In the 1980s, Ekirch was researching a book about nighttime before the industrial revolution. One day in London, wading through public records, he stumbled on references to “first sleep” and “second sleep” in a crime report from the 1600s. He had never seen the phrases before. When he broadened his search, he found mentions of first sleep in Italian (primo sonno), French (premier sommeil), and even Latin (primo somno); he found documentation in Africa, the Middle East, South Asia, and Latin America.

When sleep was divided into a two-act play, people were creative with how they spent the intermission. They didn’t have anxious conversations with imaginary doctors; they actually did something. During this dorveille, or “wake-sleep,” people got up to pee, hung out by the fire, had sex, or prayed. They reflected on their dreams and commingled with the spiritual realm, both the divine and the diabolical. In the 1550s, Martin Luther wrote of his strategies to ward off the devil: “Almost every night when I wake up … I instantly chase him away with a fart.”

Today’s sleep writers often wield Ekirch’s research to suggest that segmented sleep (or, as Ekirch calls it, biphasic—two-phase—sleep) is old, and one-sleep is new, and therefore today’s sleepers are doing it wrong. But that’s not the full story, he told me.