Humans Used to Sleep Twice Every Night. Here's Why It Vanished.

Continuous sleep is a modern habit, not an evolutionary constant, which helps explain why many of us still wake at 3 am and wonder if something's wrong. It might help to know that this is a deeply human experience.

For most of human history, a continuous eight-hour snooze was not the norm. Instead, people commonly slept in two shifts each night, often called a "first sleep" and "second sleep."

Each of these sleeps lasted several hours, separated by a gap of wakefulness for an hour or more in the middle of the night. Historical records from Europe, Africa, Asia, and beyond describe how, after nightfall, families would go to bed early, then wake around midnight for a while before returning to sleep until dawn.

Breaking the night into two parts probably changed how time felt. The qui