Did people once sleep in two shifts every night?

Possibly, and the evidence comes from two very different places. A historian has argued that before the Industrial Revolution, interrupted sleep was dominant in Western civilization: people went to bed early for a "first sleep", woke around midnight for about an hour, then returned to bed for a "second sleep". Segmented sleep is the name for sleeping in more than one chunk over 24 hours.

The historian is A. Roger Ekirch. He draws on more than 500 references to a segmented pattern in documents from the ancient, medieval and modern world, and other historians such as Craig Koslofsky have endorsed his analysis. He says the waking interval, called "the watch", was used to pray, reflect and interpret dreams, and was a favourite time for scholars and poets to write. Ekirch suggests that electric lighting is why most of us no longer do it.

The laboratory gives a partial echo. In 1992, Thomas Wehr kept seven healthy men in fourteen hours of darkness a day for a month. After about eleven hours of sleep at first, presumably to make up for sleep debt, they began sleeping for about four hours, waking for two to three, and sleeping for another four.

But the question is not closed. A 2015 study of three non-industrial equatorial societies found monophasic nighttime sleep in all three, though Ekirch questioned the authors' conclusions. Other researchers have written that whether lab results reflect the human sleep-wake system or just static experimental conditions "is open to debate".