America's obesity epidemic has been blamed on everything from excess television viewing to McDonald's Big Macs. Now researchers from around the country have found a new culprit that may be programmed into our genes.

What the researchers wanted to know: Do the genes regulating circadian rhythms (the sleep-wake cycle) affect weight gain?

What they did: These researchers looked at weight gain in two types of mice—a normal strain and a genetically altered strain, which had a mutation in a gene, called clock, that regulates the body's sleeping and waking cycles. They put a group of both normal and genetically altered mice on a normal diet and another group of both types of mice on a high-fat diet for 10 weeks and then measured changes in weight.

What they found: The genetically altered mice fed a normal diet increased in weight by 29 percent over the 10-week period, compared with a 24 percent increase for the normal mice (which was expected as a part of their typical development). For the mice fed a high-fat diet, the differences were even larger. Over the 10 weeks, the weight of genetically altered mice increased by 49 percent, versus a 38 percent increase for the normal mice. Plus, by the end of the study, all of the genetically altered mice had some symptoms of metabolic syndrome, the set of symptoms often linked to obesity and diabetes. The researchers say that they are not sure how the clock gene affects metabolism, although one theory is that the mutation makes the mice less likely to eat at the same time each day, which could have caused their bodies to metabolize food less effectively.

What it means to you: This research could lay the groundwork for further studies that might show a link between mutations in the clock gene and obesity in humans. Previous studies on humans have shown a link between getting off on your sleeping schedule—such as when working a night shift—and weight gain.

Caveats: This study was done on mice, not people. Mice behave very differently from humans and sometimes gain weight for different reasons than we do. So, while this study could be a jumping-off point for further research, it is by no means conclusive for people.

Find out more: While no human studies have been done linking mutations in the clock gene to obesity, some studies have linked it to lack of sleep. Read about one of those studies from the North American Association for the Study of Obesity.

If you want to see what human studies are being done to learn about obesity in humans, check out http://www.obesity.org from the American Obesity Association.

Read the article: Turek, F.W. et al. "Obesity and Metabolic Syndrome in Circadian Clock Mutant Mice." Science. Published online April 21 2005.

Abstract online: http://www.sciencemag.org