Working shifts may affect more than your social life: it could increase your risk of cancer. The BBC reports a new study found severe health consequences to an irregular life and sleep schedule.

Every human body has its own circadian rhythm to regulate sleep based in large part on light exposure. In the study, mice bred to be prone to breast cancer had their sleep clock delayed 12 hours once a week for a year. The mice with regular sleep disruption developed breast cancer tumors a full 8 weeks before their non-disrupted comrades.

The delayed mice also weighed an average of 20% more than their counterparts, despite eating the same amount of food.

The study suggests that irregular sleep patterns translated to an expected 2 lb weight gain and cancer development 5 years earlier for women at risk for breast cancer.

As always, the scientists were reluctant to clearly state that the lack of sleep was causing the cancer. The report suggests other related factors in people who work shifts may be to blame, such as social class, decreased activity levels, and decreased vitamin D. Still they urged caution for people to respect their body’s circadian rhythm.

Women at risk fro breast cancer were advised to never work shifts, researchers concluded, and recommended further testing in humans to clarify the results.