After waking up in the middle of the night every day for a week, today you might be diagnosed with insomnia and prescribed sleep medication. But just a few generations ago, this may hardly have been reason for concern, let alone medical intervention. 

Waking in the middle of the night was common, if not the norm, in western preindustrial cultures, according to Roger Ekirch, a professor of history at Virginia Tech whose research into segmented sleep became the basis for his book, At Day’s Close: Night in Times Past. With schedules dictated by the sun rather than clocks and electric lights, people likely retired to bed earlier and, instead of a quick, continuous eight hours, may have enjoyed a longer rest period, that included two shorter sleeps interrupted by a bout of wakefulness.

Not everyone agrees. Some research shows hunter-gatherer communities might have been sleeping in one go, much as we do now. This data could also indicate that multiple sleep sessions was never the norm in societies around the world, even before the Industrial Revolution.

Today, with electricity to lengthen our waking hours and alarms to cut short our repose, most people try to sleep in one continuous bout. But some experts debate whether intermittent sleeping is natural—and the potential benefits of different sleep patterns in modern life.

What is polyphasic sleep?

Segmented sleep consists of two (biphasic) or more (polyphasic) periods of sleep punctuated by periods of wake—both of which can range from minutes to hours depending on the species. Studies estimate that over 86 percent of mammals, including dogs, rodents, hedgehogs, and even certain whales, sleep in several bouts.

Until recently, humans were believed to be among the minority of species—including most primates—that are strictly monophasic sleepers. That hypothesis was wrong, says Russell Foster, a professor of circadian neuroscience at the University of Oxford. 

Historical records contain evidence of biphasic sleeping habits in humans dating back hundreds of years. According to Ekirch, sleep in preindustrial western civilizations happened in two shifts. People would sleep for several hours, and reawaken sometime after midnight for an hour or so of meditation, sex, and socialization before returning to bed for the second sleep.

But some experts believe that this behavior may still be in our nature. In his 1992 pioneering work on the subject, psychiatrist and scientist emeritus of the National Institute of Mental Health Thomas Wehr observed that, after several weeks of being confined to a dark room for 14 hours per day, nearly all participants had shifted into a segmented sleep cycle. 

“On average, for the whole group, it was bimodal,” Wehr says. He found that people tended to fall asleep first in the evening and again towards early morning. “The average pattern was very similar to sleep in some diurnal, day-active animals like panthers.”

Biological and psychological reasons for polyphasic sleep?

From a physiological perspective, bifurcated sleep makes sense, says Daniel Buysse, a professor of psychiatry, medicine, and clinical and translational science at the University of Pittsburgh. Dual sleep processes (homeostatic and circadian), are “smushed together” with our condensed sleep schedule, says Buysse. Given more time, he adds, the processes might separate in time, allowing us to naturally wake between cycles.

In fact, these periods of wakefulness between sleep could even serve a survival function. In his experiment, Wehr noticed that participants would wake up at slightly different times each night and that, on average, there was no time when every single person was asleep. From an evolutionary perspective, this might have served a “sentinel function” by making sure that there was always someone awake to keep watch for the group. 

Some have pushed polyphasic sleep as a way to “biohack” the body and extend waking hours. However, experts widely discourage this. Tricking the body into surviving on shorter spurts of sleep is not the same as waking naturally from well-rested slumber, says Elizabeth Klerman, who co-authored a 2021 paper with Foster analyzing the impacts of artificial polyphasic sleep. She asks, “Would you stop a washing machine before the cycle’s over?” 

Some skeptics of the natural polyphasic sleep theory point to contradictory evidence found among modern hunter-gatherer populations. Jerome Siegel, a professor of psychiatry and biobehavioral sciences at UCLA, conducted research on hunter-gatherer societies in Tanzania, Bolivia, and Namibia that revealed similar sleep patterns to humans in postindustrial societies.

Sleeping patterns data collected over hundreds of consecutive days found that, across three distinct, geographically isolated groups, people slept for roughly 5.7–7.1 continuous hours each night. For Siegel and his collaborators, these results show that modern, monophasic sleeping is a return to traditional patterns seen among the hunter-gatherers.

“They have no electric lights, they have no heating…[they] haven’t changed their environment, or their social structure for hundreds of thousands of years,” he says. “Maybe there was a period in human history when people were waking up in the middle of night, but to say that is the normal pattern just contradicts all this data.”

More than a millennium of polyphasic sleeping

Though our earliest societies may have been monophasic sleepers, Ekrich found records of segmented sleep dating back to Homer’s Odyssey, published in the late 8th or early 7th century B.C. Further digging revealed countless references to “first” and “second sleeps” in all sorts of archival documents, from diaries to medical texts. 

“The references were stated as if segmented sleep was utterly natural and did not need to be explained,” he says.

In the past, Foster says, people tended to go to sleep earlier, around nightfall, and rest—on and off—until sunrise. But everything changed with the arrival of affordable, artificial light sources, which essentially ended our dependence on sunlight, Foster says. “We’re working much later into the evening. So we’re overriding the natural darkness and therefore reducing our opportunity to sleep.”

Not everyone agrees about the history, however. Niall Boyce, an English professor at the University of London, argued that polyphasic sleep may not necessarily have been the norm. Siegel, too, questions the certainty of Ekirch’s interpretation, arguing in favor of his data on modern hunter-gatherers over the anecdotal evidence found in historical records. 

“The bimodal sleep pattern that may have existed in Western Europe is not present in traditional equatorial groups today and, by extension, was probably not present before humans migrated into Western Europe,” the authors wrote in their paper. “Rather, this pattern may have been a consequence of longer winter nights in higher latitudes.”

Whether polyphasic sleep exists among modern humans is also up for debate. While some argue for a stricter definition of the phenomenon, others include naps, siestas, and brief nighttime forays as examples of modern segmented sleep patterns.

Because sleep is influenced by environmental and social contexts, Buysse says, patterns can vary widely among individuals, as well as geographically and seasonally.

“I mainly don’t think that there’s any one pattern of sleep that is the human sleep pattern,” he says. “I think that adaptability is the main feature.”