Why We Sleep: The New Science of Sleep and Dreams

by Matthew Walker

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.

Your biological circadian rhythm coordinates a drop in core body temperature as you near typical bedtime (figure 1), reaching its nadir, or low point, about two hours after sleep onset.

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

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

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.

Also be aware that de-caffeinated does not mean non-caffeinated. One cup of decaf usually contains 15 to 30 percent of the dose of a regular cup of coffee, which is far from caffeine-free. Should you drink three to four cups of decaf in the evening, it is just as damaging to your sleep as one regular cup of coffee.

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.

If you didn’t set an alarm clock, would you sleep past that time? (If so, you need more sleep than you are giving yourself.) Do you find yourself at your computer screen reading and then rereading (and perhaps rereading again) the same sentence? (This is often a sign of a fatigued, under-slept brain.) Do you sometimes forget what color the last few traffic lights were while driving? (Simple distraction is often the cause, but a lack of sleep is very much another culprit.)

Consider the last time you fell asleep on an airplane. When you woke up, you probably checked a clock to see how long you had been asleep. Why? Because your explicit tracking of time was ostensibly lost while you slept. It is this feeling of a time cavity that, in waking retrospect, makes you confident you’ve been asleep.

The first striking result was that the signature pattern of brain-cell firing that occurred as the rats were learning the maze subsequently reappeared during sleep, over and over again. That is, memories were being “replayed” at the level of brain-cell activity as the rats snoozed. The second, more striking finding was the speed of replay. During REM sleep, the memories were being replayed far more slowly: at just half or quarter the speed of that measured when the rats were awake and learning the maze. This slow neural recounting of the day’s events is the best evidence we have to date explaining our own protracted experience of time in human REM sleep.

Let’s say that you go to bed this evening at midnight. But instead of waking up at eight a.m., getting a full eight hours of sleep, you must wake up at six a.m. because of an early-morning meeting or because you are an athlete whose coach demands early-morning practices. What percent of sleep will you lose? The logical answer is 25 percent, since waking up at six a.m. will lop off two hours of sleep from what would otherwise be a normal eight hours. But that’s not entirely true. Since your brain desires most of its REM sleep in the last part of the night, which is to say the late-morning hours, you will lose 60 to 90 percent of all your REM sleep,

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).

The relationship between the size of the nervous system, the complexity of the nervous system, and total body mass appears to be a somewhat meaningful predictor, with increasing brain complexity relative to body size resulting in greater sleep amounts.

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.

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.

(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.

Sleep, especially REM sleep and the act of dreaming, is a tenable, yet underappreciated, factor underlying many elements that form our unique human ingenuity and accomplishments, just as much as language or tool use (indeed, there is even evidence that sleep causally shapes both these latter traits as well).

Indeed, by the end of the second trimester of development (approximately week 23 of pregnancy), the vast majority of the neural dials and switches required to produce NREM and REM sleep have been sculpted out and wired up. As a result of this mismatch, the fetus brain still generates formidable motor commands during REM sleep, except there is no paralysis to hold them back. Without restraint, those commands are freely translated into frenetic body movements, felt by the mother as acrobatic kicks and featherweight punches.

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

Most striking, the very last stop on the maturational journey was the tip of the frontal lobe, which enables rational thinking and critical decision-making. 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

Sadly, neither society nor our parental attitudes are well designed to appreciate or accept that teenagers need more sleep than adults, and that they are biologically wired to obtain that sleep at a different time from their parents. It’s very understandable for parents to feel frustrated in this way, since they believe that their teenager’s sleep patterns reflect a conscious choice and not a biological edict. But non-volitional, non-negotiable, and strongly biological they are. We parents would be wise to accept this fact, and to embrace it, encourage it, and praise it, lest we wish our own children to suffer developmental brain abnormalities or force a raised risk of mental illness upon them.

These three key changes are: (1) reduced quantity/quality, (2) reduced sleep efficiency, and (3) disrupted timing of sleep.

If you spent eight hours in bed, and slept for all eight of those hours, your sleep efficiency would be 100 percent. If you slept just four of those eight hours, your sleep efficiency would be 50 percent.

Any individual, no matter what age, will exhibit physical ailments, mental health instability, reduced alertness, and impaired memory if their sleep is chronically disrupted.

The third sleep change with advanced age is that of circadian timing. In sharp contrast to adolescents, seniors commonly experience a regression in sleep timing, leading to earlier and earlier bedtimes. The cause is an earlier evening release and peak of melatonin as we get older, instructing an earlier start time for sleep. Restaurants in retirement communities have long known of this age-related shift in bedtime preference, epitomized (and accommodated) by the “early-bird special.”

Older adults may also wish to consult with their doctor about taking melatonin in the evening. Unlike young or middle-age adults, where melatonin has not proved efficacious for helping sleep beyond the circumstance of jet lag, prescription melatonin has been shown to help boost the otherwise blunted circadian and associated melatonin rhythm in the elderly, reducing the time taken to fall asleep and improving self-reported sleep quality and morning alertness.fn21

It was a saddening confirmation of my theory: the parts of our brain that ignite healthy deep sleep at night are the very same areas that degenerate, or atrophy, earliest and most severely as we age.

Poor memory and poor sleep in old age are therefore not coincidental, but rather significantly interrelated.

Perhaps, with less highfalutin language, your mother offered similar advice, extolling the benefits of sleep in healing emotional wounds, helping you learn and remember, gifting you with solutions to challenging problems, and preventing sickness and infection. Science, it seems, has simply been evidential, providing proof of everything your mother, and apparently Shakespeare, knew about the wonders of sleep.

Having observed that sleep restores the brain’s capacity for learning, making room for new memories, we went in search of exactly what it was about sleep that transacted the restoration benefit. Analyzing the electrical brainwaves of those in the nap group brought our answer. The memory refreshment was related to lighter, stage 2 NREM sleep, and specifically the short, powerful bursts of electrical activity called sleep spindles, noted in chapter 3.

We conducted the study, and that is precisely what we found: the fewer the number of spindles an elderly brain produced on a particular night, the lower the learning capacity of that older individual the next day, making it more difficult for them to memorize the list of facts we presented.

Before having slept, participants were fetching memories from the short-term storage site of the hippocampus—that temporary warehouse, which is a vulnerable place to live for any long duration of time if you are a new memory. But things looked very different by the next morning. The memories had moved.

participants were now retrieving that same information from the neocortex, which sits at the top of the brain—a region that serves as the long-term storage site for fact-based

Applying stimulation during REM sleep, or during wakefulness across the day, did not offer similar memory advantages. Only stimulation during NREM sleep, in synchronous time with the brain’s own slow mantra rhythm, leveraged a memory improvement.

Nevertheless, the findings offer a scientific explanation for the ancient practice of rocking a child back and forth in one’s arms, or in a crib, inducing a deep sleep.

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.” Participants who did not sleep showed no such impressive parsing and differential saving of the memories.fn9

The more of those spindles a participant had during a nap, the greater the efficiency with which they strengthened items tagged for remembering and actively eliminated those designated for forgetting.

The idea may invoke the premise of the Oscar-winning movie Eternal Sunshine of the Spotless Mind, in which individuals can have unwanted memories deleted by a special brain-scanning machine.

He said he was intrigued by my description of sleep as an active brain state, one in which we may review and even strengthen those things we have previously learned.

“As a pianist,” he said, “I have an experience that seems far too frequent to be chance. I will be practicing a particular piece, even late into the evening, and I cannot seem to master it. Often, I make the same mistake at the same place in a particular movement. I go to bed frustrated. But when I wake up the next morning and sit back down at the piano, I can just play, perfectly.”

When I tested participants after a night of sleep, however, my ears heard something very different. I knew what was happening even before I analyzed the data: mastery.

Their typing, post-sleep, was now fluid and unbroken.

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.

In ways your waking brain would never attempt, the sleeping brain fuses together disparate sets of knowledge that foster impressive problem-solving abilities. If you ponder the type of conscious experience such outlandish memory blending would produce, you may not be surprised to learn that it happens during the dreaming state—REM sleep.

Slowness was not the most sensitive signature of sleepiness, entirely missed responses were.

Similarly problematic is baseline resetting. With chronic sleep restriction over months or years, an individual will actually acclimate to their impaired performance, lower alertness, and reduced energy levels. That low-level exhaustion becomes their accepted norm, or baseline. Individuals fail to recognize how their perennial state of sleep deficiency has come to compromise their mental aptitude and physical vitality, including the slow accumulation of ill health. A link between the former and latter is rarely made in their mind. Based on epidemiological studies of average sleep time, millions of individuals unwittingly spend years of their life in a sub-optimal state of psychological and physiological functioning, never maximizing their potential of mind or body due to their blind persistence in sleeping too little.

Each hour of sleep lost vastly amplifies that crash likelihood, rather than incrementally nudging it up.

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.