Do you sometimes wake up in the middle of the night? Long ago, this may have been normal. A historian found many old diaries and letters from Europe that talk about a "first sleep" and a "second sleep."

People went to bed early, woke up for one or two hours, and then slept again. They prayed or talked with neighbors. But today, some groups without electric light do not sleep this way.

Waking up in the middle of the night may not always be a problem. In 2001, an American historian reported that old diaries, letters and court records from pre-industrial Britain often mentioned a "first sleep" and a "second sleep." People seem to have gone to bed early, stayed awake for an hour or two around midnight, and then slept again until morning. He believes this habit was lost after artificial light spread.

A 1992 experiment supported the idea. When participants spent weeks with 14 hours of darkness each night, many began sleeping in two parts. However, in 2015, researchers measured the sleep of three groups who live without electricity in Africa and South America, and found that this kind of sleep was not common.

Waking in the small hours is often seen as a symptom of insomnia, but historical research suggests it may once have been routine. In 2001, the historian A. Roger Ekirch published an analysis of diaries, letters, court records, medical texts and literature from pre-industrial Britain, in which references to a "first sleep" and a "second sleep" were common. People apparently retired early, spent an hour or two awake around midnight praying, reflecting on dreams or visiting neighbors, and then slept until dawn. Ekirch argued that the spread of artificial lighting erased this pattern. A 1992 experiment appeared to support him: when volunteers were exposed to 14 hours of darkness nightly for several weeks, their sleep often split into two blocks separated by one to three hours of quiet wakefulness.

But a 2015 study of three societies living without electricity in Tanzania, Namibia and Bolivia found that segmented sleep was uncommon. Even among historians the issue is contested; a 2023 reassessment questioned how widespread the practice really was, prompting a reply from Ekirch in 2024. Claims that segmented sleep is humanity's natural pattern therefore go beyond the evidence.