Researchers are discovering an increasing number of factors contributing to cancer development, some of them not even being suspected of having any connection with the disease.

A study on mice carried out by US researchers from several universities showed that nighttime exposure to artificial light stimulates breast cancer tumor development by suppressing the brain's production of a hormone called melatonin. In addition, they've found that long exposures to natural darkness significantly reduce the tumor's development.

"This is the first experimental evidence that artificial light plays an integral role in the growth of human breast cancer," David A. Schwartz, the director of one of the agencies who funded the researches said.

"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," said Les Reinlib, Ph.D.

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.

For this study, a series of mice were injected with human breast cancer cells. Once these cells developed into cancerous tumors, the tumors were implanted into female rats where they could continue to grow and develop.

Meanwhile, the researchers then took blood samples from 12 healthy volunteers. The samples were collected under three different conditions, during the daytime, during the nighttime following 2 hours of complete darkness, and during the nighttime following 90 minutes of exposure to bright fluorescent light. These blood samples were then pumped directly through the developing tumors.

"The melatonin-rich blood collected from subjects while in total darkness severely slowed the growth of the tumors."These results are due to a direct effect of the melatonin on the cancer cells," one of the researchers said.

In contrast, tests with the melatonin-depleted blood from light-exposed subjects stimulated tumor growth.

If the results are confirmed, the way in which light sources is likely to be radically changed so that melatonin production is no longer suppressed.