The distinction between day and night is disappearing in the most heavily populated regions of the Earth, a rapid shift with profound consequences for human health and the environment, according to a paper published Wednesday in the journal Science Advances.
"We're losing more and more of the night on a planetary scale," journal editor Kip Hodges said in a teleconference on the paper's findings.
From 2012 to 2016, the artificially lit area of the Earth's surface grew by 2.2 percent per year, according to the study led by Christopher Kyba of the German Research Centre for Geosciences. Kyba and his team analyzed high-resolution satellite imagery to measure the extent of artificial outdoor lighting at night. The study also found that areas of the planet already lit grew even brighter, increasing in luminosity at a rate of 2.2 percent per year.
"Earth's night is getting brighter," Kyba said. One of Kyba's images show the change in the amount of nighttime lighting from 2012 to 2016.
Much of the increase is concentrated in the Middle East and Asia. The observed "decrease" in western Australia is actually due to wildfires in 2012 that were visible from space.
These observations probably understate the true increase in lit areas and light intensity because the satellites used in the study are not sensitive to blue light wavelengths emitted by LED lights.
The trend shows no sign of relenting.
"In the near term, it appears that artificial light emission into the environment will continue to increase, further eroding Earth's remaining land area that experiences natural day-night light cycles," the paper concludes.
The past few years have seen the rapid adoption of highly efficient LED lights for indoor and outdoor use. LEDs use just a fraction of the electricity of traditional incandescent lights - a 20-watt LED bulb can generate the same amount of light as a 100-watt incandescent, representing an energy savings of 80 percent. Beyond that, LEDs also last 10 to 20 times as long as incandescents, representing more cost savings.
But the rapid increase in nighttime lighting observed by Kyba and his colleagues suggests that people are responding to cheaper lighting options by simply adding more light.
"While we know that LEDs save energy in specific projects," Kyba said at the teleconference, "when we look at our data and we look at the national and the global level, it indicates that these savings are being offset by either new or brighter lights in other places."
The shift from incandescents to LEDs has been directly observable from space.
People are particularly attuned to the short-wavelength blue light emitted by most LEDs, but it's been implicated in sleep deficiencies and other human health problems. Last year, the American Medical Association issued a warning about health risks associated with this type of light.
Bright nighttime lighting only started becoming widespread about 100 years ago, meaning we have little idea how humans or other species adapt to it at an evolutionary level. "Artificial light at night is a very new stressor," said Franz Holker, one of the paper's authors. "The problem is that light has been introduced in places, times and intensities at which it does not naturally occur and [for] many organisms, there is no chance to adapt to this new stressor."
The news isn't all bad. Studies have shown, for instance, that judicious use of low-level LED lighting can reduce light pollution without compromising peoples' sense of safety. Lighting companies have been introducing "warm" LED lights that emit much less of the potentially harmful short-wavelength blue light.
"In the longer term, perhaps the demand for dark skies and unlit bedrooms will begin to outweigh the demand for light in wealthy countries," Kyba and his colleagues write.