Women who work outside the normal work hours of 9 to 5 have an increased risk of breast cancer, according to a study published in the journal Biology.

For years, studies have shown that people who work shifts or experience frequent jet lag are at an increased risk for numerous health problems, including weight gain, diabetes, and cancer. But a multi-decade controlled study in humans to confirm whether the disruption of sleep patterns actually caused the health problems wasn't possible.

The new study is the next best thing. Dutch researchers subjected mice to disturbances in their circadian rhythm, which is our body's natural 24-hour biological clock that regulates sleeping and waking. They found that alternating light-dark cycles on a weekly basis accelerates the development of particular health problems.

In one study, female mice with a genetic predisposition to breast cancer, similar to humans who carry a BRCA mutation, were studied. These mice usually develop breast cancer at about 50 weeks, but they developed the disease eight weeks earlier when subjected to circadian rhythm disturbances. In addition, they also gained 20 percent more weight than mice who kept normal schedules.

"The conclusion is that chronic changes in light schedules are a driving factor for breast cancer development, weight gain, and other metabolic problems," said senior study author Bert van der Horst of Erasmus University Medical Center. Although other factors may be involved, such as low amounts of vitamin D or melatonin, they are only secondary factors with out-of-sync body clocks being the driving force.

"One of the questions we often get is to what extent does a mouse resemble humans," said van der Horst. "The mechanism of the circadian clock as well as carcinogenesis is comparable in mice and man, and this model opens new avenues for further exploring how circadian disruptions affect the body and how you can intervene to reduce adverse effects."