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Out of synch. People with a work or school schedule that is misaligned with their natural sleep rhythm are more likely to be overweight or obese.

Is Your Alarm Clock Making You Fat?

As if you needed another reason to despise your alarm clock. A new study suggests that, by disrupting your body's normal rhythms, your buzzing, blaring friend could be making you overweight.

The study concerns a phenomenon called "social jetlag." That's the extent to which our natural sleep patterns are out of synch with our school or work schedules. Take the weekends: many of us wake up hours later than we do during the week, only to resume our early schedules come Monday morning. It's enough to make your body feel like it's spending the weekend in one time zone and the week in another.

But is social jetlag actually bad for your health? To investigate, chronobiologist Till Roenneberg at the University of Munich in Germany and colleagues compiled data from tens of thousands of responses to an internet survey on sleep patterns and other behaviors. Previous work with such data has already yielded some clues. "We have shown that if you live against your body clock, you're more likely to smoke, to drink alcohol, and drink far more coffee," says Roenneberg.

In the new study, the team measured the social jetlag of people ages 16 to 65 by calculating how offset sleep times were on workdays and non-workdays. They then constructed a mathematical model that gauged how well biological factors, such as age, gender, sleep duration, and social jet lag could predict body weight. They found that the first three factors were important predictors of body weight for all people. In addition, for people who are already on the heavy side, greater social jet lag corresponded to greater body weight. However, social jet lag was not a good predictor for people with normal body weights, the team reports online today in Current Biology.

The researchers also found that people of all ages awoke and went to bed an average of 20 minutes later between 2002 and 2010. Work and school times have remained the same, meaning that social jetlag has increased during this period. Roenneberg says people are spending much less time outside, which gives their bodies less exposure to natural light that helps set biological rhythms for an earlier sleep schedule.

He adds that our social schedules likely influence our eating schedules: on workdays, people may eat breakfast while their bodies still think it's night. "Eating at the wrong time hits your entire digestive system at the wrong time, so it cannot efficiently do its job," he says, which may explain the link with obesity. The findings have particular relevance for teenagers, who naturally sleep much longer and later than their school schedules allow, and thus suffer more social jetlag than other age groups.

While previous studies have shown a link between sleep duration and obesity, the new work highlights the importance of sleep timing, says Kristen Knutson, a sleep researcher at the University of Chicago in Illinois. "We've known for a while that shift workers are associated with increased health risks, and shift workers have extreme variability in their sleep timing between workdays and non-workdays," Knutson says. "This paper suggests it's not just the extreme cases of irregular bed times, but even a more modest difference between weekends and weekdays of an hour or two seems to be associated with health outcomes like obesity."

While our bodies can adjust and recover from travel jetlag, social jetlag is a chronic problem that reemerges at the start of each workweek, says Nathaniel Watson, a genetic epidemiologist at the University of Washington, Seattle. "The message might be to try to make your sleep patterns more consistent," he says. "This is further evidence that we'd all feel better and be healthier if we would go to sleep when we're tired and we would wake up spontaneously when we're rested."

If only life were that simple.

Posted in Brain & Behavior, Plants & Animals