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How are humans supposed to sleep? For eight uninterrupted hours each night? In two chunks, separated by a late-night break for work, chit-chat and procreative fun? However we effing please, provided we log between six and nine hours in a given 24-hour period? 

We know that human sleep has changed over the past few millennia, but we don't know exactly how the changes relate to cognition or social and technological progress or — here's a big one — what the changes say about humans' natural mode of rest.  

The "right" way to arrange and enjoy shuteye is the subject of recurring debate in sleep science. If we discount fringe positions, two main schools of thought control the conversation. In one corner, we have experts arguing that our current concept of "normal" rest — one shuteye session, starting in the pm — is a glitchy byproduct of modernity; a wrong turn off evolutionary road. The other side kindly asks that we stop fetishizing "ancestral sleep," contending that it's not clear we'd return to the old school type of bifurcated sleep if we ditched bright lights and big tech. 

Resarchers use various types of evidence to support their positions. Evolutionary scientists look at differences between sleep in humans and other primates, namely the six other species classified as great apes (aka hominids). Other researchers have tackled the issue by analyzing sleep habits in contemporary hunter-gatherer groups. And then other experts draw heavily on historical records of human sleep, seeking to understand how social and technological advancements have influenced shifts in resting habits. Unsurprisingly, different approaches yield different answers. While both sides boast strong evidence and credible proponents, no one has an airtight case. In a recent review paper, a team of evolutionary anthropologists and psychiatrists at Duke University summarized this debate, among other lingering what-ifs in evolutionary sleep. 

Naked Apes in Repose: An Evolutionary Refresher

About two million years ago homo erectus grew too large for his tree-branch bed and started sleeping on the ground, a shift considered a turning point for cognitive and social development. "Freed from the disadvantages of arboreal sleep they could have achieved longer duration and higher quality sleep, which would have improved waking cognition," wrote Charles Nunn, David Samson and Andrew Krystal, who co-authored the Duke paper. 

Humans share certain sleep-related behaviors with other great apes, all united by our most recent common ancestor — a term of art in evolutionary anthropology — who lived about 14 million years ago. For example, while we're the only species to doze on pillow-topped mattresses, we're hardly the only species to build elevated sleep environments. All great apes make platforms or nests to sleep on, a practice that probably emerged between 18 and 14 million years ago.

But, in some ways, human sleep is unique: All humans sleep on the ground. In other great-ape species, large males (and large males alone) only hit the floor when risk of predation is low. 

Humans also stand out in terms of sleep architecture, a term for the structure and arrangement of sleep stages. Compared to other primate spacies, humans spend the least amount of time sleeping (seven-or-so hours, on average) and spend the greatest proportion of their rest in REM sleep, the phase when most vivid dreaming occurs. 

When it comes to sleep, humans are outliers.

While sleep has changed across all great apes, human sleep has evolved disproportionately. We know this because scientists can use statistical modeling to determine whether or not one species in a family (e.g., human beings in the great-ape family) has evolved in a comparable manner to other species in the family. In other words, could we look at changes in the sleep (schedule, environment, architecture, etc.) of chimps & company over the past few millennia and accurately predict the course of human sleep? Nope. When it comes to sleep, humans are outliers. Evolution-based predictions of how humans should sleep in 2016 differ considerably from how we actually sleep. 

The Debate Over Dozing "Naturally"

So, we know that human sleep has evolved — extensively — in concert with changes in physical size, social structure, living environments and other behaviors and characteristics. Now we're getting to the big questions: Was human sleep supposed to change so much? Can we point to a time in history when humans slept the way we "should?" What's the natural way for homo-homos to hit the sack-sack? Is there one?

Here's one theory, introduced into mainstream thought in 2005 by the historian E. Roger Ekirch: Biphasic (or segmented) sleep is "natural." Artificial light killed humans' natural habit of sleeping in twice-daily segments (just like video killed the radio star). Let's backtrack. At some point, in some civilizations, our ancestors maintained segmented (or biphasic) sleep schedules. They turned in for their "first sleep" around 9pm or so, woke up in the middle of the night, hung out for a few hours, and then went back to bed for a "second sleep." Biphasic sleep got phased out during the industrial revolution, when artificial light became cheap and easily accessible.

Segmented sleep may have arisen when humans migrated away from the equator and experienced longer periods of darkness.

But, while the demise of biphasic sleep (as a norm) is well-documented, its origins are less clear. It might be hard to refute that segmented sleep was all the rage leading up to the advent of machine tools. But what about before then? Do we have a good reason to believe that early humans slept in shifts? Evolutionary scientists aren't so sure. 

Here's another theory, favored by the evolutionary crew and based on contemporary comparisons of "primitive" and "modernized" societies: With or without electrical outlets or augmented reality, humans would sleep a lot like most people do today — in one stretch, every night, for about seven hours. Evidence for this view comes courtesy of a UCLA study published last year. A research team, lead by psychiatrist and sleep researcher Jerome Siegel, compared the sleep habits of three modern-day hunter-gatherer peoples who, despite being around in 2016, maintain a tech-free lifestyle similar to that of pre-industrial humans. Data from actigraphy trackers (i.e. fitbits), the study found, suggested that all three groups slept a lot like today's i-phone toting humans: in one, nightly seven-to-eight hour block, with little daytime napping. The study also showed that bedtimes varied considerably more than wake-times did. 

They see adoption of seven-hour monophasic sleep as a necessary step in evolution.

If we accept this theory, what can we make of the pre-industrial biphasic craze? Segmented sleep may have arisen when humans migrated away from the equator and experienced longer periods of darkness, as Siegel told The Atlantic last year: “The long nights caused this pathological sleep pattern and the advent of electric lights and heating restored the primal one.”

Here's more support for this theory: The Duke team came up with the "sleep intensity hypothesis," which says that early humans started to get less, effcient sleep when they moved to the ground. (efficient meaning densely packed with REM and slow-wave sleep). Due to new pressures associated with ground-living (human competition, animal predators), humans needed more waking hours and sharper brains. In turn, denser sleep reinforced social and cognitive development. So, they see ancestral adoption of seven-hour monophasic sleep as a necessary step in evolution. 

While researchers who believe modern-day monophasic sleep emerged for a reason may reject the idea that artificial light ruined our natural sleep, they still recognize that external factors, including technology and social progress, have affected how and how well we sleep. In the Duke paper, researchers laid out three premises underlying their work, the second being that we can blame some of the evolved sleep differences on increased acess to electrical lighting (in the developed world) and the use of separate bedrooms, soft beds and "cultural norms against daytime napping." 

Finally, there's a middle-ground position that takes into account historical records as well as contemporary sleep patterns across diverse societies and groups within them: Humans are flexible — far more flexible than their primate relatives — when it comes to how and when they sleep. The above-mentioned UCLA findings considered the sleep habits of three modern-day hunter-gatherer societies, but not every technology-rejecting social group boasts the same seven-hour norm. One South-American hunter-gatherer group, the Pirahas, take daytime and nighttime naps, ranging from 15 minutes to two hours, and carry out activities in the central village all night long. Pennsylvania Older Amish are also big on naps, with research suggesting that more than half of the population takes a daytime nap at least once per week. 

Regardless of how humans slept 175 years ago, or 175 million years ago, we may have evolved into a species that, when it comes to shuteye, doesn't uniformly beat to the same rhythm, and can go with the flow.