Results of new research in laboratory mice injected with human cancer cells show that nighttime exposure to artificial light stimulated the growth of human breast tumors.
The same federally funded study also showed that extended periods of nighttime darkness dramatically slowed the growth of these tumors. Apparently, artificial light depletes levels of the hormone melatonin.
Authors of the study, published in the Dec. 1 issue of the journal Cancer Research, say the findings might explain why female night-shift workers have a higher rate of breast cancer and why incidence of breast cancer is much greater in industrialized nations than in Third World countries.
“This is the first experimental evidence that artificial light plays an integral role in the growth of human breast cancer,” said Dr. David A. Schwartz, director of the National Institute of Environmental Health Sciences.
NIEHS and the National Cancer Institute, both agencies of the Bethesda-based National Institutes of Health, funded the research. The experimentation was conducted by investigators with the Bassett Research Institute of the Mary Imogene Bassett Hospital in Cooperstown, N.Y., and with Thomas Jefferson University in Philadelphia.
Said Lee Reinlib, a program administrator with the NIEHS grants division: “The risk of developing breast cancer is about five times higher in industrialized nations than it is in underdeveloped countries. These results suggest that the increasing nighttime use of electric lighting, both at home and in the workplace, may be a significant factor.”
In a press release, NIEHS noted that previous research has found that artificial light suppresses the production of melatonin, a hormone that helps regulate a person’s sleeping and waking cycles.
The federal institute says this new study “shows that melatonin also plays a key role in [preventing] the development of cancerous tumors.”
Dr. David Blask , a neuroendocrinologist with the Bassett Research Institute and lead author of the study, explained that “many tumors are largely dependent on a nutrient called linoleic acid … in order to grow.”
Melatonin, he said, “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 lab mice. After the cells became cancerous tumors, the tumors were transplanted into adult female rats, where they were allowed to grow.
The researchers then took blood samples from 12 healthy premenopausal women, then pumped the blood directly into the tumors. Some of the samples were collected during the daytime, some during the night after two hours of complete darkness and some during the night after 90 minutes of exposure to bright fluorescent light.
“The melatonin-rich blood collected from subjects in total darkness severely slowed the tumors. These results are due to a direct effect of the melatonin on the cancer cells,” Dr. Blask said.
In contrast, he said, tests with the melatonin-depleted blood from light-exposed women triggered tumor growth.
“The effects we are seeing are of greatest concern to people who routinely stay in a lighted environment during times when they would prefer to be sleeping,” said Mark Rollag, a visiting research scientist at the University of Virginia and another of the study’s authors.