top of page


Screen Shot 2019-12-08 at 3.45.27 PM.png

Book:  Why We Sleep

Author:  Matthew Walker

Purchase:  PrinteBookAudiobook

Citation:  Walker, M. (2017). Why we sleep : unlocking the power of sleep and dreams. New York, NY: Scribner, an imprint of Simon & Schuster, Inc.

Three Big Takeaways:
  1. There are two main factors that determine when you want to sleep and when you want to be awake. The first factor is a "circadian rhythm" - an internal twenty-four-hour clock located deep within your brain. The clock creates a cycling day/night rhythm that makes you feel tired or alert at regular times during the night and day. The second factor is a chemical substance that builds up in your brain and creates "sleep pressure." The longer you've been awake, the more that chemical sleep pressure accumulates, and the sleepier you feel. (pg. 13)

  2. We flip-flop back and forth between NREM and REM sleep throughout the night every 90 minutes. In the first half of the night, the vast majority of our ninety-minute cycles are NREM sleep. But as we transition through into the second half of the night, most of the time is dominated by REM sleep.. A key function of NREM sleep, which predominates early in the night, is to do the work of weeding out and removing any unnecessary memory and brain storage space. In contrast, the dreaming stage of REM sleep, which prevails later in the night, plays a role in strengthening memory and refining brain storage space. (pg. 44)

  3. Any individual, no matter what age, will exhibit physical ailments, mental health instability reduced alertness, and impaired memory if their sleep is chronically disrupted. Alternately, sleep enhances your memory, makes you more creative, makes you more attractive, keeps you slim, lowers food cravings, protects you from cancer, wards of colds and the flu, lowers your risk of heart attack and stroke, and makes you feel happier, less depressed, and less anxious. (pg. 97)


Other Key Ideas:

Drowsy driving is the cause of hundreds of thousands of traffic accidents and fatalities each year. Tragically, one person dies in a traffic accident every hour in the United State due to a fatigue-related error. Vehicular accidents caused by drowsy driving exceed those caused by alcohol and drugs combined. There are two main culprits of drowsy-driving accidents. The first is people completely falling asleep at the wheel. This happens infrequently, however, and usually requires an individual to be acutely sleep-deprived (having gone without sleeping for twenty-plus hours). The second, more common lapse in concentration, is called a microsleep. These last for just a few seconds, during which time the eyelids will either or partially close. They are usually suffered by individuals who are chronically sleep restricted, defined as getting less than seven hours of sleep a night on a routine basis. Ten days of six hours of sleep a night impairs driving as much as it would be to go without sleep for twenty-four hours straight. After being awake for nineteen hours, people who were sleep-deprived are as cognitively impaired as those who are legally drunk. Said another way, if you wake up at seven am 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 am you are as cognitively impaired in your ability to drive as a legally drunk driver. Finally, 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 and you are 11.5 times more likely to be involved in a car accident. (pg. 5 & 134)

Some people are "morning types" and make up about 40 percent of the populace. Others are "evening types" and account for approximately 30 percent of the population. The remaining 30 percent of people lie somewhere in between. These "types" are known as their chronotype, and are largely determined by genetics. If you are a night owl, it's likely that one (or both) of your parents is a night owl. Sadly, society treats night owls rather unfairly. First is the label of being lazy, based on a night owl's want to wake up later in the day. Others will chastise night owls on the erroneous assumption that such preferences are a choice. However, night owls are not owls by choice. Also, there is an unlevel playing field in society's work scheduling, which is strongly biased toward early start times that punish owls and favor larks. Job performance of night owls is far less optimal in the mornings, and many night owls are sleep-deprived - forced to wake up early but unable to fall asleep until later in the evening. (pg. 21)

Melatonin is released into your bloodstream after dusk, essentially telling your body "it's dark!" At this moment, we have been told that it's nighttime and a biological command for sleep has begun. And while many people take melatonin pills, there may be little, if any, quality in the pill (other than the placebo effect...which is real). And keep in mind that over-the-counter melatonin is not commonly regulated by governing bodies. (pg. 23)

For every day you are in a different time zone, your body can only adjust your internal clock by about an hour. Therefore, it will take you eight days to adjust being in Europe which is eight hours ahead. Sadly, after taking eight days to adjust to the new time zone, odds are you probably are due to return home. Similarly, it is more difficult to acclimate to a new time zone when traveling east than west. Falling asleep earlier (when you travel east) is more difficult than falling asleep later (when you travel west). (pg. 25)

Your 24 hour circadian rhythm is the first of the two factors determining wake and sleep. The second is sleep pressure. At this very moment, a chemical called adenosine is building up in your brain. It will continue to increase in concentration with every waking minute that elapses. The longer you are awake, the more adenosine will accumulate. Think of adenosine as a chemical barometer that continuously registers the amount of elapsed time since you woke up this morning. When adenosine concentrations peak, an irresistible urge for slumber will take hold. It happens to most people after twelve to sixteen hours of being awake. (pg. 27)

You can, however, artificially mask the sleep signal of adenosine by using a chemical that makes you feel more alert and awake; caffeine. Levels of caffeine peak approximately thirty minutes after being consumed. What can be problematic, however, is the persistence of caffeine in your system. Caffeine has an average half-life of seven hours. Let's say you have a cup of coffee at 7:30pm. This means by 1:30am, 50 percent of that caffeine may still be active and circulating throughout your brain tissue. Sleep will not come easily or be smooth throughout the night as your brain continues its battle against the opposing force of caffeine. Also, beware that decaffeinated does not mean non-caffeinated. One cup of decaf usually contains 15 to 30 percent of the dose of a regular cup of coffee. Finally, the older we are, the longer it takes our brain and body to remove caffeine. (pg. 28)

What happens to all of the accumulated adenosine once you do fall asleep? During sleep, a mass evacuation gets under way as the brain has the chance to remove the day's adenosine. Across the night, sleep lifts the weight of sleep pressure, lightening the adenosine load. After approximately eight hours of healthy sleep, the adenosine purge is done. However, when you don't get enough sleep, one consequence is that adenosine concentrations remain high. Like an outstanding debt, come the morning, some quantity on yesterday's adenosine remains. You then carry that outstanding sleepiness balance throughout the following day. And like a loan in arrears, this sleep debt will continue to climb. The debt will roll over into the next cycle, producing a condition of prolonged, chronic sleep deprivation from one day to another. (pg. 33)

When you pull an "all-nighter" the next day you will go through times when you feel sleepy and other times when you feel more alert. This is because the circadian rhythm cycles on, oblivious to your ongoing lack of sleep. You might actually feel pretty awake during the middle of the day, but by the time night number two rolls around the desire to sleep will be almost irresistible. (pg. 33)

Most people in developed nations take a long, single sleep at night. However, visit cultures untouched by electricity and you see something different. Not only do they have the long slumber at night, they also have a 30 to 60 minute nap in the afternoon. This is deeply biological, as all humans have a genetically hardwired dip in alertness that occurs in the mid-afternoon hours. This brief descent from high-degree wakefulness reflects an innate drive to be asleep and napping in the afternoon, and not working. Data indicates that regular napping results in a variety of positive results, including better health and longer life-spans. (pg. 68)

REM sleep allows us to make more intelligent decisions and helps us to regulate our emotions each day. Emotional IQ depends on getting sufficient REM sleep night after night. REM sleep also fuels creativity. While NREM sleep helps transfer and make safe newly learned information into long-term storage inside the brain, REM sleep takes these freshly minted memories and helps spark new creative insights. (pg. 74)

Asking your teenage son or daughter to go to bed and fall asleep at 10:00pm is the circadian equivalent of asking you, their parent, to go to sleep at seven or eight pm. No matter how loud you enunciate the order, and no matter how much that teenager truly wishes to obey your instructions, the circadian rhythm of a teenager will not be miraculously coaxed into a change. Furthermore, asking that same teenager to wake up at seven the next morning and function with intellect and good mood is the equivalent of asking you, their parent, to do the same at four or five am. 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 than their parents. Instead of believing this is a conscious choice and not a biological edict, we would be wise to accept this fact, embrace it, and encourage it. (pg. 93)

Sleep has proven itself time and again as a memory aid: both before learning, to prepare your brain for initially making new memories, and after learning, to cement those memories and prevent forgetting. Sleep helps clear out your short term information repository with plentiful free space. Overall, a memory retention benefit between 20 and 40 percent is offered by sleeping, compared to being awake. For fact-based, textbook-like memory, early night sleep in the NREM stage will provide significant memory retention. Even daytime naps as short as twenty minutes can offer a memory consolidation advantage. 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, you awake the next morning able to locate and retrieve those once unavailable memories with ease and precision. Your brain will continue to improve memory in the absence of any other learning during the day. This "offline" learning occurs across a period of sleep, and not across equivalent time periods spent awake. (pg. 107)

Millions of people unwittingly spend years of their life in a sub-optimal state of psychological and physiological functioning, never maximizing their potential due to their blind persistence in sleeping too little. Sixty years of scientific research prevent me from accepting anyone who tells me that he or she can "get by on just four or five hours of sleep a night just fine." (pg. 137)

The "recycle" rate of a human being is around sixteen hours. After sixteen hours of being awake, the brain begins to fail. 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. "Catching up on the weekend" is 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. (pg. 140)

If you notice yourself feeling drowsy while driving, or actually falling asleep at the wheel, stop for the night. If you really must keep going, then pull off the road into a safe spot for a short time. Take a brief nap (twenty or thirty minutes). When you wake up, do NOT start driving as you may be suffering from sleep inertia. Wait for another twenty or thirty minutes, perhaps having a cup of coffee, and only then start driving again. (pg. 142)

When short sleeping, the very same individuals eat 300 calories more each day compared to when they get a full night of sleep. Add that over the could of a working year, and you have consumed 70,000 extra calories. Based on caloric estimates, that would cause 10 to 15 pounds of weight gain a year, each and every year. (pg. 173)

The less sleep an individual is getting in the week before facing the active common cold virus, the more likely it is that they will be infected and catch a cold. In those sleeping sleeping five hours on average, the infection rate is 50 percent. In those sleeping seven hours or more a night the week prior, the infection rate is just 18 percent. (pg. 182)

Better REM sleep quality at night provides superior comprehension of the social world the following day. Furthermore, REM sleep and the act of dreaming result in intelligent information processing that inspires creativity and promotes problem solving. (pg. 208)

All of us will experience difficulty sleeping every now and then, which may last just one night or several. That is normal. Insomnia is specifically for those people who experience sleeping for several months at a time.  Approximately one out of every nine people will meet this strict clinical criteria for insomnia, which translates to 40 million Americans. The two most common triggers for chronic insomnia are worry and anxiety. (pg. 242)

Artificial evening light - most notably through LED-powered laptop screens, smartphones, and tablets - delays the release of melatonin, and makes it considerably less likely that you'll be able to fall asleep at a reasonable time. Approximately 90 percent of American adults regularly use some form of portable electronic device sixty minutes or less before bedtime. Compared to reading a printed book, reading on an iPad suppresses melatonin release by over 50 percent at night, delaying the rise of melatonin by up to three hours. Not only do people have a harder time falling asleep, the quality of their sleep is also reduced. (pg. 268)

Alcohol fragments sleep, littering the night with brief awakenings. Most people do not remember these awakenings, but it does cause next-day exhaustion. Also, 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 all-important dream sleep. (pg. 271)

A bedroom temperature of around 65 degrees is ideal for the sleep of most people. This surprises many, as it sounds just a little too cold for comfort. It is because of this that most of us fall into a category setting of a controlled bedroom temperature that is too high: between 70 and 72 degrees. Sleep clinicians treating insomnia patients will advise them to drop their current thermostat by 3 to 5 degrees. Regardless, you will always find it easier to fall asleep in a room that is too cold than too hot. (pg. 275)

Never lie awake in bed for a significant time period; rather, get out of bed and do something quiet and relaxing until the urge to sleep returns. If you find yourself awake after staying in bed for more than twenty minutes or if you are starting to feel anxious or worried, get up and do some relaxing activities until you feel sleepy. The anxiety of not being able to sleep can make it harder to fall asleep. (pg. 291)

Going to bed and waking up at the same time no matter what day it is is perhaps the single most effective way of helping improve your sleep. (pg. 293)

It is clear that a sedentary life is one that does not help with sound sleep, and all of us should try to engage in some degree of regular exercise to help maintain not only the fitness of our bodies but also the quality and quantity of our sleep. (pg. 294)

A study found that insufficient sleep cost almost $2,000 per year in lost productivity. While this may sound trivial, multiply that times the number of employees you have (250) and this totals up to half a million dollars in lost productivity. Those individuals who have obtained less sleep in the preceding days are the employees who will consistently have less creativity and will select less challenging problems, generating fewer solutions. (pg. 298)

Nike and Google have both allowed employees to time their daily work hours to match their individual circadian rhythms. The change in mind-set is so radical that these corporations allow workers to sleep on the job. Littered throughout their corporate headquarters are dedicated "relaxation" rooms with places to take naps. Such changes reflect a marked departure from the days when employees found catnapping on the clock were disciplined or fired. Sadly, most CEOs and managers still reject the importance of a well-slept employee. (pg. 304)

More than 80 percent of public high schools in the US begin before 8:15am, with 50 percent starting before 7:20am, meaning that some kids get picked up on the bus close to 5:45am. It is the lack of REM sleep - that critical stage occurring in the final hours of sleep that we strip from our children and teenagers by way of early school start times - that creates the difference between a stable and unstable mental state. Numerous school districts have shifted the start of schools to a later hour and their students experienced significantly higher academic achievement outputs. Furthermore, school districts are reporting a significant reduction in morning traffic accidents. (pg. 308)

An added reason for making sleep a top priority in the education and lives of our children concerns the link between sleep deficiency and the epidemic of ADHD. Children with this diagnosis are irritable, moodier, more distractible, and unfocused on learning during the day. If you strip away the label of ADHD, these symptoms are nearly identical to those caused by a lack of sleep. Take an under-slept child to a doctor and describe these symptoms without mentioning the lack of sleep, which is not uncommon, and the doctor very well may diagnose them with ADHD. Based on recent surveys, we estimate that more than 50 percent of all children with an ADHD diagnosis actually have a sleep disorder. (pg. 314)

bottom of page