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While the scientific study of sleep only emerged in the 19th century, historical records, diaries, and literature have long hinted at how people rested before electric light. History professor Roger Ekirch examined these non‑scientific sources and concluded that humans once followed a biphasic pattern: a few hours of sleep after sunset, a wakeful interlude of about an hour in the middle of the night, and a final sleep period until dawn. According to Ekirch, the widespread adoption of artificial light disrupted this natural rhythm.

Ekirch’s argument, published in the American Historical Review, challenges the modern notion that consolidated, uninterrupted sleep is biologically natural. While the humanities provide rich contextual evidence, they cannot alone define what “natural” sleep truly is. Modern scientific research offers a more nuanced picture.

Evidence Supporting Biphasic Sleep

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Most of Ekirch’s evidence derives from literary and epistolary accounts—records that do not meet contemporary experimental standards. Nonetheless, several empirical studies corroborate his observations:

• A 1992 experiment altered the photoperiod of seven participants from 16 to 10 hours of light over four weeks. All subjects adopted a polyphasic pattern, with the majority exhibiting biphasic sleep.
• A 2017 field study of 21 rural inhabitants in Madagascar—living without electricity—revealed a default biphasic sleep pattern.
• A 2015 Journal of Biological Rhythms paper compared two closely related indigenous groups in Argentina, one with electric lighting and one without. The group lacking electricity tended to sleep earlier and accumulated more total sleep due to the absence of artificial light.

Arguments Against a Universal Biphasic Pattern

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Despite these findings, the evidence is not unequivocal. The same 1992 study placed participants in a dark environment for 14 hours, a condition that may have artificially induced biphasic sleep. The Madagascar research also reported lower total sleep time and reduced sleep quality compared to electrified peers, suggesting that environmental factors, rather than biology, drove the pattern.

Additional research challenges the notion that biphasic sleep is the default. A 2015 study examined three pre‑industrial societies across Africa and South America, all of which displayed monophasic sleep. Evolutionary biologists note that monophasic sleep is typical among higher primates, while biphasic patterns are more common in species like elephants.

A 2016 review on human sleep evolution argued that ecological factors—predation risk, food acquisition, and social interaction—are the primary determinants of sleep architecture, not an inherent biological timetable. In other words, human sleep may be adaptable rather than fixed to either biphasic or monophasic patterns.

Ekirch’s analysis focuses largely on the U.K. and Western Europe—regions north of 40° latitude. Here, winter nights can last 15 hours or more, making a biphasic schedule a practical adaptation to variable daylight. While this explains the historical prevalence, it does not prove that such a pattern is “natural” for all humans.