Why We Sleep: The New Science of Sleep and Dreams

by Matthew Walker

eyes have evolved separately and independently numerous times across different phyla throughout evolution for the common purpose of visual perception.

When a theme repeats in evolution, and independently across unrelated lineages, it often signals a fundamental need.

First, and of little surprise, sleep duration is far longer on the recovery night (ten or even twelve hours in humans) than during a standard night without prior deprivation (eight hours for us). Responding to the debt, we are essentially trying to “sleep it off,” the technical term for which is a sleep rebound.

Second, NREM sleep rebounds harder. The brain will consume a larger portion of deep NREM sleep than of REM sleep on the first night after total sleep deprivation, expressing a lopsided hunger.

Should you keep recording sleep across a second, third, and even fourth recovery night, there’s a reversal. Now REM sleep becomes the primary dish of choice with each returning visit to the recovery buffet table, with a side of NREM sleep added.

It is important to note, however, that regardless of the amount of recovery opportunity, the brain never comes close to getting back all the sleep it has lost.

That humans (and all other species) can never “sleep back” that which we have previously lost is one of the most important take-homes of this book,

Take cetaceans, such as dolphins and whales, for example. Their sleep, of which there is only NREM, can be unihemispheric, meaning they will sleep with half a brain at a time!

The flock will first line up in a row. With the exception of the birds at each end of the line, the rest of the group will allow both halves of the brain to indulge in sleep.

If you compare the electrical depth of the deep NREM slow brainwaves on one half of someone’s head relative to the other when they are sleeping at home, they are about the same.

But if you bring that person into a sleep laboratory, or take them to a hotel—both of which are unfamiliar sleep environments—one half of the brain sleeps a little lighter than the other, as if it’s standing guard with just a tad more vigilance due to the potentially less safe context that the conscious brain has registered while awake.

Humans always have to sleep with both halves of our brain in some state of NREM sleep.

Whatever the functions of REM-sleep dreaming—and there appear to be many—they require participation of both sides of the brain at the same time, and to an equal degree.

Individuals who are deliberately fasting will sleep less as the brain is tricked into thinking that food has suddenly become scarce.

(If you’re wondering, humans have no such similar ability.)

Ultra power naps

You can now imagine why the US government continues to have a vested interest in discovering exactly what that biological suit of armor is: their hope for developing a twenty-four-hour soldier.

It of course means the middle of the night, or, more technically, the middle point of the solar cycle.

The practice of biphasic sleep is not cultural in origin, however. It is deeply biological. All humans, irrespective of culture or geographical location, have a genetically hardwired dip in alertness that occurs in the midafternoon hours.

Both you and the meeting attendees are falling prey to an evolutionarily imprinted lull in wakefulness that favors an afternoon nap, called the post-prandial alertness dip (from the Latin prandium, “meal”).

Should you ever have to give a presentation at work, for your own sake—and that of the conscious state of your listeners—if you can, avoid the midafternoon slot.

Around the close of the early modern era (circa late seventeenth and early eighteenth centuries), historical texts suggest that Western Europeans would take two long bouts of sleep at night, separated by several hours of wakefulness.

Yet the fact that no pre-industrial cultures studied to date demonstrate a similar nightly split-shift of sleep suggests that it is not the natural, evolutionarily programmed form of human sleep.

Rather, it appears to have been a cultural phenomenon that appeared and was popularized with the western European migration.

Instead, the true pattern of biphasic sleep—for which there is anthropological, biological, and genetic evidence, and which remains measurable in all human beings to date—is one consisting of a longer bout of continuous sleep at night, followed by a shorter midafternoon nap.

It is perhaps unsurprising that in the small enclaves of Greece where siestas still remain intact, such as the island of Ikaria, men are nearly four times as likely to reach the age of ninety as American males.

From a prescription written long ago in our ancestral genetic code, the practice of natural biphasic sleep, and a healthy diet, appear to be the keys to a long-sustained life.

(Homo sapiens—Latin derivative, “wise person”)?

Homo erectus was the first to use fire, and fire was one of the most important catalysts—if not the most important—that enabled us to come out of the trees and live on terra firma.

For the first time in our evolution, hominids could consume all the body-immobilized REM-sleep dreaming they wanted, and not worry about the lasso of gravity whipping them down from treetops.

Our sleep therefore became “concentrated”: shorter and more consolidated in duration, packed aplenty with high-quality sleep. And not just any type of sleep, but REM sleep that bathed a brain rapidly accelerating in complexity and connectivity.

From these clues, I offer a theorem: the tree-to-ground reengineering of sleep was a key trigger that rocketed Homo sapiens to the top of evolution’s lofty pyramid.

I posit that both have been beneficially and causally shaped by the hand of sleep, and specifically our intense degree of REM sleep relative to all other mammals: (1) our degree of sociocultural complexity, and (2) our cognitive intelligence. REM sleep, and the act of dreaming itself, lubricates both of these human traits.

More specifically, the coolheaded ability to regulate our emotions each day—a key to what we call emotional IQ—depends on getting sufficient REM sleep night after night.

From this REM-sleep-enhanced emotional IQ emerged a new and far more sophisticated form of hominid socioecology across vast collectives, one that helped enable the creation of large, emotionally astute, stable, highly bonded, and intensely social communities of humans.

The second evolutionary contribution that the REM-sleep dreaming state fuels is creativity.

NREM sleep helps transfer and make safe newly learned information into long-term storage sites of the brain. But it is REM sleep that takes these freshly minted memories and begins colliding them with the entire back catalog of your life’s autobiography.

Sleep cycle by sleep cycle, REM sleep helps construct vast associative networks of information within the brain.

REM sleep can even take a step back, so to speak, and divine overarching insights and gist: something akin to general knowledge—that is, what a collection of information means as a whole, not just an inert back catalogue of facts. We can awake the next morning with new solutions to previously intractable problems or even be infused with radically new and original ideas.

Nevertheless, the superior emotional brain gifts that REM sleep affords should be considered more influential in defining our hominid success than the second benefit, of inspiring creativity.

Our shift from tree to ground sleeping instigated an ever more bountiful amount of relative REM sleep compared with other primates, and from this bounty emerged a steep increase in cognitive creativity, emotional intelligence, and thus social complexity.

REM-sleep dreaming therefore represents a tenable new contributing factor, among others, that led to our astonishingly rapid evolutionary rise to power, for better and worse—a new (sleep-fueled), globally dominant social superclass.

Chapter 5 Changes in Sleep Across the Life Span

Prior to birth, a human infant will spend almost all of its time in a sleep-like state, much of which resembles the REM-sleep state.

But in utero, the immature fetus’s brain has yet to construct the REM-sleep muscle-inhibiting system adults have in place.

Think of REM sleep like an Internet service provider that populates new neighborhoods of the brain with vast networks of fiber-optic cables. Using these inaugural bolts of electricity, REM sleep then activates their high-speed functioning.

An infant brain without sleep will be a brain ever underconstructed.

Imbalances in synaptic connections are common in autistic individuals: excess amounts of connectivity in some parts of the brain, deficiencies in others.

Of course, no expecting mother has to worry about scientists disrupting the REM sleep of their developing fetus. But alcohol can inflict that same selective removal of REM sleep.

Alcohol is one of the most powerful suppressors of REM sleep that we know of.

The newborns of heavy-drinking mothers spent far less time in the active state of REM sleep compared with infants of similar age but who were born of mothers who did not drink during pregnancy.