“How insupportable would be the days, if the night with its dews and darkness did not come to restore the drooping world.” Henry David Thoreau
The hours when the earth is dark are frequently thought of as a time when not much of significance happens. But as certain Indigo Buntings demonstrated in the 1960s, that isn’t really true. It was known that during migration these birds flew at night to reach their winter home in the Caribbean, but no one understood how they knew which way to go. Behavioral ecologist Stephen Emlen utilized a planetarium to determine that the birds used the night stars to navigate, much as ancient human mariners used to do.
For wildlife, many important activities are carried on at night, and those activities depend on the night being dark.
Light pollution, the excess of artificial light that washes out the night sky is a serious problem for wild animals of all types and sizes. That’s no surprise when you consider that 69 percent of mammals, 30 percent of all vertebrates, and 60 percent of invertebrates are nocturnal. Man-made nocturnal lighting can be a big problem for them.
Migrating birds, for example, are attracted to bright city lights and often collide with windows, as City Wildlife’s Lights Out DC program has documented. Insects are also attracted to artificial light, and about a third of the insects who are attracted to an artificial light will die before dawn of predation, collision, or exhaustion. Many bat species avoid lights, so that areas lit at night become dead zones for them where they cannot hunt. And many animals’ breeding behaviors are compromised by nights that are no longer dark.
The good news is that, as with many environmental problems, individuals can make changes to their lifestyle to mitigate the harm. External lights can be placed only where they are needed and they can be aimed at the ground (“shielding” can be added to further direct lights downward), not at the stars and the moon whose lights they obscure. Timers and motion sensors can be used to turn lights on when they are needed and keep them off when they are not.
Moreover, not all light is the same. We can and should use the lowest possible brightness and the warmest color available. Warm colors (amber and yellowish light) are less disruptive to humans and animals than cool colors (bright white and blue). To make our choice easier, we can check the label on light bulbs, whether incandescent or LED; they are marked to show the brightness (lumens) and appearance (light color as measured in Kelvins). Go for bulbs with the lowest lumens you can use (note that watts measures electricity used; lumens measure brightness) and ones that emit 2700 or fewer Kelvins. Or use yellow-colored bulbs outside for color.
To find other ways you can help keep the night dark, visit the website of DarkSky International, darksky.org. And check out Lights Out: Recovering Our Night Sky, an exhibit at the Smithsonian Museum of Natural History (open until December 2025).