December 19, 2005 -- Results from a new study in laboratory mice show that nighttime exposure to artificial light stimulated the growth of human breast tumors by suppressing the levels of a key hormone called melatonin. The study also showed that extended periods of nighttime darkness greatly slowed the growth of these tumors.

The study results might explain why female night shift workers have a higher rate of breast cancer. It also offers a promising new explanation for the epidemic rise in breast cancer incidence in industrialized countries like the United States.

The National Cancer Institute and the National Institute of Environmental Health Sciences, agencies of the federal National Institutes of Health, provided funding to researchers at the Bassett Research Institute of the Mary Imogene Bassett Hospital in Cooperstown, New York and The Thomas Jefferson University in Philadelphia, Pa. The results are published in the December 1, 2005 issue of the scientific journal Cancer Research.
"This is the first experimental evidence that artificial light plays an integral role in the growth of human breast cancer," said NIEHS Director David A. Schwartz, M.D. "This finding will enable scientists to develop new strategies for evaluating the effects of light and other environmental factors on cancer growth."

"The risk of developing breast cancer is about five times higher in industrialized nations than it is in underdeveloped countries," said Les Reinlib, Ph.D., a program administrator with the NIEHS' grants division. "These results suggest that the increasing nighttime use of electric lighting, both at home and in the workplace, may be a significant factor."

Previous research showed that artificial light suppresses the brain's production of melatonin, a hormone that helps to regulate a person's sleeping and waking cycles. The new study shows that melatonin also plays a key role in the development of cancerous tumors.

"We know that many tumors are largely dependent on a nutrient called linoleic acid, an essential fatty acid, in order to grow," said David Blask, M.D., Ph.D., a neuroendocrinologist with the Bassett Research Institute and lead author on the study. "Melatonin interferes with the tumor's ability to use linoleic acid as a growth signal, which causes tumor metabolism and growth activity to shut down."

To test this hypothesis, the researchers injected human breast cancer cells into laboratory mice. Once these cells developed into cancerous tumors, the tumors were implanted into female rats where they could continue to grow and develop.

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