Ever since the world first formed, night has followed day. It is a certainty hardwired into living things. Plants will boost their scent or close up based on the time of day. Animals will hunt or mate, give birth or hatch, at a particular moment relative to dawn or dusk. “This biological clock, the circadian rhythm is ancient, shared and completely fundamental,” ecologist Johan Eklöf writes in The Darkness Manifesto.

It is also under threat. “Light pollution”—a term first coined by astronomers searching for ever more remote places to study the night sky—has come to be used by ecologists, physiologists and neurologists who study the effects of disappearing night. Eklöf gives the example of the dramatic decline in insect populations over the past 30 years, one of the biggest...