Mice that ate a high-fat diet gained weight and experienced a disruption in their circadian clocks, which regulate metabolic functions such as when they go to sleep, wake up and become hungry.

The disruption threw off the timing of the animals’ internal signals, including appetite control. As a result, the mice ate extra calories during the time when they would have otherwise been asleep or resting. For humans, this would be the equivalent of raiding the refrigerator in the middle of the night.

The high-fat diet and resulting weight gain also triggered diminished expression of genes that encode the clock in the brain and in peripheral tissues.

The findings suggest that changes in metabolic state that occur with obesity and diabetes affect not only circadian rhythms of behavior but also physiology.

Past studies have found that a misaligned body clock can throw off your metabolism, and increase your risk of obesity and diabetes.

This represents a “vicious loop,” according to researchers, because once weight is gained, your internal clock is disrupted, and a disrupted clock makes the original problem worse.

"Timing and metabolism evolved together and are almost a conjoined system," said one of the study’s authors Joe Bass, M.D., assistant professor of medicine and neurobiology and physiology at Northwestern and head of the division of endocrinology and metabolism at ENH. "If we perturb the delicate balance between the two, we see deleterious effects."

• Science Daily November 7, 2007