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On average, humans in industrialized societies sleep for one, seven-hour block each night. Critiques of modern-day sleep patterns have become a common refrain — some contending that modernity itself ruined our rest. But new information says the way we sleep might actually be a purposeful process nearly two million years in the making, rather than a byproduct of technological dependence. 

First, some backstory.

The History

Evolution First Stage

Historians and sleep researchers have said that the introduction of artificial light during the Industrial Revolution coincided with a shift from sleeping in two or more segments (biphasic or polyphasic sleep) to the single period of rest we’ve come to recognize as standard (monophasic sleep). Historian Roger Ekirch, a leading proponent of this polyphasic sleep theory, hinged his argument on documents dating back to ancient civilization.

There’s also persuasive evidence that we would sleep differently without blue light in our lives. In one 2013 study, for example, enterprising volunteers gave up their gadgets and spent a week sleeping in nature. Uncuffed from electronics, their body clocks naturally synced up to light-and-dark cycles.

How we sleep is neither an accident nor a side effect of technology.

Earlier this year, however, a tide-turning study on modern-day hunter-gatherer societies effectively called bullshit on the idealized notion of re-embracing “ancestral sleep.” According to UCLA researchers, our sleep patterns hew pretty damn close to those of our hallowed ancestors.

In the study, researchers monitored sleep activity in three tribes  — we’ll call them “traditional populations” — who live without artificial light and other technological fripperies. Given that they still party like it’s 1699, researchers believed the traditional populations’ sleep patterns would match those of pre-industrialized humans, at least in theory.

But all three populations actually slept like modern Westerners. In fact, they slept slightly less (marginally so) than we do. In the end, study authors concluded that ancient humans probably slept a lot like modern ones.

Caught up? Good.

The New Theory

Evolution Second Stage

What if sleeping in seven-hour chunks is not only natural for humans, but actually a vital evolutionary behavior? What if single sessions of efficient sleep are fundamental to those cognitive capabilities that distinguish us from other species and make us, well, human?

That’s what two Duke University researchers believe.

Evolutionary anthropologists David Samson and Charles Nunn fielded an extensive cross-species study comparing sleep duration and architecture between humans and 21 of our closest primate relatives. Apes, lemurs, chimps and the like, they found, get at least twice as much sleep as we do. Samson and Nunn also determined that we spend more time in the two deepest stages of sleep (slow-wave sleep, or SWS, and REM) than any other species.

How we sleep, they concluded, is neither an accident nor a side effect of technology. In fact, in their “Sleep Intensity Hypothesis,” Samson and Nunn describe the shift to efficient sleep as a critical link in the long chain of human evolution.  

The Hypothesis

Evolution Third Stage

Two million years ago, our earliest ancestors slept in trees, and these tree-sleeping humans got the long, shallow sleep they needed for survival. Specifically, they spent a lot of time in the first two stages of non-REM sleep, a period of “light” sleep characterized by a low arousal threshold. They were, therefore, more likely to startle awake when, say, a tree-climbing predator approached or if they were about to fall off their branch.

With time and evolution, homo erectus grew too large for his leafy loft bed and, bending to gravity’s will, moved to the ground. Terrestrial sleep came with a new set of concerns. Vulnerability to both predators and rival hominids, working together became an important defense tactic. One consequence of increased socialization was sleeping less.

As a newly social species, these first humans needed to negotiate hunting-and-gathering schedules with their neighboring cavemen. But their brains weren’t up to snuff; they hadn’t yet developed the frontal lobes required for higher-level thinking, and the light sleep of their recent past wasn’t doing their cognition any favors. This is when SWS and REM sleep stages take center stage. Humans may be sleeping less, but they’re getting more out of it. They’re sleeping deeply.

Samson and Nunn identified three cognitive benefits from deep sleep:

  1. REM facilitated threat priming.
    During dreams, we rehearse scenarios, including threatening situations that could happen during waking hours. Basically, we practice throwing down.
  2. REM sparked creativity and innovation.
    Heating up raw meat may not seem innovative today, but the first hominid to roast a rodent over a spitfire was his generation’s Steve Jobs.
  3. SWS became critical for memory consolidation
    This helped humans convert the knowledge and skills they acquired by day into long-term memories.

Long story short, early humans needed to sleep more efficiently to maximize their chance of survival. Their altered sleep architecture, in turn, enhanced cognitive abilities.

Sleeping Today

Evolution Fourth Stage

More than a million years later, we still rest in short, intense, REM-heavy blocks.

“Relative to other primates,” Samson told Van Winkle’s, “humans have adapted a quality over quantity approach to sleep.”

Operating under the assumption that our sleep patterns emerged for a good reason, the Sleep Intensity Hypothesis fills in some unknowns about the #hardeight.

“When you’re asleep, you can’t eat, you can’t mate and you are vulnerable,” said Samson. “Evolution would never be so sloppy as to allow for a universal behavior such as sleep to emerge if it wasn’t absolutely critical to the survival of life on this planet.”

Of course, this is still a hypothesis, and it could be proven wrong. For example, as the paper states, research showing that modern-day traditional populations actually sleep more than post-industrialized societies would undermine the hypothesis. As would evidence that, under the same conditions, humans sleep less efficiently (less time in REM and SWS) than other primate species.

Until recently, it would have been difficult to perform the type of research necessary to affirm or invalidate the sleep intensity hypothesis — that is, measuring sleep in traditional populations. But, that’s no longer the case, said Samson, thanks to actigraphy (i.e. Fitbits and other wearables). Samson recently measured sleep in rural Madagascar to assess variations in sleep architecture among different non-industrial societies.

But if we’re serious about getting these measurements, the time is now to dispatch Fitbits around the world. “Time is of the essence,” said Samson, “because many societies are experiencing rapid change in development from globalization."