The night is not what it was. Once, the Earth was cast perpetually half in shadow. Man and beast slept beneath inky skies, dotted with glittering stars. Then came fire, the candle, and the light bulb, gradually drawing back the curtain of darkness and giving us unprecedented control over our lives.
But a brighter world, it is becoming increasingly clear, has its drawbacks. A study released last month finding that breast cancer is nearly twice as common in brightly lit communities as in dark ones only added to a growing body of evidence that artificial light threatens not just stargazing but also public health, wildlife, and possibly even safety.
Those findings are all the more troubling considering that an estimated 30 percent of outdoor lighting—plus even some indoor lighting—is wasted. Ill-conceived, ineffective, and inefficient lighting costs the nation about $10.4 billion a year, according to Bob Gent of the International Dark-Sky Association, a nonprofit that aims to curtail light pollution, and it generates 38 million tons of carbon dioxide a year.
Motivated by such trends, more than two dozen cities worldwide will go dim on March 29 in an hourlong demonstration. At 8 p.m. local time, Atlanta's and Chicago's tallest towers, the Phoenix Suns' arena, and San Francisco's Golden Gate Bridge will join many other sites in turning off their lights. According to the World Wildlife Fund, which is organizing the event, an estimated 2.2 million Australians switched off their lights or took other action during "Earth Hour" last year in Sydney, briefly reducing that city's energy use by more than 10 percent.
A number of groups are trying to measure light pollution and assess its detrimental effects on the environment in the hope that people will reduce their own contribution to the problem. Last week, as part of an annual program called GLOBE at Night, thousands of students and amateur scientists stared up at the constellation Orion from locations across the country and reported how many of its stars they could see. No data are yet available, but in dark, rural areas, says Gent, about 2,000 stars are typically visible at night, compared with "maybe five" in a bright city square—and about 5,000 in centuries past. "One of the goals," says Steve Pompea of the National Optical Astronomy Observatory in Tucson, Ariz., "is to identify urban oases—places in our cities that are dark enough to see the sky."
People who are working while others are stargazing may face the greatest risks. Hormonal disturbances triggered by nighttime exposure to white or bluish light can disrupt circadian rhythms and fuel the growth of tumors, experiments show. Two decades of research indicate that women who work night shifts have unusually high rates of breast cancer, and some data suggest a parallel effect on male workers' prostate cancer rates. Last December, a unit of the World Health Organization deemed shift work a probable human carcinogen.
Yet light and cancer may be even more fundamentally linked. In last month's study, a team that included Richard Stevens, the University of Connecticut Health Center epidemiologist who first proposed the connection, compared satellite images of Israel at night with maps showing where cancers are most common. Its analysis suggests that 73 percent more breast cancers occur in the country's brightest communities than in its darkest.
Beaming up. Light beamed into the sky is squandered, since it's not illuminating any target. Yet many fixtures—like old-fashioned spherical streetlamps—send plenty of photons upward and outward. "If you fly into a city at night and you can see the streetlights from the airplane," says Chad Moore, leader of the National Park Service's Night Sky Program, "that light is counterproductive." As the light bounces off particles in the air, it casts a far-reaching "sky glow," he says. "We have documented light from distant cities traveling roughly 200 miles into national parks."
And while lighting is often installed in the name of safety, says Gent, it may ironically benefit criminals. A pedestrian temporarily blinded by the glow of an ATM, for instance, may be an easier target for a mugger hiding in the shadows. In fact, most light that goes directly from its source to a person's eye is worse than worthless. Such glare—from a car's high beams, a poorly aimed porch light, or even an unshielded window—inhibits night vision, paradoxically making it harder to see. That can endanger drivers, not to mention hapless deer.