Why We Sleep: Unlocking the Power of Sleep and Dreams

by Matthew Walker

to looping your favorite songs in a repeating playlist at night, we cherry-pick specific slices of your autobiographical past, and preferentially strengthen them by using the individualized sound cues during sleep.VIII I’m sure you can imagine innumerable uses for such a method. 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.

The results were clear. 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.”

The term “muscle memory” is a misnomer. Muscles themselves have no such memory: a muscle that is not connected to a brain cannot perform any skilled actions, nor does a muscle store skilled routines. Muscle memory is, in fact, brain memory. Training and strengthening muscles can help you better execute a skilled memory routine. But the routine itself—the memory program—resides firmly and exclusively within the brain.

In other words, your brain will continue to improve skill memories in the absence of any further practice. It is really quite magical. Yet, that delayed, “offline” learning occurs exclusively across a period of sleep, and not across equivalent time periods spent awake, regardless of whether the time awake or time asleep comes first. Practice does not make perfect. It is practice, followed by a night of sleep, that leads to perfection.

day. Perhaps more relevant to the modern world is the time-of-night effect we discovered. Those last two hours of sleep are precisely the window that many of us feel it is okay to cut short to get a jump start on the day. As a result, we miss out on this feast of late-morning sleep spindles.

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brain damage, we have now discovered that the slow, day-by-day return of motor function in stroke patients is due, in part, to the hard night-by-night work of sleep.

There is much that sleep can do that we in medicine currently cannot. So long as the scientific evidence justifies it, we should make use of the powerful health tool that sleep represents in making our patients well.

A final benefit of sleep for memory is arguably the most remarkable of all: creativity. Sleep provides a nighttime theater in which your brain tests out and builds connections between vast stores of information. This

called a microsleep.

During a microsleep, your brain becomes blind to the outside world for a brief moment—and not just the visual domain, but in all channels of perception.

But it was the two partial sleep deprivation groups that brought the most concerning message of all. After four hours of sleep for six nights, participants’ performance was just as bad as those who had not slept for twenty-four hours straight—that

Most worrying from a societal perspective were the individuals in the group who obtained six hours of sleep a night—something that may sound familiar to many of you. 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. And like the total sleep deprivation group, the

Returning to Dinges’s study results, you may have predicted that optimal performance would return to all of the participants after a good long night of recovery sleep, similar to many people’s notion of “sleeping it off” on the weekends to pay off their weeknight sleep debt. However, even after three nights of ad lib recovery sleep, performance did not return to that observed at the original baseline assessment when those same individuals had been getting a full eight hours of sleep regularly. Nor did any group recover all the sleep hours they had lost in the days prior. As we have already learned, the brain is incapable of that.

They believed that by inserting a nap at the front end of an incoming bout of sleep deprivation, you could insert a buffer, albeit temporary and partial, that would protect the brain from suffering catastrophic lapses in concentration. They were right. Pilots suffered fewer microsleeps at the end stages of the flight if the naps were taken early that prior evening, versus if those same nap periods were taken in the middle of the night or later that next morning, when the attack of sleep deprivation was already well under way.

They had discovered the sleep equivalent of the medical paradigm of prevention versus treatment. The

“power nap” was born. The problem, however, is that people, especially those in such positions, came to erroneously believe that a twenty-minute power nap was all you needed to survive and function with perfect, or even acceptable, acumen.

No matter what you may have heard or read in the popular media, there is no scientific evidence we have suggesting that a drug, a device, or any amount of psychological willpower can replace sleep. Power naps may momentarily increase basic concentration under conditions of sleep deprivation, as can caffeine up to a certain dose. But in the subsequent studies that Dinges and many other researchers (myself included) have performed, neither naps nor caffeine can salvage more complex functions of the brain, including learning, memory, emotional stability, complex reasoning, or decision-making.

One day we may discover such a counteractive method. Currently, however, there is no drug that has the proven ability to replace those benefits that a full night of sleep infuses into the brain and body.

Part of the explanation appears to lie in their genetics, specifically a sub-variant of a gene called BHLHE41.

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.

We discovered that different deep emotional centers in the brain just above and behind the amygdala, called the striatum—associated with impulsivity and reward, and bathed by the chemical dopamine—had become hyperactive in sleep-deprived individuals in response to the rewarding, pleasurable experiences. As with the amygdala, the heightened sensitivity of these hedonic regions was linked to a loss of the rational control from the prefrontal cortex.

You may think that the former counter-balances

I firmly believe that sleep loss and mental illness is best described as a two-way street of interaction, with the flow of traffic being stronger in one direction or the other, depending on the disorder.

“The best bridge between despair and hope is a good night’s sleep.”VII

with a fine-dining experience, it is far more preferable to separate the educational meal into smaller courses, with breaks in between to allow for digestion, rather than attempt to cram all of those informational calories down in one go.

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. 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.

The two most feared diseases throughout developed nations are dementia and cancer. Both are related to inadequate sleep.

with several other toxic metabolites. Phrased differently, and perhaps more simply, wakefulness is low-level brain damage, while sleep is neurological sanitation.

I was once fond of saying, “Sleep is the third pillar of good health, alongside diet and exercise.” I have changed my tune. Sleep is more than a pillar; it is the foundation on which the other two health bastions sit. Take away the bedrock of sleep, or weaken it just a little, and careful eating or physical exercise become less than effective, as we shall see.

Like water from a burst pipe in your home, the effects of sleep deprivation will seep into every nook and cranny of biology, down into your cells, even altering your most fundamental self—your DNA.

the shorter your sleep, the shorter your life.

I should note that in many of these studies, the relationship between short sleep and heart failure remains strong even after controlling for other known cardiac risk factors, such as smoking, physical activity, and body mass. A lack of sleep more than accomplishes its own, independent attack on the heart.

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.

Part of the reason the heart suffers so dramatically under the weight of sleep deprivation concerns blood pressure. Have

One night of modest sleep reduction—even just one or two hours—will promptly speed the contracting rate of a person’s heart, hour upon hour, and significantly increase the systolic blood pressure within their vasculature.I You

As your sleep-deprived heart beats faster,

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.

The less you sleep, the more you are likely to eat. In addition, your body becomes unable to manage those calories effectively, especially the concentrations of sugar in your blood.

After participants had been restricted to four to five hours of sleep for a week, the cells of these tired individuals had become far less receptive to insulin.

When your sleep becomes short, you will gain weight. Multiple forces conspire to expand your waistline. The first concerns two hormones controlling appetite: leptin and ghrelin.

individuals were far more ravenous when sleeping four to five hours a night.

But feeling hungry and actually eating more are not the same thing. Do you actually eat more when sleeping less? Does your waistline really swell as a consequence of that rise in appetite? With another landmark study, Van Cauter proved this to be the case.

Of relevance to this behavior is a recent discovery that sleep loss increases levels of circulating endocannabinoids, which, as you may have guessed from the name, are chemicals produced by the body that are very similar to the drug cannabis. Like marijuana use, these chemicals stimulate appetite and increase your desire to snack, otherwise known as having the munchies.

Some argue that we eat more when we are sleep-deprived because we burn extra calories when we stay awake. Sadly, this is not true.

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.

We found that a full night of sleep repairs the communication pathway between deep-brain areas that unleash hedonic desires and higher-order brain regions whose job it is to rein in these cravings. Ample sleep can therefore restore a system of impulse control within your brain, putting the appropriate brakes on potentially excessive eating.

Sleep’s role in redressing the balance of the body’s nervous system, especially its calming of the fight-or-flight sympathetic branch, improves the bacterial community known as your microbiome, which is located in your gut (also known as the enteric nervous system).

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. Lean

short sleep duration is significantly associated with smaller-sized testicles.