If you're pulling an all-nighter, you're not doing it in the dark, and several studies have found that women exposed to artificial light at night (particularly night shift workers) develop breast cancer at a higher rate than other women. The research is so convincing, the World Health Organization has labeled night shift work a probable carcinogen. Researchers believe the phenomenon is linked to melatonin, the hormone that controls your sleep and wake cycles. Melatonin levels decrease when light is present, and melatonin is also partly responsible for suppressing estrogen. Scientists say less melatonin means more estrogen, which increases breast cell production. More breast cells mean more possibilities for abnormal (breast cancer-causing) cells.