Studies show that women who worked night shifts face a significantly higher risk of breast cancer -- up to 60 percent higher, according to one study published by the National Cancer Institute. The thinking is that nighttime work, which entails exposure to light at night, interferes with the body's production of melatonin, the sleep-inducing hormone. Sleep deprivation and melatonin interruption in turn stimulate increased estrogen production, researchers believe.

Additional studies have linked low melatonin levels in general with breast cancer. The connection is strong enough that researchers are exploring whether taking supplemental melatonin can protect against breast cancer.