The consequences of global light pollution

 LEDs produce more blue and green light compared to older high-pressure sodium lights. Astronauts aboard the International Space Station can see the difference from space.

All of this artificial light has serious consequences for human health.

It disrupts internal biological clocks that evolved to a 24-hour night/day cycle, impairing sleep and the body's ability to produce hormones that are governed by daylight. The sleep hormone melatonin, for example, is produced during daylight and released when it's dark. This release triggers other hormones that help the body rejuvenate and cope with diseases.

Light pollution also disturbs wildlife and ecosystems. It can even be considered a form of habitat loss, as animals often avoid brightly lit areas.

It's an additional stressor and can be the direct cause of mortality.

Animal behaviors, life stages and predator/prey interactions are adapted to the 24-hour day/night cycle and the 12-month seasonal cycle, according to the report. Too much light at the wrong time confuses them and interferes with mating, reproduction and migration, among other ecological impacts.

Sea turtles, for example, use light as their guide, laying and burying their eggs on beaches at night. When too much artificial light is present, hatchlings, who are meant to find their way to the sea, become disoriented and move in the wrong direction. Lights also confuse insects, which gather around streetlamps and become easy prey. And birds that migrate at night frequently crash into communications towers or brightly lit windows.

Light pollution affects plants, too, disrupting key life cycle stages governed by the presence of light. That can have cascading effects on animals that depend on them.

Dark skies are also significant for cultural reasons.

Storytelling, religious traditions and calendars have depended on the night sky throughout human history. 

The Declaration in Defense of the Night Sky and the Right to Starlight, adopted at the Starlight Conference held in La Palma, Spain, in 2007, considers it a human right to see an unpolluted night sky. The conference included astronomers, conservation biologists and experts in tourism and cultural preservation.

Prior to the 17th century, human civilizations burned oil or fat for light and had no streetlights, so light pollution was nonexistent. With the rise of electrification in the 1870s, outdoor light spread at an accelerating pace.

Astronomers recognized the light pollution problem in the 1970s. Skyglow was impairing research, obstructing constellations and worsening the quality of telescope observations. A truly dark sky is still visible in remote research stations and some outdoor locations, but even those places are threatened.

While some light is necessary to maintain the safety and productivity of modern cities, increases in efficiency and reductions in cost have led to its misuse.

Perhaps surprisingly, there is also no conclusive evidence that brightly lit streets increase nighttime safety for drivers and pedestrians. Excessive glare blocks hazards on the road, distracts drivers and reduces night vision, which lowers overall awareness.

Light pollution may seem like a secondary issue compared to other climate change impacts, but it's easy to reduce. Simply dimming outdoor lighting or directing it only where necessary would make a big difference.

Other solutions include changing the color temperature of outdoor lightning. Light in the blue part of the spectrum—the kind often emitted by LEDs—is especially harmful. It scatters the most and causes the greatest disruptions to people and animals. Warm tones such as amber-colored light are the least harmful and would aid night vision for animals, including people. Timers and motion sensors that turn off lights when they're not needed would also help.

The World at Night: portals.iucn.org/library/sites … ments/PAG-033-En.pdf