Anyone who has struggled to get a teenager out of bed on Monday morning—or wake themselves—knows how difficult it can be to make the transition from lazy weekend days to the work-and-school week. Now a new study from the University of Munich suggests that the often-sharp contrast between weekday and weekend sleep habits may also contribute to obesity.
Researchers led by Till Roenneberg of the University of Munich analyzed sleep logs of more than 65,000 Europeans ages 10 to 80. They found that weekly average sleep plummets between ages 10 and 20, largely because of increased social and work obligations. More than 80 percent, the researchers found, used an alarm clock to wake during the week, indicating that they had to wake earlier than their natural sleep cycle, and those with a later circadian rhythm, such as teenagers, had even more disrupted sleep patterns.
Most of the study participants tried to pay off their sleep deficit on the weekend, staying out later at night and sleeping and eating later on Saturday and Sunday. This created what Roenneberg and his colleagues called "social jetlag," because the Monday-morning drag was similar in both cause and effect to flying West several timezones on Saturday and flying back again Monday morning:
"The symptoms of jetlag (e.g., problems in sleep, digestion and performance) are manifestations of a misaligned circadian system. In travel-induced jetlag, they are transient until the clock re-entrains. In contrast, social jetlag is chronic throughout a working career."
The researchers found one in three people experienced two hours or more of social jetlag every week, and teenagers suffered the most extreme social jetlag of any age group. Moreover, the greater the disconnect between week and weekend sleep cycles, the more likely a person was to be overweight or obese, and the risk was independent of how much total sleep someone had.
With schools under pressure to help stem the rising child obesity epidemic, this study suggests that rethinking the school day—particularly in middle and and high school—may be as important as getting students moving more at recess.
The study is available in this month's issue of Current Biology, and here's Roenneberg (who has a very soothing voice) talking about the findings.