Social jetlag（时差综合症） -- a syndrome related to the mismatch between the body's internal clock and the realities of our daily schedules -- does more than make us sleepy. It is also contributing to the growing tide of obesity, according to a large-scale epidemiological study reported online on May 10 in Current Biology, a Cell Press publication. "We have identified a syndrome in modern society that has not been recognized until recently," said Till Roenneberg of the University of Munich. "It concerns an increasing discrepancy between the daily timing of the physiological clock and the social clock. As a result of this social jetlag, people are chronically sleep-deprived. They are also more likely to smoke and drink more alcohol and caffeine. Now, we show that social jetlag also contributes to obesity; the plot that social jetlag is really bad for our health is thickening."

Each of us has a biological clock, he explained. We can't set those clocks according to our whims（奇想，怪念头） like watches. They are rather entrained by daylight and night-darkness to provide the optimal window for sleep and waking. In modern society, we listen to those clocks "less and less due to the increasing discrepancy（不符，矛盾） between what the body clock tells us and what the boss tells us."

To find out how big this problem really is, Roenneberg's team is compiling a vast database on human sleeping and waking behavior, which they'll eventually use to produce a world sleep map. Now 10 years into the effort, they already have lots of information, including participants' height, weight, and sleep patterns.

Their analysis shows that people with more severe social jetlag are also more likely to be overweight. In other words, it appears that living "against the clock" may be a factor contributing to the epidemic of obesity, the researchers say.

The findings should weigh in on decisions about Daylight Saving Time, not to mention work and school times, they add. It would also help if people began spending more time outdoors in open daylight or at least sitting by a window. As people fail to do this for one reason or another, their body clocks get set later and later, leaving them awake into the night and tired by day.

"Waking up with an alarm clock is a relatively new facet of our lives," Roenneberg says. "It simply means that we haven't slept enough and this is the reason why we are chronically tired. Good sleep and enough sleep is not a waste of time but a guarantee for better work performance and more fun with friends and family during off-work times." And slimmer waistlines, too.