Jet-Lagged By Your Social Calendar? Better Check Your Waistline

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It doesn't take a transcontinental flight to end up out of sync with your body clock. It might just be that you stay up too late.

According to German researcher Till Roenneberg, the disconnect between our social calendars and our biological clocks is creating a kind of jet lag — he's dubbed it "social jet lag."

And the consequence? Expanding waistlines. "The larger the discrepancy between social time and what your biological clock tells you to do, the more likely it is you are [overweight or obese]," Roenneberg tells The Salt.

So who's in this category of the socially jet-lagged? If your weekday work schedule is significantly different from your weekend schedule, it could be you we're talking about here.

Say you wake up at the crack of dawn Monday through Friday, but during the weekend you shift your schedule to later wake-up times and later sleep times. Roenneberg says the effect is as if you are switching time zones on the weekend.

If you listen to my story, you'll hear how the theory jells with a bunch of 20-something professionals — who say they generally are up much later on the weekends. None of them were overweight — or sleepy — but perhaps social jet lag catches up with people eventually.

In his paper, published in Current Biology, Roenneberg estimates that for every hour of social jet lag, the risk of being overweight or obese rises about 33 percent.

Whether disrupting your body clock has an effect on your weight — over and above the fact that it reduces the amount of sleep you get — is unclear. But there's certainly a whole body of evidence linking too little sleep to weight problems.

"As sleep researchers, we do believe that there's an intimate relationship between insufficient sleep and the drive to store fat," Dr. Helene Emsellem of the Center for Sleep and Wake Disorders in Chevy Chase, Md., told us.

The connection between poor sleep and higher body weights has been documented in shift workers such as nurses, in mothers of infants, and even in toddlers and teens. In some cases, people do eat more when their schedules are wacky. But Emsellem says it's also possible that something more primitive is at play here.

"Unfortunately, we have caveman's hard-core wiring," Emsellem says, "and insufficient sleep in primitive times was read by the body: Danger, store fat," she says.

Experts say this may be just one of several complicated mechanisms linking sleep and weight, but the important take-home message is this: Get your ZZZZ's!

"We know if people sleep less, even starting in infants, that this leads to a greater risk of obesity," says Dr. Matthew Gillman, director of the Obesity Prevention Program at Harvard Medical School.

Gillman acknowledges that the "sleep longer, sleep better" message is easier said than done. "I guess I'm a big offender, because I got up this morning at 4 a.m. to make a 6 o'clock flight."

Dr. Clete Kushida, a sleep expert at Stanford Sleep Medicine Center, says more studies are needed to investigate the effects of sleep "timing" separately from sleep duration, though the two are inextricably linked. But he says this new study does offer "some evidence" that living "against the clock" is associated with higher BMI (body mass index).

So, in our crazy, go-go society, even the experts who have a hard time living it say it may be wise to start paying more attention to that internal clock.

Copyright 2012 National Public Radio. To see more, visit http://www.npr.org/.

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