If you do shift work, and frequently work at night when the rest of the world is sleeping, you may be putting yourself at a higher risk for breast cancer. The World Health Organization has found enough cause to list ultraviolet light during sleep as a "probable carcinogen," putting it in the same category as toxic chemicals such as PCB's (polychlorinated biphenyls).

Scientists have known for years that a rat that sleeps in the dark has a much lower risk of cancer than a rat that sleeps with the light on, and new studies are showing similar results in women. Women who habitually work night shifts, such as nurses and flight attendants, have a 60% higher rate of breast cancer even when diet and lifestyles factors were taken into account.

The key seems to be melatonin, a hormone secreted by the pineal gland located in the brain. Melatonin regulates sleep patterns and also inhibits the growth and development of malignant tumors. When the pineal gland produces less melatonin, tumors utilize linolenic acid more efficiently and can grow more rapidly. Although more research is required, it appears that those who regularly work the night shift are at a higher risk because their bodies are prevented from producing the required amounts of melatonin necessary to inhibit tumor growth and development.

If the link can be confirmed, our use of artificial lighting in the United States and other industrialized countries may be significantly affected. Dr. David Blask, a neuroendocrinologist with the Bassett Research Institute, suggests the possibility of altering the wavelength and intensity of light so as not to disrupt melatonin production in workers who work at night.