The fascinating science of sleep and its role in human evolution

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Evolutionary pyramid showing humans sleeping on the top and chimpanzee at bottom.

Table of Contents

Some 2 million years ago, some of our hairy, tree-living ancestors descended from sleeping on the trees to sleeping on the ground. Fast-forward to the present day, their descendants have split the atom, walked on the moon, and sent probes on interstellar voyages. Not to mention, colonized the entire planet. What drastically changed was the amount of time they now spend dreaming in deep sleep. Ask yourself ‘why do we dream’? And you might stumble upon an answer to why we are at the zenith of the evolutionary pyramid on planet Earth.

So what gave way to this evolution?

REM sleep is the answer. It’s this state of deep sleep in which we become as good as dead, and dream of marvelous visions each night. This state conferred some unique benefits to our species. These benefits made some significant changes to our jumbo brains, and these changes are the very reason why we are at the top.

Let’s go back to the beginning to find out what happened.

Descent from the trees

A chimpanzee thinking versus a human thinking in front of a laptop
Our brain and the way we think and solve problems has changes drastically because of REM sleep

We often like to think of our species as unique on this planet. We’re not though. We belong to a group of Primates with over 200 species. Our first cousins (like the Neanderthals) might have gone extinct but our second cousins – monkeys, lemurs, and apes are pretty much still around. Our closest living relatives today are chimpanzees and bonobos, with whom we share 99% of our DNA.

Did you know?
Just six million years ago, a single female ape had two daughters. One of those daughters is the ancestor of all chimpanzees, and the other one is our own grandmother!

Life of our closest cousins

So like our closest cousins, we also lived an arboreal life millions of years ago. We built nests on trees to sleep each night (a tedious task that took up hours of our ancestor’s time). But then as we evolved from hominids to Homo erectus, our bodies became unfit from living on trees. And that was when we moved to sleeping on the ground. This is where our common histories began to diverge.

The transition to terrestrial sleeping, first and foremost, eliminated the biggest danger of arboreal sleeping – lethal falls. One bad dream, one wrong turn while sleeping, and you would have been out of the gene pool.

A group of early humans sleeping around a fire while one person is awake
Sentineled groups were an evolutionary advantage

Second, it brought with it several physiological and cultural changes like beds, shelters, variation in chronotypes, and controlled use of fire.

Controlled use of fire is probably one of the most important factors as it did two things. One, saved us from both flesh-eating and blood-sucking predators while we slept on the ground. Two, we began sleeping in large, sentineled groups on stable ground beds, protected by fire, which would have fostered a safe sense of community.

For the first time in history, there was no danger of falling off a tree. There was fire as a weapon against predators, and the safety brought by sleeping in groups in which someone was always keeping guard.

For the first time in history, our ancestors experienced the advantage of deep, efficient sleep. 

This put them in a unique position. They could now capitalize on the advantages of deeper, more intense, REM-dominant sleep. And that’s where our journey of a different type of dreaming began.

Why do we dream? Evolution of unique sleep of humans.

Human sleep is unique. When compared to other primates, human sleep duration is the shortest, being an average of 7 hours. Our primate relatives clock in somewhere between 13 and 17 hours of sleep each night. Good for them, but our sleep is way more efficient.

A graph showing that humans sleep the least number of total hours versus other species
When compared to other species, humans sleep the least number of total hours

When we say higher sleep efficiency, we mean that human sleep is shorter, deeper, and exhibits a higher proportion of REM. We spend a staggering 25% of our sleep in REM, which also means that we dream more. In fact, the human REM to NREM ratio (22:78) is the highest proportion of REM to NREM of all primates.

Why do we dream so much? Why do we dream when the deep REM state leaves us vulnerable to predators and dangers?

But first, how did human sleep evolve to be more efficient?

Two evolutionary anthropologists, Charles Nunn and David Samson, have done some groundbreaking research in this department. They have also postulated Sleep Intensity Hypothesis as an answer to this question.

According to the sleep intensity hypothesis, “early humans experienced selective pressure to fulfill sleep needs in the shortest time possible.”

An early man hiding behind the rocks and bushes to be safe from a tiger
The risk of predators was one factor that pressured early humans to pack more efficient sleep in less time

What this means is that early humans had the pressure to sleep more efficiently in less time because of several factors. For one, the risk of predators was higher in terrestrial environments. So the sooner they completed their quota of REM sleep, the better the chances of protection from incoming threats. For another, less sleep meant more time to engage in pursuits of skills and knowledge. And deep sleep meant effective consolidation of these skills, which eventually led to higher cognitive intelligence, a point we’ll discuss later in more detail.

Our unique sleep architecture

Research has revealed we have four 90-minute long sleep cycles. And these sleep cycles have three distinct stages of human sleep :

  • Stage 1 – Light N2 sleep – This comprises NREM 1 and NREM 2.
  • Stage 2 – Deep N3 sleep – Characterized by slow wave activity or SWA in the brain
  • Stage 3 – REM
A visual representation of our sleep architecture
The proportion of REM and NREM sleep stages keep on varying throughout our sleep cycle

What happens in NREM sleep?

NREM sleep has three stages. The N1 stage is the shortest (usually lasting less than 10 minutes) and occurs right after you drift into sleep. The N2 stage marks the beginning of slow-wave brain activity. Your body becomes more relaxed. This stage lasts for around 30 to 60 minutes. The last NREM stage is the N3 stage, which lasts between 20 and 40 minutes. It’s the hardest to wake someone up from this stage.

Each of these stages takes us into deeper depths of sleep. During this phase, the body repairs and regrows tissue; builds muscles and bones; and boosts the immune system.

What happens in REM sleep?

REM sleep on the other hand is associated with our bodies becoming paralysed and our minds becoming more awake as we dream. Recent studies of MRI scans show parts of the brain that are up to 30% more active during REM sleep. There’s high neural activity, pretty much the same as when we are awake. And it’s in this deep sleep that learning, consolidation of the information learnt, and sieving of important information in long-term memory takes place.

A woman dreaming in REM sleep, in which her brain is experiencing many benefits
REM sleep conferred multiple benefits to the early human brain, which then helped in their evolution at a rapid pace

However, with terrestrial sleeping, early humans evolved to dedicate more time to SWS and REM sleep. This led to less time to light N2 sleep (NREM stages 1-2). This increased the total proportion of time we spent in deep sleep in comparison with light sleep.

From an evolutionary perspective, even though deep sleep stages were vulnerable, they shortened the total time early humans needed to sleep and be inactive. Considering the evolutionary advantages that REM sleep bestowed upon humans, it would be safe to say that this shift worked out just fine. So if you’re wondering why do we dream, it’s because dreaming gave our primitive brains some evolutionary gifts.

Three unique gifts of REM sleep

After we fall asleep, we enter this deep sleep stage. This stage conferred three special benefits on us.

The first of which is called threat priming.

The ability of threat priming is the answer to why do we dream.

Through the ability to dream, REM enables sleepers by rehearsing events or social scenarios that might occur in their waking environments. Our brain has this unique ability to imagine scenarios that haven’t even taken place yet, in phenomenal detail. In a way, day-dreaming has evolutionary advantages.

So today, we might be priming our brains to rehearse that presentation or practice that Olympic-worthy gymnast routine. But ages ago, this ability gave our ancestors the ability to become exceptionally better at warding off threats and fighting them.

Even today, performers, musicians, sportspersons – basically people from all walks of life – practice this technique of mentally rehearsing a particular action before actually performing them.

No other species can imagine a future event and rehearse it mentally like us.

An early man experiencing the benefits of REM sleep.
Threat priming, increased innovation to build tools and shelter, along with more free time to interact and form social bonds, are some of the key benefits provided to early humans by REM sleep

The second benefit conferred by REM sleep on us, is increased innovation.

REM sleep and its contents allowed the human brain to form a wider network of ideas. This resulted in greater frequency of creativity, insight, and innovation. A high creative association was required to do many tasks. These included building tools, whittling the ends of spears to make some decent hunting gear, developing them with time to make survival easy, etc.

Plus, increased sleep intensity also enhanced memory consolidation. A genius creative idea wasn’t of much use if our ancestors got it and forgot it the next day. Without the ability to consolidate creative ideas and innovation in the brain, we’d probably still be tending a bonfire outside caves, flinting stones.

Did you know?
Slow Wave Sleep and REM sleep play an important role in processing daily information into long term memory stores. 

SWS is especially associated with consolidation of procedural memories, episodic memory, and processing emotions. So let’s say that Steve from a clan had a realization. Knapping a certain kind of stone with a certain kind of technique makes excellent blades for chopping that hunted dinner (procedural memory). He would need to remember that information, consolidate it in his brain before passing it on to other clan members. This is another answer to why do we dream.

The third benefit given to us by this deep sleep was an increase in available time.

Now that we slept more efficiently and deeply, the number of hours required for us to sleep decreased. With more awake time, we could indulge in more social activities, which eventually had important consequences for how we interacted as a society. Now our hominin ancestors had more time to bond with their communities. They could talk, gossip, tell stories as a way of transmitting cultural information, and increase cognitive abilities.

Still wondering why do we dream when we can do so much more while being awake? Think of all the benefits dreams and deep sleep gave us.

Cognitive intelligence and sociocultural complexity

People of all nationalities, races, and backgrounds standing together
Our unique ability to collaborate with people of different nations, backgrounds, religions, and languages is one of our unique trait as humans

What separates us from the rest of the animal kingdom is our cognitive intelligence and our sociocultural complexity. And both these markers are a result of the stage of deep sleep and dreaming, and how they changed our brains.

By bestowing us with these two defining features, REM sleep set in motion a different course of history for us. This course would diverge from anything any species had done in the animal kingdom. In a way, REM sleep laid the foundation for what became today’s modern society.

Our cognitive intelligence allowed us to reason, question, solve complex problems, apply logic and use language to communicate complex ideas. A monkey might know that if he drops a fruit, it will fall on the ground. But a human knew this and discovered gravity and a mathematical formula for it. Also, how the gravity on Earth is different from, say, that of a black hole 12 light years away.

Our sociocultural complexity is also unparalleled.

Humans of one nation can cooperate with humans of another nation easily based on their shared interests and agendas. The same can’t be said for, let’s say, forest to forest cooperation between American and African chimpanzees.

Did you know?
The Beatles’ classic hits ‘Yesterday’ and ‘Let it be’ came to Paul McCartney in his sleep?

Our cognitive intelligence also allowed us to do something called myth making, which led to our complex sociocultural coordination and cooperation. This myth making made our society and culture way more complex than any other in the animal kingdom. Our ability to imagine, use language and converse complex ideas gave way for us to create shared myths or ideas. These myths and ideas are socially constructed like religion, money, heaven and hell, etc. 

For example, a monkey might have been able to communicate a danger to its group. Like, there’s a lion near the pond, and it’s dangerous to go there. But humans evolved to communicate something more complex. Like “Oh hey, saw a lion near the pond last night so maybe not go there if you don’t wanna end up dead? And also, since this beast is so fierce, it should be the guardian spirit of our tribe. And any tribe that worships it is our friend. Any tribe that worships an Eagle, shall not have help from us if they were ever in trouble.”

No other animal in the animal kingdom did something even remotely close to this. The entire foundation of what we have achieved today begins with this question of why do we dream.

This myth making was the beginning of how our nation-states and modern society would come into being by sharing certain beliefs.

Why do we dream: What REM sleep and dreaming gave us

Two astronauts on Mars standing in front of a colony, looking at Earth
Humans have started their journey of colonizing Mars, while our nearest relatives – the Chimps, are still living the same way they did millions of years ago

So today, our nearest cousins are still living in jungles, building their nests the same way our hairy ancestors did. And us? Well, thanks to REM sleep and dreaming, some specimens of our species are thinking of building a colony on another planet, and decoding the secrets of the universe.

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