What is “light pollution”? Is it really a factor in breast cancer?

— Gudrun Smythe, Madison, Wis.

The glow of city lights blotting out stars in the night sky has frustrated many a stargazer, but recent studies have shown that “light pollution” — defined as excess or obtrusive light at night — can actually have serious health effects. Researchers have found that exposure to bright nocturnal light can decrease the human body’s production of melatonin, a hormone secreted at night that regulates our sleep-wake cycles. Decreased melatonin production has in turn been linked to higher rates of breast cancer in women.

“Light at night is now clearly a risk factor for breast cancer,” says David Blask, a researcher at the Cooperstown, N.Y.-based Mary Imogene Bassett Research Institute. “Breast tumors are awake during the day, and melatonin puts them to sleep at night,” he adds.

Epidemiologist Richard Stevens of the U.S. Department of Energy’s Pacific Northwest National Laboratory first discovered the link between breast cancer and light pollution in the late 1980s. Stevens found that breast cancer rates were significantly higher in industrialized countries, where nighttime lighting is prevalent, than in developing regions.

Lending credence to Stevens’ research are the findings of another researcher, William Hrushesky of the South Carolina-based Dorn Veterans Affairs Medical Center, who discovered that female night shift workers have a 50 percent greater risk of developing breast cancer than other working women. He also found that blind women have high melatonin concentrations and unusually low rates of breast cancer.

To reduce breast cancer risks from light pollution, Prevention magazine recommends nine hours of sleep nightly in a dark room devoid of both interior (computer screens) and exterior (street lamps) light sources. A study of 12,000 Finnish women found that those who slept nine hours nightly had less than one-third the risk of developing a breast tumor than those who slept only seven or eight hours. Even bright light from a trip to the bathroom can have an effect, so dim nightlights are recommended for night lighting.

Light pollution causes other problems besides increased cancer risks. According to the Sierra Club, birds and animals can be confused by artificial lighting, leading them away from familiar foraging areas and disrupting their breeding cycles. And the photosynthetic cycles of deciduous trees (those that shed their leaves in the fall) have been shown to be disrupted due to the preponderance of artificial nighttime lights.

Another environmental impact of excessive use of artificial light is, of course, energy waste. The International Dark-Sky Association computes that unnecessary nighttime lighting wastes upward of $1.5 billion in electricity costs around the world each year while accounting for the release of more than 12 million tons of carbon dioxide, the leading greenhouse gas, into the atmosphere. Individuals can do their part by keeping lights dim or off at home at night — and convincing their employers and local government offices to do the same.