may open the door to weight gain and other health problems.
Researchers in Europe have coined the term "social jet lag" to describe the all-too common practice of following a different sleep schedule on weekdays versus the weekend. Our circadian rhythms are out of sync with our hectic work schedules, the theory goes, so each weekend we're effectively flying back and forth between time zones without ever leaving the ground.
Social jet lag is "the discrepancy between what our body clock wants us to do and what our social clock wants us to do," says Till Roenneberg, Ph.D., a professor at the University of Munich's Institute of Medical Psychology, in Germany. "It almost looks as if people on a Friday evening fly from Paris to New York, and on Monday morning they fly back again."
This weekly disruption is more than just a nuisance. In a new study published today in the journal Current Biology, Roenneberg and his colleagues surveyed the sleep habits of more than 65,000 adults and found that people with different weekday and weekend sleep schedules had triple the odds of being overweight.
What's more, the body mass index (BMI) of overweight people tended to rise as the gap between their weekday and weekend "time zones" widened.
The findings echo previous research linking higher BMI to sleep deprivation and irregular sleep schedules. In particular, numerous studies have found an increased risk of obesity -- as well as chronic diseases such as diabetes -- among shift workers. Social jet lag may be harmful in the same way, says David J. Earnest, Ph.D., a neurobiologist and body-clock expert at the Texas A&M Health Science Center College of Medicine, in Bryan.
"The schedules we keep on the weekend are much different than those we keep during the week," Earnest says. "This potentially has ramifications for disrupting circadian rhythms and translating into the same sorts of things associated with shift work, such as an increased risk for cancer and diabetes and so on."
Some of the proposed explanations for the link between shift work and obesity, such as irregular meal times and metabolism disruptions at the cellular level, may help explain the social jet lag findings as well, Roenneberg says.
"With social jet lag, we're forced to eat at times when the body doesn't want to eat, or isn't prepared for digesting food properly," he says. "All these things coming together might influence the way you digest food and how you incorporate it into your body fat. The result is that you become overweight or obese."
Most people experience at least some social jet lag. Two-thirds of the study participants reported at least one hour's difference in their average weekday and weekend sleep schedules, and more than 10 percent reported three-plus hours.
Paying more attention to our body clocks may be good for the economy as well as our health, Roennenberg suggests. Rather than bending early birds and night owls to the same work schedule, why not encourage personalized schedules based on each individual's circadian rhythms? The result would be a better-rested, healthier, and doubtless more productive workforce, Roennenberg says.
"Living against our body clocks is detrimental for our health," says Roenneberg, who spoke to Health.com from Munich at 11 p.m. local time, just before heading to bed. "On an epidemiological level, we pay an enormous price for not being within our natural clocks."