If you’ve ever woken up at 3 a.m. and stared into the ceiling, wondering what’s wrong with you, the answer might be nothing. For most of human history, that interruption was built into the night. People slept in two shifts: a “first sleep” and a “second sleep,” with an hour or two of wakefulness between them.

Letters, diaries, and even ancient poetry describe this pattern. Families in pre-industrial Europe would sleep for several hours, wake around midnight, stoke the fire, check animals, pray, or talk. Some read by candlelight. Some had sex. Then, when the body felt ready, they’d drift back into the “second sleep” until dawn. It wasn’t a sleep disorder. It was normal.

According to historians and sleep scientists, continuous eight-hour sleep is a modern creation. Research shows the shift

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