Why We Sleep: Unlocking the Power of Sleep and Dreams

by Matthew Walker

There is perhaps no better illustration highlighting the smarts of REM-sleep dreaming than the elegant solution to everything we know of, and how it fits together. I am not trying to be obtuse. Rather, I am describing the dream of Dmitri Mendeleev on February 17, 1869, which led to the periodic table of elements: the sublime ordering of all known constituent building blocks of nature.

In Mendeleev’s own words:II I saw in a dream a table where all the elements fell into place as required. Awakening, I immediately wrote it down on a piece of paper. Only in one place did a correction later seem necessary.

The creative muse of dreaming has also sparked countless literary ideas and epics.

When awake, we see only a narrow set of all possible memory interrelationships. The opposite is true, however, when we enter the dream state and start looking through the other (correct) end of the memory-surveying telescope. Using that wide-angle dream lens, we can apprehend the full constellation of stored information and their diverse combinatorial possibilities, all in creative servitude.

It is sleep that builds connections between distantly related informational elements that are not obvious in the light of the waking day. Our participants went to bed with disparate pieces of the jigsaw and woke up with the puzzle complete. It is the difference between knowledge (retention of individual facts) and wisdom (knowing what they all mean when you fit them together). Or, said more simply, learning versus comprehension. REM sleep allows your brain to move beyond the former and truly grasp the latter.

After time spent awake across the day, despite the chance to consciously deliberate on the problem as much as they desired, a rather paltry 20 percent of participants were able to extract the embedded shortcut. Things were very different for those participants who had obtained a full night of sleep—one dressed with late-morning, REM-rich slumber. Almost 60 percent returned and had the “ah-ha!” moment of spotting the hidden cheat—which is a threefold difference in creative solution insight afforded by sleep!

Little wonder, then, that you have never been told to “stay awake on a problem.” Instead, you are instructed to “sleep on it.” Interestingly, this phrase, or something close to it, exists in most languages (from the French dormir sur un problem, to the Swahili kulala juu ya tatizo), indicating that the problem-solving benefit of dream sleep is universal, common across the globe.

What we do know, however, is that Edison was a habitual daytime napper. He understood the creative brilliance of dreaming, and used it ruthlessly as a tool, describing it as “the genius gap.” Edison would allegedly position a chair with armrests at the side of his study desk, on top of which he would place a pad of paper and a pen. Then he would take a metal saucepan and turn it upside down, carefully positioning it on the floor directly below the right-side armrest of the chair. If that were not strange enough, he would pick up two or three steel ball bearings in his right hand. Finally, Edison would settle himself down into the chair, right hand supported by the armrest, grasping the ball bearings. Only then would Edison ease back and allow sleep to consume him whole. At the moment he began to dream, his muscle tone would relax and he would release the ball bearings, which would crash on the metal saucepan below, waking him up. He would then write down all of the creative ideas that were flooding his dreaming mind. Genius, wouldn’t you agree?

The participants were allowed to fall asleep in the MRI scanner, entering REM sleep where they could dream. During REM sleep, however, all voluntary muscles are paralyzed, preventing the dreamer from acting out ongoing mental experience. Yet, the muscles that control the eyes are spared from this paralysis, and give this stage of sleep its frenetic name. Lucid dreamers were able to take advantage of this ocular freedom, communicating with the researchers through eye movements. Pre-defined eye movements would therefore inform the researchers of the nature of the lucid dream (e.g., the participant made three deliberate leftward eye movements when they gained lucid dream control, two rightward eye movements before clenching their right hand, etc.). Non-lucid dreamers find it difficult to believe that such deliberate eye movements are possible while someone is asleep, but watch a lucid dreamer do it a number of times, and it is impossible to deny.

There could be no question. Scientists had gained objective, brain-based proof that lucid dreamers can control when and what they dream while they are dreaming. Other studies using similar eye movement communication designs have further shown that individuals can deliberately bring themselves to timed orgasm during lucid dreaming, an outcome that, especially in males, can be objectively verified using physiological measures by (brave) scientists.

It is possible that lucid dreamers represent the next iteration in Homo sapiens’ evolution. Will these individuals be preferentially selected for in the future, in part on the basis of this unusual dreaming ability—one that may allow them to turn the creative problem-solving spotlight of dreaming on the waking challenges faced by themselves or the human race, and advantageously harness its power more deliberately?

The term “somnambulism” refers to sleep (somnus) disorders that involve some form of movement (ambulation). It encompasses conditions such as sleepwalking, sleep talking, sleep eating, sleep texting, sleep sex, and, very rarely, sleep homicide.

Trapped between the two worlds of deep sleep and wakefulness, the individual is confined to a state of mixed consciousness—neither awake nor asleep. In this confused condition, the brain performs basic but well-rehearsed actions, such as walking over to a closet and opening it, placing a glass of water to the lips, or uttering a few words or sentences.

Since he could remember only vague fragments of the murder (e.g., flashes of his mother-in-law’s face with a “help me” look on it), had no motive, and had a long history of sleepwalking (as did other members of his family), a team of defense experts concluded that Ken Parks was asleep when he committed the crime, suffering a severe episode of sleepwalking. They argued that he was unaware of his actions, and thus not culpable. On May 25, 1988, a jury rendered a verdict of not guilty. This defense has been attempted in a number of subsequent cases, most of which have been unsuccessful.

The two most common triggers of chronic insomnia are psychological: (1) emotional concerns, or worry, and (2) emotional distress, or anxiety.

Since psychological distress is a principal instigator of insomnia, researchers have focused on examining the biological causes that underlie emotional turmoil. One common culprit has become clear: an overactive sympathetic nervous system, which, as we have discussed in previous chapters, is the body’s aggravating fight-or-flight mechanism.

Emotions make us do things, as the name suggests (remove the first letter from the word).

Don’t worry if you have had an episode of sleep paralysis at some point in your life. It is not unique to narcolepsy. Around one in four healthy individuals will experience sleep paralysis, which is to say that it is as common as hiccups. I myself have experienced sleep paralysis several times, and I do not suffer from narcolepsy.

As Corke approached the six-month mark of no sleep, he was bedridden and approaching death. Despite his young age, Corke’s neurological condition resembled that of an elderly individual in the end stages of dementia. He could not bathe or clothe himself. Hallucinations and delusions were rife. His ability to generate language was all but gone, and he was resigned to communicating through rudimentary head movements and rare inarticulate utterances whenever he could muster the energy.

Soon after turning forty-two years old, Michael Corke died of a rare, genetically inherited disorder called fatal familial insomnia (FFI). There are no treatments for this disorder, and there are no cures. Every patient diagnosed with the disorder has died within ten months, some sooner. It is one of the most mysterious conditions in the annals of medicine, and it has taught us a shocking lesson: a lack of sleep will kill a human being.

Then there was the tragic death of a Bank of America intern, Moritz Erhardt, who suffered a life-ending epileptic seizure after acute sleep deprivation from the work overload that is so endemic and expected in that profession, especially from the juniors in such organizations. Nevertheless, these are simply case studies, and they are hard to validate and scientifically verify after the fact.

any adult sleeping an average of 6.75 hours a night would be predicted to live only into their early sixties: very close to the median life span of these tribespeople.

When food becomes scarce, sleep becomes scarce, as animals try to stay awake longer to forage.

Epidemiological evidence suggests that the relationship between sleep and mortality risk is not linear, such that the more and more sleep you get, the lower and lower your death risk (and vice versa). Rather, there is an upward hook in death risk once the average sleep amount passes nine hours, resulting in a tilted backward J shape:

should you explore those studies in detail, you learn that the causes of death in individuals sleeping nine hours or longer include infection (e.g., pneumonia) and immune-activating cancers. We know from evidence discussed earlier in the book that sickness, especially sickness that activates a powerful immune response, activates more sleep. Ergo, the sickest individuals should be sleeping longer to battle back against illness using the suite of health tools sleep has on offer.

Keep in mind that food, oxygen, and water are no different, and they, too, have a reverse-J-shape relationship with mortality risk. Eating to excess shortens life.

There is an adaptive balance to be struck between wakefulness and sleep. In humans, that appears to be around sixteen hours of total wakefulness, and around eight hours of total sleep, for an average adult.

(1) constant electric light as well as LED light, (2) regularized temperature, (3) caffeine (discussed in chapter 2), (4) alcohol, and (5) a legacy of punching time cards.

The loss of daylight informs our suprachiasmatic nucleus that nighttime is now in session; time to release the brake pedal on our pineal gland, allowing it to unleash vast quantities of melatonin that signal to our brains and bodies that darkness has arrived and it is time for bed. Appropriately scheduled tiredness, followed by sleep, would normally occur several hours after dusk across our human collective. Electric light put an end to this natural order of things.

To contextualize that, let’s say you are reading this book at eleven p.m. in New York City, having been surrounded by electric light all evening. Your bedside clock may be registering eleven p.m., but the omnipresence of artificial light has paused the internal tick-tocking of time by hindering the release of melatonin. Biologically speaking, you’ve been dragged westward across the continent to the internal equivalent of Chicago time (ten p.m.), or even San Francisco time (eight p.m.).

What of a petite bedside lamp? How much can that really influence your suprachiasmatic nucleus? A lot, it turns out. Even a hint of dim light—8 to 10 lux—has been shown to delay the release of nighttime melatonin in humans. The feeblest of bedside lamps pumps out twice as much: anywhere from 20 to 80 lux. A subtly lit living room, where most people reside in the hours before bed, will hum at around 200 lux. Despite being just 1 to 2 percent of the strength of daylight, this ambient level of incandescent home lighting can have 50 percent of the melatonin-suppressing influence within the brain.

As a consequence, evening blue LED light has twice the harmful impact on nighttime melatonin suppression than the warm, yellow light from old incandescent bulbs, even when their lux intensities are matched.

A recent survey of over fifteen hundred American adults found that 90 percent of individuals regularly used some form of portable electronic device sixty minutes or less before bedtime. It has a very real impact on your melatonin release, and thus ability to time the onset of sleep.

One of the earliest studies found that using an iPad—an electronic tablet enriched with blue LED light—for two hours prior to bed blocked the otherwise rising levels of melatonin by a significant 23 percent. A more recent report took the story several concerning steps further.

But did reading on the iPad actually change sleep quantity/quality above and beyond the timing of melatonin? It did, in three concerning ways. First, individuals lost significant amounts of REM sleep following iPad reading. Second, the research subjects felt less rested and sleepier throughout the day following iPad use at night. Third was a lingering aftereffect, with participants suffering a ninety-minute lag in their evening rising melatonin levels for several days after iPad use ceased—almost like a digital hangover effect. Using LED devices at night impacts our natural sleep rhythms, the quality of our sleep, and how alert we feel during the day. The societal and public health ramifications, discussed in the penultimate chapter, are not small. I, like many of you, have seen young children using electronic tablets at every opportunity throughout the day . . . and evening. The devices are a wonderful piece of technology. They enrich the lives and education of our youth. But such technology is also enriching their eyes and brains with powerful blue light that has a damaging effect on sleep—the sleep that young, developing brains so desperately need in order to flourish.I Due to its omnipresence, solutions for limiting exposure to artificial evening light are challenging. A good start is to create lowered, dim light in the rooms where you spend your evening hours. Avoid powerful overhead lights. Mood lighting is the order of the night. Some committed individuals will even...

Short of prescription sleeping pills, the most misunderstood of all “sleep aids” is alcohol. Many individuals believe alcohol helps them to fall asleep more easily, or even offers sounder sleep throughout the night. Both are resolutely untrue.

I am very deliberately avoiding the term “sleep,” however, because sedation is not sleep. Alcohol sedates you out of wakefulness, but it does not induce natural sleep. The electrical brainwave state you enter via alcohol is not that of natural sleep; rather, it is akin to a light form of anesthesia.

More than its artificial sedating influence, alcohol dismantles an individual’s sleep in an additional two ways. First, alcohol fragments sleep, littering the night with brief awakenings. Alcohol-infused sleep is therefore not continuous and, as a result, not restorative. Unfortunately, most of these nighttime awakenings go unnoticed by the sleeper since they don’t remember them. Individuals therefore fail to link alcohol consumption the night before with feelings of next-day exhaustion caused by the undetected sleep disruption sandwiched in between. Keep an eye out for that coincidental relationship in yourself and/or others. Second, alcohol is one of the most powerful suppressors of REM sleep that we know of.

People consuming even moderate amounts of alcohol in the afternoon and/or evening are thus depriving themselves of dream sleep. There is a sad and extreme demonstration of this fact observed in alcoholics who, when drinking, can show little in the way of any identifiable REM sleep. Going for such long stretches of time without dream sleep produces a tremendous buildup in, and backlog of, pressure to obtain REM sleep. So great, in fact, that it inflicts a frightening consequence upon these individuals: aggressive intrusions of dreaming while they are wide awake. The pent-up REM-sleep pressure erupts forcefully into waking consciousness, causing hallucinations, delusions, and gross disorientation. The technical term for this terrifying psychotic state is “delirium tremens.”II

In contrast, those who had their sleep laced with alcohol on the first night after learning suffered what can conservatively be described as partial amnesia seven days later, forgetting more than 50 percent of all that original knowledge. This fits well with evidence we discussed earlier: that of the brain’s non-negotiable requirement for sleep the first night after learning for the purposes of memory processing.

Framed practically, let’s say that you are a student cramming for an exam on Monday. Diligently, you study all of the previous Wednesday. Your friends beckon you to come out that night for drinks, but you know how important sleep is, so you decline. On Thursday, friends again ask you to grab a few drinks in the evening, but to be safe, you turn them down and sleep soundly a second night. Finally, Friday rolls around—now three nights after your learning session—and everyone is heading out for a party and drinks. Surely, after being so dedicated to slumber across the first two nights after learning, you can now cut loose, knowing those memories have been safely secured and fully processed within your memory banks. Sadly, not so. Even now, alcohol consumption will wash away much of that which you learned and can abstract by blocking your REM sleep. How long is it before those new memories are finally safe? We actually do not yet know, though we have studies under way that span many weeks.

The politically incorrect advice I would (of course never) give is this: go to the pub for a drink in the morning. That way, the alcohol will be out of your system before sleep. Glib advice aside, what is the recommendation when it comes to sleep and alcohol? It is hard not to sound puritanical, but the evidence is so strong regarding alcohol’s harmful effects on sleep that to do otherwise would be doing you, and the science, a disservice. Many people enjoy a glass of wine with dinner, even an aperitif thereafter. But it takes your liver and kidneys many hours to degrade and excrete that alcohol, even if you are an individual with fast-acting enzymes for ethanol decomposition. Nightly alcohol will disrupt your sleep, and the annoying advice of abstinence is the best, and most honest, I can offer.

To successfully initiate sleep, as described in chapter 2, your core temperature needs to decrease by 2 to 3 degrees Fahrenheit, or about 1 degree Celsius. For this reason, you will always find it easier to fall asleep in a room that is too cold than too hot, since a room that is too cold is at least dragging your brain and body in the correct (downward) temperature direction for sleep.

Environmental light and temperature therefore synergistically, though independently, dictate nightly melatonin levels and sculpt the ideal timing of sleep.

It is no evolutionary coincidence that we humans have developed the pre-bed ritual of splashing water on one of the most vascular parts of our bodies—our face, using one of the other highly vascular surfaces—our hands. You may think the feeling of being facially clean helps you sleep better, but facial cleanliness makes no difference to your slumber. The act itself does have sleep-inviting powers, however, as that water, warm or cold, helps dissipate heat from the surface of the skin as it evaporates, thereby cooling the inner body core.

The need to dump heat from our extremities is also the reason that you may occasionally stick your hands and feet out from underneath the bedcovers at night due to your core becoming too hot, usually without your knowing.

A bedroom temperature of around 65 degrees Fahrenheit (18.3°C) is ideal for the sleep of most people, assuming standard bedding and clothing. This surprises many, as it sounds just a little too cold for comfort.

Selectively warming the feet and hands by just a small amount (1°F, or about 0.5°C) caused a local swell of blood to these regions, thereby charming heat out of the body’s core, where it had been trapped. The result of all this ingenuity: sleep took hold of the participants in a significantly shorter time, allowing them to fall asleep 20 percent faster than was usual, even though these were already young, healthy, fast-sleeping individuals.

A luxury for many is to draw a hot bath in the evening and soak the body before bedtime. We feel it helps us fall asleep more quickly, which it can, but for the opposite reason most people imagine. You do not fall asleep faster because you are toasty and warm to the core. Instead, the hot bath invites blood to the surface of your skin, giving you that flushed appearance. When you get out of the bath, those dilated blood vessels on the surface quickly help radiate out inner heat, and your core body temperature plummets. Consequently, you fall asleep more quickly because your core is colder. Hot baths prior to bed can also induce 10 to 15 percent more deep NREM sleep in healthy adults.IV

Sleeping pills do not provide natural sleep, can damage health, and increase the risk of life-threatening diseases.