Dimming the lights in the city that never sleeps

Every year on Sept. 11, two columns of bright blue light shine from lower Manhattan into the night sky. Until recently, this memorial massively disrupted bird migration, influencing the behavior of over 150,000 birds each year. The installation was “one of the most egregious examples” of how urban nighttime lights endanger animals, says George Wittemyer, who studies human impacts on wildlife at Colorado State University.

Finding a solution was no easy task; there’s now a program to mitigate the impacts of the lights, but it requires immense manpower and planning. Through all types of weather, employees and volunteers at the NYC Bird Alliance monitor the lights with binoculars from the ground and, every 20 minutes, manually count the number of birds flying in the beams. If they count more than 1,000 birds, the lights get turned off for a short period. When the lights are turned back on, the process starts again, and so it goes from dusk until dawn. 

The Tribute in Light only shines in a small area once a year, but it’s a clear example of the effort needed to counteract light pollution in New York City. And as light at night gets worse, the city has a vested interest in figuring out how to dull its effects — not only because of the impact on birds, but also because too much light at night isn’t healthy for humans either. 

There are no simple or universal solutions, but the circumstances are pressing. Light pollution is increasing by 2% per year globally, enough to obstruct animals’ dependence on natural light cycles. Artificial light at night is not always a bad thing, says Carrie Ann Adams, a Colorado State University researcher who studies how artificial light affects birds. “It’s allowed us to navigate the dark safely and extend our social and working hours,” she explains. “We just have so much more of it than we actually need.”

This extra light has devastating impacts on migrating birds. Most apparent is how it alters their migratory patterns. Birds tend to migrate at night because the air is calmer, so “in the spring and fall, when you’re sleeping, there’s probably birds flying overhead,” Adams says. When birds encounter intense light, they may alter or abandon their flight paths, or collide with buildings.

Excess light also changes how migrating birds interact with other animals. Well-lit areas like cities make great hunting grounds for bird predators, for example. One study found that peregrine falcons around the Empire State Building are able to take advantage of the influx of their prey — migratory birds — around bright skyscrapers. The light causes disorientation, which in turn may make them “more vulnerable to predation by falcons,” the researchers wrote. 

Attraction to cities can also harm migratory birds that eat rodents. “As you saw with Flaco in New York City, rodenticides can be a big problem as well,” and they’re more likely to be present in densely populated areas, Adams explains. 

Birds aren’t the only animals affected by light pollution. Nighttime light suppresses melatonin in birds, fish, amphibians, reptiles and mammals — including humans. Melatonin controls sleep rhythms, immune responsiveness and more in these animals, so superfluous light is a health risk. Studies have even linked melatonin suppression to increased cancer risk in humans.

There are other impacts on people, too, including higher taxes to pay for excess lighting from streetlights, schools and other public spaces, says Christopher Kyba, an artificial light researcher at the GFZ Helmholtz Centre for Geosciences in Germany. And if the light pollution comes from advertising or other commercial sources, customers foot the bill through higher prices, he says. 

There’s no shortage of reasons to limit light at night, but regulating it remains a challenge since public awareness of the issue is still low. “People tend not to think about light very much,” Kyba says. In cities across the country — even the city that never sleeps — most people go to bed by midnight, so they don’t see whether lights stay on or turn off at night, according to Kyba.

The negative effects of light at night are not visible to most people, adds Adams, so most have no reason to advocate for change. Even with deaths from collisions possibly topping one billion birds per year in the US, most bodies go unnoticed. “Collision victims can be removed by scavenging predators or city streetsweepers, fly away before succumbing to their injuries or decompose in dense vegetation without ever being noticed,” Adams explained.

Another major roadblock is the upfront cost of retrofitting existing buildings with devices that can decrease light pollution, like light shields and timers, Colorado State’s Wittemyer says. Saman Mahmood, who leads advocacy at NY Bird Alliance, agrees. Her team finds that commercial groups are often willing to consider these changes once they learn about the benefits, but are stymied by costs. The largest DarkSky-compliant shields cost hundreds of dollars each, with installation costs of hundreds of dollars more per hour. For a large building, these numbers could add up to many thousands.

In addition to cost concerns from building managers, some groups worry that darkness is accompanied by crime. This anxiety isn’t totally misguided, as the National Bureau of Economic Research found that extra lighting at night around public housing projects can reduce the number of serious crimes. 

But light pollution experts push back against the idea that curbing light-at-night would necessarily increase crime. Kyba questions whether the NBER study is sound because the lighting used in the experiment was concentrated and extremely bright — at least six times brighter than an ordinary streetlight in a pedestrian area — and the generators used were very loud. “People like the idea that light can prevent crime, but the unfortunate reality is that crime doesn’t stop in the daytime, and the sun is far, far brighter than any streetlights,” he says. 

Wittemyer notes that there are many ways to decrease excess lighting while keeping safety top of mind. The typical retrofits, like shields and timers, protect wildlife while ensuring that human activity is still well lit.

Despite these obstacles, progress is possible — some cities have implemented successful policies to manage light at night. Flagstaff, Arizona, is often cited as the model for government response. There, local regulations require that outside light is shielded, limited to certain wavelengths and not excessively bright. And these regulations seem to work; Flagstaff emits almost 14 times less light than Cheyenne, Wyoming, a city comparable in size. 

Because this oversight is so effective, Flagstaff was the first ever designated International Dark Sky Place in 2001. The city’s policies have “been very successful at allowing astronomy to continue to be an important part of the city, even as it’s grown,” Adams says, noting that Flagstaff is home to the famed Lowell Observatory, where Pluto was first observed in 1930.

While few American cities have followed Flagstaff’s lead so far, many European countries have imposed strict rules on night lighting. French businesses are required to turn off illuminated window displays and advertising overnight, for example, and local municipalities in Austria temporarily ensured that less than 50% of street lighting is on at any given time overnight.

But New York City is a different beast. The city is often on lists of the brightest cities in the world (depending on which measurement is used) and is known globally for its illuminated nighttime skyline. It’s still worth trying to decrease light at night there, the NY Bird Alliance’s Mahmood says. “Progress happens one step at a time,” she says. “Every bird that we can save matters.”

Some regulations to limit hours of outdoor and excessive lighting were passed in 2022 by the City Council, but they only apply to city-owned or -leased buildings, so advocates are pushing the city government to take bigger steps. “We’re not trying to turn off Times Square, but we can turn off an [unoccupied] building,” Mahmood says.

Researchers have many suggestions for what may be effective in New York on a larger scale. The most obvious approaches rely on restricting unnecessary light, similar to most European laws. The city could require that bright billboards be turned off overnight when traffic is low, Germany’s Kyba says. As is, “it’s not doing the advertiser any good because there’s no one to see it.”

Shielding is another way to ensure that light only shines where intended, so Wittemyer would like to see shielding requirements. “If you’re actually just targeting the place you want and you’re not wasting this light, it’d be less expensive for you,” he says, noting the energy savings that come with these types of policies. Colorado State’s Adams agrees, adding that she would limit the amount of light that shines towards the sky.

Adams also thinks laws protecting parks and other green spaces from light pollution would make meaningful change for migrating birds. Green spaces provide the few spots where birds can rest and eat without dealing with the impacts of bright lights while passing through New York. “Ensuring that light from the surrounding buildings and from the surrounding streetlights isn’t shining into those parks and green spaces is really important,” she says. Even with concerns about crime, shielding can keep light focused in human paths and away from resting birds in places like Central Park.

Kyba suggests a second, more unconventional way of regulating light at night. He thinks cities like New York could benefit from only allowing facades along the iconic skyline to be lit up on certain days, which would both decrease light pollution and attract tourists. He used to live in Potsdam, Germany, where public buildings are only lit a few days a year, becoming more of a spectacle, he says. “If you were to say, for example, ‘We’re going to have a lit skyline, but it’s only going to be on Fridays and Saturdays,’” he says, “then it becomes an event.” 

The bottom line for everyone involved is the same, though: the darker the sky is, the better birds and humans will fare, so local legislation is critical — especially in a place like New York City. “What happens here is essential,” Mahmood says.