Why We Sleep: Unlocking the Power of Sleep and Dreams

by Matthew Walker

Even daytime naps as short as twenty minutes can offer a memory consolidation advantage, so long as they contain enough NREM sleep.V

following a night of sleep you regain access to memories that you could not retrieve before sleep. Like a computer hard drive where some files have become corrupted and inaccessible, sleep offers a recovery service at night. Having repaired those memory items, rescuing them from the clutches of forgetting, you awake the next morning able to locate and retrieve those once unavailable memory files with ease and precision.

By boosting the electrical quality of deep-sleep brainwave activity, the researchers almost doubled the number of facts that individuals were able to recall the following day, relative to those participants who received no stimulation.

a Swiss research team recently suspended a bedframe on ropes from the ceiling of a sleep laboratory (stick with me here). Affixed to one side of the suspended bed was a rotating pulley. It allowed the researchers to sway the bed from side to side at controlled speeds. Volunteers then took a nap in the bed as the researchers recorded their sleeping brainwaves. In half of the participants, the researchers gently rocked the bed once they entered NREM sleep. In the other half of the subjects, the bed remained static, offering a control condition. Slow rocking increased the depth of deep sleep, boosted the quality of slow brainwaves, and more than doubled the number of sleep spindles.

Sleep powerfully, yet very selectively, boosted the retention of those words previously tagged for “remembering,” yet actively avoided the strengthening of those memories tagged for “forgetting.” Participants who did not sleep showed no such impressive parsing and differential saving of the memories.IX

Sleep had systematically identified where the difficult transitions were in the motor memory and smoothed them out. This finding rekindled the words of the pianist I’d met: “but when I wake up the next morning and sit back down at the piano, I can just play, perfectly.”

Rather than a transfer from short- to long-term memory required for saving facts, the motor memories had been shifted over to brain circuits that operate below the level of consciousness.

daytime naps that contain sufficient numbers of sleep spindles also offer significant motor skill memory improvement, together with a restoring benefit on perceived energy and reduced muscle fatigue.

Obtain anything less than eight hours of sleep a night, and especially less than six hours a night, and the following happens: time to physical exhaustion drops by 10 to 30 percent, and aerobic output is significantly reduced. Similar impairments are observed in limb extension force and vertical jump height, together with decreases in peak and sustained muscle strength. Add to this marked impairments in cardiovascular, metabolic, and respiratory capabilities that hamper an underslept body, including faster rates of lactic acid buildup, reductions in blood oxygen saturation, and converse increases in blood carbon dioxide, due in part to a reduction in the amount of air that the lungs can expire. Even the ability of the body to cool itself during physical exertion through sweating—a critical part of peak performance—is impaired by sleep loss.

sleep loss inflicts such devastating effects on the brain, linking it to numerous neurological and psychiatric conditions (e.g., Alzheimer’s disease, anxiety, depression, bipolar disorder, suicide, stroke, and chronic pain), and on every physiological system of the body, further contributing to countless disorders and disease (e.g., cancer, diabetes, heart attacks, infertility, weight gain, obesity, and immune deficiency). No facet of the human body is spared the crippling, noxious harm of sleep loss. We are, as you will see, socially, organizationally, economically, physically, behaviorally, nutritionally, linguistically, cognitively, and emotionally dependent upon sleep.

Ten days of six hours of sleep a night was all it took to become as impaired in performance as going without sleep for twenty-four hours straight.

if you wake up at seven a.m. and remain awake throughout the day, then go out socializing with friends until late that evening, yet drink no alcohol whatsoever, by the time you are driving home at two a.m. you are as cognitively impaired in your ability to attend to the road and what is around you as a legally drunk driver.

Operating on less than five hours of sleep, your risk of a car crash increases threefold. Get behind the wheel of a car when having slept just four hours or less the night before and you are 11.5 times more likely to be involved in a car accident.

After sixteen hours of being awake, the brain begins to fail. Humans need more than seven hours of sleep each night to maintain cognitive performance. 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.

vehicle accidents caused by drowsy driving exceed those caused by alcohol and drugs combined. Drowsy driving alone is worse than driving drunk.

If you really must keep going—and you have made that judgment in the life-threatening context it genuinely poses—then pull off the road into a safe layby for a short time. Take a brief nap (twenty to thirty minutes). When you wake up, do not start driving. You will be suffering from sleep inertia—the carryover effects of sleep into wakefulness. Wait for another twenty to thirty minutes, perhaps after having a cup of coffee if you really must, and only then start driving again.

It was as though, without sleep, our brain reverts to a primitive pattern of uncontrolled reactivity. We produce unmetered, inappropriate emotional reactions, and are unable to place events into a broader or considered context.

Insufficient sleep does not, therefore, push the brain into a negative mood state and hold it there. Rather, the under-slept brain swings excessively to both extremes of emotional valence, positive and negative.

Studies of adolescents have identified a link between sleep disruption and suicidal thoughts, suicide attempts, and, tragically, suicide completion in the days after. One more reason for society and parents to value plentiful sleep in teens rather than chastise it, especially considering that suicide is the second-leading cause of death in young adults in developed nations after car accidents.

Sleep disturbance is a recognized hallmark associated with addictive substance use.IV Insufficient sleep also determines relapse rates in numerous addiction disorders, associated with reward cravings that are unmetered, lacking control from the rational head office of the brain’s prefrontal cortex.V Relevant from a prevention standpoint, insufficient sleep during childhood significantly predicts early onset of drug and alcohol use in that same child during their later adolescent years, even when controlling for other high-risk traits, such as anxiety, attention deficits, and parental history of drug use.VI

There is no major psychiatric condition in which sleep is normal.

otherwise healthy people can experience a neurological pattern of brain activity similar to that observed in many of these psychiatric conditions simply by having their sleep disrupted or blocked.

Major depression has as much to do with absence of positive emotions, a feature described as anhedonia: the inability to gain pleasure from normally pleasurable experiences, such as food, socializing, or sex.

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

improvement. In other words, if you don’t sleep the very first night after learning, you lose the chance to consolidate those memories, even if you get lots of “catch-up” sleep thereafter.

over 60 percent of patients with Alzheimer’s disease have at least one clinical sleep disorder.

wakefulness is low-level brain damage, while sleep is neurological sanitation.

Without sufficient sleep, amyloid plaques build up in the brain, especially in deep-sleep-generating regions, attacking and degrading them. The loss of deep NREM sleep caused by this assault therefore lessens the ability to remove amyloid from the brain at night, resulting in greater amyloid deposition. More amyloid, less deep sleep, less deep sleep, more amyloid, and so on and so forth.

Adults forty-five years or older who sleep fewer than six hours a night are 200 percent more likely to have a heart attack or stroke during their lifetime, as compared with those sleeping seven to eight hours a night.

Think of your deep NREM sleep as a natural form of nighttime blood-pressure management—one that averts hypertension and stroke.

In the Northern Hemisphere, the switch to daylight savings time in March results in most people losing an hour of sleep opportunity. Should you tabulate millions of daily hospital records, as researchers have done, you discover that this seemingly trivial sleep reduction comes with a frightening spike in heart attacks the following day. Impressively, it works both ways. In the autumn within the Northern Hemisphere, when the clocks move forward and we gain an hour of sleep opportunity time, rates of heart attacks plummet the day after. A similar rise-and-fall relationship can be seen with the number of traffic accidents, proving that the brain, by way of attention lapses and microsleeps, is just as sensitive as the heart to very small perturbations of sleep. Most people think nothing of losing an hour of sleep for a single night, believing it to be trivial and inconsequential. It is anything but.

sleeping less than seven or eight hours a night will increase your probability of gaining weight, being overweight, or being obese, and significantly increases your likelihood of developing type 2 diabetes.

Beyond the average treatment cost of more than $85,000 per patient (which contributes to higher medical insurance premiums), diabetes lops ten years off an individual’s life expectancy.

It was a classic case of physiological double jeopardy: participants were being punished twice for the same offense of short sleeping: once by having the “I’m full” signal removed from their system, and once by gaining the “I’m still hungry” feeling being amplified. As a result, participants just didn’t feel satisfied by food when they were short sleeping.

Inadequate sleep is the perfect recipe for obesity: greater calorie intake, lower calorie expenditure.

When you are not getting enough sleep, the body becomes especially stingy about giving up fat. Instead, muscle mass is depleted while fat is retained.

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.

Women who do become pregnant and routinely sleep less than eight hours a night are also significantly more likely to suffer a miscarriage in the first trimester, relative to those consistently sleeping eight hours or more a night.

A large European study of almost 25,000 individuals demonstrated that sleeping six hours or less was associated with a 40 percent increased risk of developing cancer, relative to those sleeping seven hours a night or more. Similar associations were found in a study tracking more than 75,000 women across an eleven-year period.

Poor sleep quality therefore increases the risk of cancer development and, if cancer is established, provides a virulent fertilizer for its rapid and more rampant growth. Not getting sufficient sleep when fighting a battle against cancer can be likened to pouring gasoline on an already aggressive fire. That may sound alarmist, but the scientific evidence linking sleep disruption and cancer is now so damning that the World Health Organization has officially classified nighttime shift work as a “probable carcinogen.”

Not sleeping enough, which for a portion of the population is a voluntary choice, significantly modifies your gene transcriptome—that is, the very essence of you, or at least you as defined biologically by your DNA.

we could predict with confidence the form of someone’s dream—would it be visual, would it be motoric, would it be awash with emotion, would it be completely irrational and bizarre?—before the dreamers themselves reported their dream experience to the research assistant.

A meaningful, psychologically healthy life is an examined one,

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 is necessary, but REM sleep alone is not sufficient. Dreams are not the heat of the lightbulb—they are no by-product.

REM sleep is the only time during the twenty-four-hour period when your brain is completely devoid of this anxiety-triggering molecule.

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.

It was not, therefore, time per se that healed all wounds, but instead it was time spent in dream sleep that was providing emotional convalescence.

In a series of publications that I still revisit with admiration to this day, Cartwright demonstrated that it was only those patients who were expressly dreaming about the painful experiences around the time of the events who went on to gain clinical resolution from their despair, mentally recovering a year later as clinically determined by having no identifiable depression. Those who were dreaming, but not dreaming of the painful experience itself, could not get past the event, still being dragged down by a strong undercurrent of depression that remained.

Deprive an individual of their REM-sleep dreaming state, and the emotional tuning curve of the brain loses its razor-sharp precision.