Matthew Walker, PhD is a world renowned sleep researcher who shares some astonishing, well-kept secrets in his 2017 New York Times Bestseller, Why We Sleep: Unlocking The Power Of Sleep And Dreams. In this fascinating book, Walker starts with the basics such as what sleep actually is, how widespread sleep is in the animal kingdom, and why it is needed. He goes on to explain what the various stages of sleep do for us and what happens when they are missed. He closes with an enlightening discussion of how our societal norms help and harm us with respect to sleep. To accomplish this, he breaks the book into four main parts including:
- This Thing Called Sleep
- Why Should You Sleep?
- How and Why We Dream
- From Sleeping Pills to Society Transformed
He draws a startling and compelling conclusion at the end. He also provides a helpful appendix drawn from the NIH Guide to Healthy Sleep with 12 common sense tips.
Walker’s book packs insights from decades of research and over three million years of evolution into under 350 pages, and it’s worth reading every word. He is a masterful storyteller citing numerous studies and expanded material via footnotes throughout the book to support his assertions. In format, while the index is extensive, the one thing missing is a consolidated bibliography that would help readers refer back to those informative studies.
In Part 1, Walker provides an intensive look at what makes up sleep. All animals sleep although the amounts vary dramatically from species to species. As an example, he states:
“Elephants need half as much sleep as humans, requiring just four hours of slumber each day. Tigers and lions devour fifteen hours of daily sleep. The brown bat outperforms all other mammals, being awake for just five hours each day while sleeping nineteen hours.”
He further demonstrates that the amount of sleep needed isn’t governed by whether an animal is a predator or a prey, or even by size. He gives more examples of the disparities among closely related species such as the differences between squirrels and degus. Both are rodents, but the former sleeps 15.9 hours while the latter only sleeps for 7.7 hours per day. Ultimately as more studies accumulated over the years, sleep scientists have determined that at best, the amount of sleep a species needs is based on numerous factors from dietary type to nervous system complexity and social frameworks.
Where humans fall in this spectrum depends on their age to some degree, although not as much as one might suspect. It is common knowledge that children need more sleep than adults, but what is less well-known is that once adulthood is reached, the need for sleep remains constant although the timing may shift slightly. In other words, the common belief that older adults need less sleep is completely wrong. With very few exceptions, adults need between 7 and 9 hours of sleep per night and failure to get it can manifest in myriad ways, none of them beneficial.
Walker carefully describes the different sleep states which form cycles throughout the night with the proportion of each stage of sleep shifting in each cycle. It’s widely known that three basic sleep states exist: Wake (yes, the absence of sleep is technically a sleep state), REM, and NREM. REM is short for rapid eye movement, something that is indicative of dreaming. By extension, NREM means non-rapid eye movement sleep or non-dreaming sleep. With that in mind, the spectrum of sleep states includes Wake, REM, and NREM stages 1-4. The stages of NREM sleep vary in depth with each NREM stage becoming progressively deeper sleep. A human sleep cycle nominally lasts about 90 minutes, and the average adult completes five such cycles through a representative night. The interesting revelation that has been shown through myriad studies is that the proportion of those stages shifts from a majority of NREM stages 3 and 4 in the early sleep cycles to a majority of REM sleep in the later sleep cycles.
Before leaving the first part of the book, Walker provides an extensive look at the sleep patterns that humans need throughout their lives. As embryos and infants, humans sleep almost as much as the brown bat, and most of that is REM sleep. As children grow, they start sleeping a little less, though by the time they reach adolescence, they still require somewhere around 9 or 10 hours per night. The problem for many families is that adolescent circadian rhythms shift later making their sleep start and stop times somewhat incompatible with societal norms, though Walker comes back to that in Part 4 when he discusses some of the societal impacts on sleep. By the time we reach adulthood, we’ve normally settled into a 7-9 hour sleep need. The reason that some might assume that older adults need less sleep is because aging can create various difficulties in both entering sleep and maintaining it, but the need still remains.
So what does sleep accomplish? Far more than initially thought, and it’s not dependent just on the amount of sleep one gets but also on sleep quality including the percentages of each stage of sleep. In Part 2 of the book, Walker explains what sleep accomplishes and how little sleep loss it takes to develop a sleep debt that can never be repaid. Of particular note though is that the benefits of each phase of sleep differ, and those benefits were recognized, unscientifically, by Shakespeare of all people. In Macbeth, act two, scene two, Macbeth himself states:
“Sleep that knits up the ravell’d sleeve of care,
The death of each day’s life, sore labour’s bath,
Balm of hurt minds, great nature’s second course,
Chief nourisher in life’s feast, …”
But what does sleep nourish, and how? First stop, the brain. Walker discusses at length the benefits that the brain derives from sleep from memory, to better comprehension and attention, to emotional wellness. He stresses that not all phases of sleep perform the same functions, yet all are essential for brain health. He delves deeply into how this is accomplished and which parts of sleep are needed for which brain functions, but suffice it to say that a deficit in any stage of sleep can cause some kind of cognitive deterioration.
It turns out that for learning sleep is crucial not just after learning something new, but it is also necessary before to condition the brain to accept that new information. This is true not just for intellectual challenges but for physical ones as well. In the context of mastering physical skills such as those of an athlete or a musician, there is a common belief that one develops something called muscle memory. Walker refutes that entirely as muscles have no memory. That thing that people call muscle memory is really the brain learning to provide the correct impulses to command the nerves to stimulate the appropriate muscles for a given task. Walker shows that a crucial piece of that learning involves sleep.
As an example, a musician can practice time and time again, but it’s not until after sleeping that they master the task at hand. While working on a challenging piece of music for hours at a time they can seemingly get stuck at a certain point, yet magically after a good night’s sleep, they’ll resume their practice to find that their proficiency has lunged forward since their last session. The same thing happens with athletes, and this phenomenon isn’t limited to physical tasks. It also applies when someone is faced with an intellectual challenge such as pondering a difficult problem.
Walker discusses many of the mechanics of how this magic happens explaining which parts of the brain and body are affected during various stages of sleep. Beyond significant cognitive deterioration, the body suffers other physical affects from sleep deprivation. He cites numerous studies that have shown a direct correlation between sleep deprivation and damage to the cardiovascular system, metabolic functions, immune efficiency, and reproductive health. In other words, the entire body is damaged from too little sleep in the correct proportions. Although written before the current pandemic, Walker shows that sleep plays a direct role regarding the efficacy of vaccinations, so be sure to get a good night’s sleep prior to receiving any vaccination whether it’s a flu shot or to protect against COVID-19.
In Part 3, Walker discusses the intricacies of dreaming, what it is, what it is not, and how it affects us all. Dreams have been shown to spark creativity in many ways, freeing us from the constraints of the real world. In that way, dreams allow our brains to explore connections we cannot easily make while we retain rational control. More importantly, beyond those creative insights, Walker terms dreams “the soothing balm” with respect to emotional health. The chemical changes involved during REM sleep are key to this soothing effect, and they reduce the emotional pain of traumatic experiences.
It’s not time so much as sleep that heals emotional wounds. In fact, dreams act in seemingly contradictory ways to help us remember salient experiences and information while simultaneously helping us to forget that which carries an emotional sting. Walker discusses later studies that further identify the types of dreams that are necessary to achieve emotional healing. Dreams about the emotional themes and sentiments of experienced trauma are the cure. That mechanism can break down in some people after a significant trauma resulting in PTSD which is characterized by an inability to forget or dull the emotional toll from their traumatic events. In PTSD, there’s a drastic shift in brain chemistry that both increases the frequency of nightmares while blocking the healing effect that dreaming should have.
Beyond discussing dreams’ healing and creative effects, Walker also introduces the fascinating realm of lucid dreaming, i.e. controlling one’s dreams which is something that less than 20% of the population can do. He describes recent studies using MRI scans of study participants definitively proving this remarkable ability exists although the question of whether this is a beneficial or harmful ability is still up for debate.
In Part 4, Walker turns his attention to societal influences on sleep. He comes back to his earlier discussion on how much sleep children need and how adolescents’ circadian rhythms shift later putting their sleep needs in direct opposition to early school scheduling. He strongly supports moving school later to improve children’s learning potential and overall health and provides irrefutable scientific data to support that position.
In this section, Walker also discusses several sleep disorders. After acknowledging that there are numerous disorders that affect human sleep, he focuses on the science of just a few, namely somnambulism, insomnia, narcolepsy, and fatal familial insomnia. He uses this discussion not so much to recommend treatment or cures, but more to further explain what happens during sleep along with the consequences when that goes awry. The stories of how these disorders manifest is both riveting and in some cases terrifying.
Beyond legitimate clinical disorders that can interrupt or prevent sleep, several societal norms put sleep in jeopardy as well. One of the most damaging is the attitude that to succeed in one’s career, they need to sacrifice sleep to spend more time at work in environments where the most sleep-deprived wins. The sad reality is that by sacrificing sleep, people hurt their performance and their health. It would be in corporate America’s best interests to reduce work hours and prioritize adequate sleep which would improve overall productivity and efficiency in addition to employee health.
Another contributor to bad sleep is reliance on chemical sleep aids which do not promote sleep but instead induce sedation. Sedation is not sleep, and one does not benefit from the various stages of sleep that serve to heal and recharge the brain and the body when they are sedated. He stresses that even short term use has deleterious effects. In a startling chart, he shows that those taking as little as ½ – 18 sleeping pills in a year were 3.6 times more likely to die than somebody not taking any sleeping pills. Taking 18-132 sleeping pills in a year makes someone 4.3 times more likely to die, and taking over 132 sleeping pills in a year increases that risk by 5.3 times.
Beyond sleeping pills, Walker discusses at length the negative effects of alcohol on sleep and memory. Sleeping under the effects of alcohol destroys the quality of sleep. While someone may fall asleep marginally faster under the influence of alcohol, they’ll have far more trouble maintaining sleep and will suffer far more fragmented and low quality sleep not reaching the stages necessary for the positive regeneration that sleep provides. As a result, alcohol’s impediments on learning and memory are far more dramatic than once thought. In studies to determine the effects of alcohol on learning new material, not only were people only able to retain less than 50% of that new material compared to an alcohol-free control group the night following learning, there were even significant losses in memory when alcohol was introduced before sleep three days after learning. The upshot is that if one wishes to imbibe, they should ensure that any alcohol is completely out of their system before sleeping.
Walker provides some insights into how we might change ourselves and society to align with better sleep practices, but acknowledges that in industrialized nations it is a challenge. Still, following the examples of a few forward thinking companies like Nike and Google could help. In addition to offering flexible schedules that allow employees to better align their work schedules with their circadian rhythms, a few leading companies also encourage naps to support creativity and wellness while at work. They are working hard to change attitudes toward sleep.
The inescapable conclusion is that sleep is the secret to health and success despite our developed world seemingly challenging what over three million years of evolution has built.
Finally, here’s the condensed list of those tips for getting healthy sleep. This list is expanded in the Why We Sleep Appendix. The expanded guide can also be found in the NIH Guide to Healthy Sleep starting on page 27:
- Stick to a sleep schedule.
- Exercise is great, but not too late in the day.
- Avoid caffeine and nicotine.
- Avoid alcoholic drinks before bed.
- Avoid large meals and beverages late at night.
- If possible, avoid medications that delay or disrupt your sleep.
- Don’t take naps after 3 p.m.
- Relax before bed.
- Take a hot bath before bed.
- Have a good sleeping environment.
- Have the right sunlight exposure.
- Don’t lie in bed awake.
- (Not in the appendix) See a doctor if you continue to have trouble sleeping.
Why We Sleep by Matthew Walker, PhD