Working graveyard shift linked to cancer Soon the International Agency for Research on Cancer, the cancer arm of the World Health Organization, will add overnight shift work as a probable carcinogen. The agency is basing its decision on a number of previous studies that found women working overnight shifts more prone to breast cancer, men more prone to prostate cancer, and animals that have their light schedules reversed, more likely to develop tumors. While the higher cancer rates don't prove working overnight can cause cancer, scientists suspect overnight work disrupts the circadian rhythm, the body's biological clock and impedes the development of the hormone melatonin, which can suppress tumor development. An exact tie-in has not been confirmed and some who attribute the rise in cancer to other factors remain skeptical.