Nashville is never truly dark. When the sun sets, an artificial glow rises, blocking out most of the celestial wonders above and disrupting the lives of organisms below. 

This phenomenon is called light pollution, and the city is about to confront one of its worst culprits. 

Nashville is planning to replace 55,000 street lamps with light-emitting diodes in the next five years. The new, LED lightbulbs will have smart technology, allowing the city to control brightness, remotely turn them on or off and identify streetlight outages. 

“We have much greater control over the light network itself,” said Kendra Abkowitz, Nashville’s sustainability chief. 

Courtesy Metropolitan Government of Nashville

LED lights do emit heat, just in a different way than other lightbulbs.

As part of the project, which was proposed this week and still needs council approval, the city will also replace the fixtures surrounding bulbs to ensure that light is directed downwards – instead of bleeding in all directions. These fixtures are required in some states, but Tennessee does not have any laws to reduce light pollution. 

LEDs last longer, emit less heat and use less energy than incandescent alternatives, which the U.S. Department of Energy effectively began phasing out last year.  

“This lighting conversion will save energy, reduce carbon emissions, and provide the technology to reduce light pollution,” Councilmember Burkley Allen, lead sponsor of the legislation proposing this project, said in a press release. 

Light pollution has three main factors. In addition to luminosity and direction, color is critical. 

Wave(lengths) of pollution

The color of the LEDs impacts how it affects the surrounding ecosystem. Blue wavelengths spread more easily, which is why the daytime sky is blue — sunlight contains all colors, but the atmosphere scatters blue wavelengths the most effectively. 

LEDs with more blue wavelengths can make light pollution worse. In the past decade, this has happened in cities around the world.

Nashville’s plan is to use “warm white” lightbulbs that are 3,000 Kelvin, the unit that reflects the color temperature, which Abkowitz said is compliant with the best industry standards to control light pollution.

About 99% of people living in the U.S. experience light pollution.

In 2016, the American Medical Association warned that blue-rich LEDs were “associated with reduced sleep times, dissatisfaction with sleep quality, excessive sleepiness, impaired daytime functioning, and obesity.”

Blue light radiation suppresses melatonin production in humans and animals, affecting both individual physiology and ecosystem function, and is causing “substantial biological impacts” across Earth, according to a study published in Science Advances last year. 

The lack of darkness could also affect mental health.

In a way, light pollution is a form of social isolation on a grander scale. 

“Loss of views of the natural night sky may have impacts on people’s sense of ‘nature’ and of their place in the universe,” the study authors said. 

Natural skies are lit by the Moon, natural atmospheric emissions, the stars and the galaxy. 

In the U.S., 99% of people live under light-polluted skies. The vast majority, including most of the population in Tennessee, cannot see the Milky Way.