For much of his life, the Swedish conservationist Johan Eklöf has studied bats, work often best done when both the scientist and his subjects are in the dark. But as more artificial light spreads into the nocturnal landscape, true darkness is getting harder to come by. In “The Darkness Manifesto,” Mr. Eklöf argues that light pollution, which he simply defines as “unnecessary artificial light,” also has consequences for plants and many animals, including humans. “While artificial lighting today makes up just a tenth of our combined energy usage, only an extremely small part of that light is of actual benefit to us,” he writes. “Most of it spills out into the sky instead of lighting walkways and outer doors as intended.”

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