A mismatch between when our internal clock wants us to wake up and when the alarm clock rings to get people to work and school on time could be fuelling obesity, a European study suggests.

Tight work schedules and a hectic social calendar structure modern societies. The result is "social jet lag" — a syndrome related to the mismatch between the body's internal clock and the realities of our daily schedules that makes people sleepy.

In Thursday's online issue of the journal Current Biology, researchers in Germany analyzed sleep, height, weight, age and sex data submitted by 65,000 Europeans.

"Beyond sleep duration, social jet lag is associated with increased body mass index," a measure of overweight and obesity, Professor Till Roenneberg of Ludwig-Maximilians-Universität München in Munich and his co-authors concluded.

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Social jet lag means most people feel like they are working the early shift. (Frank Gunn/Canadian Press)

"It is thought the majority of the population is working the early shift. Here, we identify this discrepancy between biological and social timing as one of many factors contributing to the epidemic of overweight and obesity."

The researchers say that ideal sleep timing is incompatible with the work schedules that most people follow.

Those tired weekdays build up into a sleep debt that the data showed people often try to compensate for by sleeping in on free days.

Some of the participants showed different "chronotypes" — like being a morning lark or a night owl in when they go to bed or wake up.

People who wake up too early or stay up too late are more likely to smoke, be depressed, drink and eat more, which causes our metabolism to react in ways that are linked to weight gain, Roenneberg said.

Dr. Elliott Lee, a sleep specialist at the Royal Ottawa Health Care Group, called it a sophisticated research paper.

What's not clear is causation, Lee said.

Are obese people more likely to be shifted against the normal tendency of their internal clock or does the shifting lead to obesity as the authors suggest?

Amanda Bartlett, who manages a bakery in Toronto's Kensington Market, sometimes has to wake up at 1 a.m. or 4 a.m. for an eight-hour shift. She said before moving to a building with a gym, the schedule made it hard to fit in a work-out, which is another part of the problem.

"I would find myself not wanting to go to the gym because I'd be tired from work or I'd get home too late," Bartlett said.

The study was funded by the European Commission, the Netherlands Organization for Scientific Research, the University of Groningen Rosalind Franklin Program, and Hersenstichting Nederland.

With files from CBC's Aaron Saltzman