Enea Xharja Logo

Why We Sleep

Written by Matthew Walker, "Why we sleep" is the biography of John D. Rockefeller, world's first billionaire and among the greatest philanthropists in history.

Below, are my favorite quotes from the book:

Routinely sleeping less than six or seven hours a night demolishes your immune system, more than doubling your risk of cancer.

Insufficient sleep is a key lifestyle factor determining whether or not you will develop Alzheimer’s disease. Inadequate sleep—even moderate reductions for just one week—disrupts blood sugar levels so profoundly that you would be classified as pre-diabetic.

Too little sleep swells concentrations of a hormone that makes you feel hungry while suppressing a companion hormone that otherwise signals food satisfaction. Despite being full, you still want to eat more.

Add the above health consequences up, and a proven link becomes easier to accept: the shorter your sleep, the shorter your life span. The old maxim “I’ll sleep when I’m dead” is therefore unfortunate.

Sadly, human beings are in fact the only species that will deliberately deprive themselves of sleep without legitimate gain.

Tragically, one person dies in a traffic accident every hour in the United States due to a fatigue-related error.

It is disquieting to learn that vehicular accidents caused by drowsy driving exceed those caused by alcohol and drugs combined.

No matter what vantage point you take, sleep would appear to be the most foolish of biological phenomena. When you are asleep, you cannot gather food. You cannot socialize. You cannot find a mate and reproduce. You cannot nurture or protect your offspring. Worse still, sleep leaves you vulnerable to predation. Sleep is surely one of the most puzzling of all human behaviors.

sleep is the single most effective thing we can do to reset our brain and body health each day—Mother Nature’s best effort yet at contra-death.

However, night owls are not owls by choice. They are bound to a delayed schedule by unavoidable DNA hardwiring. It is not their conscious fault, but rather their genetic fate.

For every day you are in a different time zone, your suprachiasmatic nucleus can only readjust by about one hour. It therefore took me about eight days to readjust to London time after having been in San Francisco, since London is eight hours ahead of San Francisco.

Caffeine works by successfully battling with adenosine for the privilege of latching on to adenosine welcome sites—or receptors—in the brain.

The upshot: caffeine tricks you into feeling alert and awake, despite the high levels of adenosine that would otherwise seduce you into sleep.

In pharmacology, we use the term “half-life” when discussing a drug’s efficacy. This simply refers to the length of time it takes for the body to remove 50 percent of a drug’s concentration.

Caffeine has an average half-life of five to seven hours. Let’s say that you have a cup of coffee after your evening dinner, around 7:30 p.m. This means that by 1:30 a.m., 50 percent of that caffeine may still be active and circulating throughout your brain tissue. In other words, by 1:30 a.m., you’re only halfway to completing the job of cleansing your brain of the caffeine you drank after dinner.

Aging also alters the speed of caffeine clearance: the older we are, the longer it takes our brain and body to remove caffeine, and thus the more sensitive we become in later life to caffeine’s sleep-disrupting influence.

First, after waking up in the morning, could you fall back asleep at ten or eleven a.m.? If the answer is “yes,” you are likely not getting sufficient sleep quantity and/or quality. Second, can you function optimally without caffeine before noon? If the answer is “no,” then you are most likely self-medicating your state of chronic sleep deprivation.

When it comes to information processing, think of the wake state principally as reception (experiencing and constantly learning the world around you), NREM sleep as reflection (storing and strengthening those raw ingredients of new facts and skills), and REM sleep as integration (interconnecting these raw ingredients with each other, with all past experiences, and, in doing so, building an ever more accurate model of how the world works, including innovative insights and problem-solving abilities).

Without exception, every animal species studied to date sleeps, or engages in something remarkably like it. This includes insects, such as flies, bees, cockroaches, and scorpions;fn1 fish, from small perch to the largest sharks;fn2 amphibians, such as frogs; and reptiles, such as turtles, Komodo dragons, and chameleons. All have bona fide sleep. Ascend the evolutionary ladder further and we find that all types of birds and mammals sleep: from shrews to parrots, kangaroos, polar bears, bats, and, of course, we humans. Sleep is universal. Even invertebrates, such as primordial mollusks and echinoderms, and even very primitive worms, enjoy periods of slumber.

Why did life ever bother to wake up? Considering how biologically damaging the state of wakefulness can often be, that is the true evolutionary puzzle here, not sleep.

Adopt this perspective, and we can pose a very different theory: sleep was the first state of life on this planet, and it was from sleep that wakefulness emerged.

Every species in which we can measure sleep stages experiences NREM sleep—the non-dreaming stage. However, insects, amphibians, fish, and most reptiles show no clear signs of REM sleep—the type associated with dreaming in humans.

Only birds and mammals, which appeared later in the evolutionary timeline of the animal kingdom, have full- blown REM sleep.

It suggests that dream (REM) sleep is the new kid on the evolutionary block. REM sleep seems to have emerged to support functions that NREM sleep alone could not accomplish, or that REM sleep was more efficient at accomplishing.

Regardless of when true REM sleep emerged in evolution, we are fast discovering why REM-sleep dreaming came into being, what vital needs it supports in the warm-blooded world of birds and mammals (e.g., cardiovascular health, emotional restoration, memory association, creativity, body-temperature regulation), and whether other species dream. As we will later discuss, it seems they do.

NREM sleep was first to appear in evolution.

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. This is true for total sleep time, just as it is for NREM sleep and for REM sleep.

humans (and all other species) can never “sleep back” that which we have previously lost

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.

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

The changes in deep NREM sleep always precede the cognitive and developmental milestones within the brain by several weeks or months, implying a direction of influence: deep sleep may be a driving force of brain maturation, not the other way around.

Therefore, the back of the brain of an adolescent was more adult-like, while the front of the brain remained more child-like at any one moment during this developmental window of time.fn14 His findings helped explain why rationality is one of the last things to flourish in teenagers, as it is the last brain territory to receive sleep’s maturational treatment.

That said, you may also feel ethically uncomfortable about the prospect, considering that you would have the power to write and rewrite your own remembered life narrative or, more concerning, that of someone else. This moral dilemma is somewhat far in the future, but should such methods continue to be refined, it is one we may face.

In this way, sleep helps you retain everything you need and nothing that you don’t, improving the ease of memory recollection. Said another way, forgetting is the price we pay for remembering.

Instead, sleep is able to offer a far more discerning hand in memory improvement: one that preferentially picks and chooses what information is, and is not, ultimately strengthened.

Practice does not make perfect. It is practice, followed by a night of sleep, that leads to perfection.

After ten days of just seven hours of sleep, the brain is as dysfunctional as it would be after going without sleep for twenty-four hours. Three full nights of recovery sleep (i.e., more nights than a weekend) are insufficient to restore performance back to normal levels after a week of short sleeping. Finally, the human mind cannot accurately sense how sleep-deprived it is when sleep-deprived.

As a result, 1.2 million accidents are caused by sleepiness each year in the United States. Said another way: for every thirty seconds you’ve been reading this book, there has been a car accident somewhere in the US caused by sleeplessness.

Indeed, many of the brain regions commonly impacted by psychiatric mood disorders are the same regions that are involved in sleep regulation and impacted by sleep loss. Further, many of the genes that show abnormalities in psychiatric illnesses are the same genes that help control sleep and our circadian rhythms.

the American entrepreneur E. Joseph Cossman: “The best bridge between despair and hope is a good night’s sleep.”

In terms of memory, then, sleep is not like the bank. You cannot accumulate a debt and hope to pay it off at a later point in time. Sleep for memory consolidation is an all-or-nothing event. It is a concerning result in our 24/7, hurry-up, don’t-wait society.

getting too little sleep across the adult life span will significantly raise your risk of developing Alzheimer’s disease.

Sleep, it turns out, is an intensely metabolically active state for brain and body alike. For this reason, theories proposing that we sleep to conserve large amounts of energy are no longer entertained.

The upshot of all this work can be summarized as follows: short sleep (of the type that many adults in first-world countries commonly and routinely report) will increase hunger and appetite, compromise impulse control within the brain, increase food consumption (especially of high-calorie foods), decrease feelings of food satisfaction after eating, and prevent effective weight loss when dieting.

If increasing your risk for developing Alzheimer’s disease, cancer, diabetes, depression, obesity, hypertension, and cardiovascular disease weren’t sufficiently disquieting, chronic sleep loss will erode the very essence of biological life itself: your genetic code and the structures that encapsulate it.

Neglect sleep, and you are deciding to perform a genetic engineering manipulation on yourself each night, tampering with the nucleic alphabet that spells out your daily health story. Permit the same in your children and teenagers, and you are imposing a similar genetic engineering experiment on them as well.

The scientists were able to predict with significant accuracy the content of participants’ dreams at any one moment in time using just the MRI scans, operating completely blind to the dream reports of the participants. Using the template data from the MRI images, they could tell if you were dreaming of a man or a woman, a dog or a bed, flowers or a knife.

There may well be a time in the not-too-distant future where we can accurately “read out” and thus take ownership of a process that few people have volitional control over—the dream.fn1 When this finally happens, and I’m sure it will, do we hold the dreamer responsible for what they dream? Is it fair to judge what it is they are dreaming, since they were not the conscious architect of their dream? But if they were not, then who is? It is a perplexing and uncomfortable issue to face.

Dreams are not, therefore, a wholesale replay of our waking lives. We do not simply rewind the video of the day’s recorded experience and relive it at night, projected on the big screen of our cortex.

If there is a red-thread narrative that runs from our waking lives into our dreaming lives, it is that of emotional concerns.

REM-sleep dreaming takes the painful sting out of difficult, even traumatic, emotional episodes you have experienced during the day, offering emotional resolution when you awake the next morning.

This was the theory of overnight therapy. It postulated that the process of REM-sleep dreaming accomplishes two critical goals: (1) sleeping to remember the details of those valuable, salient experiences, integrating them with existing knowledge and putting them into autobiographical perspective, yet (2) sleeping to forget, or dissolve, the visceral, painful emotional charge that had previously been wrapped around those memories. If true, it would suggest that the dream state supports a form of introspective life review, to therapeutic ends.

Reality and perceived reality were no longer the same in the “eyes” of the sleepless brain. By removing REM sleep, we had, quite literally, removed participants’ levelheaded ability to read the social world around them.

Aside from being a stoic sentinel that guards your sanity and emotional well-being, REM sleep and the act of dreaming have another distinct benefit: intelligent information processing that inspires creativity and promotes problem solving.

future?” Different from solidifying memories, which we now realize to be the job of NREM sleep, REM sleep, and the act of dreaming, takes that which we have learned in one experience setting and seeks to apply it to others stored in memory.

The two most common triggers of chronic insomnia are psychological: (1) emotional concerns, or worry, and (2) emotional distress, or anxiety. In this fast-paced, information-overloaded modern world, one of the few times that we stop our persistent informational consumption and inwardly reflect is when our heads hit the pillow. There is no worse time to consciously do this.

Warm hands and feet help your body’s core cool, inducing inviting sleep quickly and efficiently.

Sleeping pills, old and new, target the same system in the brain that alcohol does—the receptors that stop your brain cells from firing—and are thus part of the same general class of drugs: sedatives.

sleep is not like a credit system or the bank. The brain can never recover all the sleep it has been deprived of. We cannot accumulate a debt without penalty, nor can we repay that sleep debt at a later time.

Under-slept employees are not only less productive, less motivated, less creative, less happy, and lazier, but they are also more unethical.

No amount of years on the job helps a doctor “learn” how to overcome a lack of sleep and develop resilience. How could it? Mother Nature spent millions of years implementing this essential physiological need. To think that bravado, willpower, or a few decades of experience can absolve you (a surgeon) of an evolutionarily ancient necessity is the type of hubris that, as we know from the evidence, costs lives.

“I’ve always loathed the necessity of sleep. Like death, it puts even the most powerful men on their backs.”