“Scientists have discovered a revolutionary new treatment that makes you live longer. It enhances your memory and makes you more creative. It makes you look more attractive. It keeps you slim and lowers food cravings. It protects you from cancer and dementia. It wards off colds and flu. It lowers your risk of heart attacks and stroke, not to mention diabetes. You’ll even feel happier, less depressed, and less anxious. Are you interested?” – Matthew Walker
Why We Sleep: The New Science of Sleep and Dreams is a clarion call to reevaluate preconceived ideas about sleep and its importance. Packed with current scientific findings, it is anything but dry, discussing subjects from sleep deprivation’s effect on the development of serious diseases to testicle shrinkage, lucid dreaming and why schools should start later in the day.
Walker starts the book with a discussion of circadian rhythms and how the urge to sleep develops. Build up of a chemical called adenosine throughout the day increases sleep pressure until it is difficult to remain awake. During eight-hours of sleep the adenosine is broken down and the cycle continues. At the same time, another process called the circadian wake drive fluctuates all day, with sleep coming more easily when the sleep drive is high and the wake drive is low.
He uses the example of pulling an all-nighter to demonstrate how the fluctuating wake drive can allow a person to stay up for so long and even feel like they don’t need sleep, briefly. I’m sure many people have vague memories of sitting in a greasy spoon at 10 a.m. the morning after a big night, feeling a second wind of renewed energy, despite having been awake for over 24 hours. Eventually though, because the levels of adenosine, without sleep will not have been broken down, once the wake drive wavers, the urge for shut-eye will become unbearable.
The architecture of sleep is explained next. A typical eight-hours sleep from 11 p.m. until 7 a.m. is divided into five 90 minute cycles, fluctuating between two main types of sleep – rapid eye movement sleep (REM) and non-rapid eye movement sleep (NREM). Each cycle oscillates down through REM to NREM stages 1-4 (NREM stages 3-4 being the deepest sleep known as slow-wave sleep) and back up to REM, before the next cycle continues. NREM sleep dominates the first half of a nights sleep, with very little REM sleep. The balance however tips as we move into the second half of the night, with REM taking over the majority of the time in the cycles. Walker cautions us here that cutting sleep short by either going to bed late or waking up early we are losing a portion of either NREM or REM sleep, respectively.
NREM sleep is described as a process where brain waves slow down and become unified into a mantra-like throbbing rhythm throughout the brain. Walker also describes another feature called ‘sleep spindles’ a quick trill of brainwave activity, one of the functions of which are to protect the brain from being woken up by external noises.
Consolidation of short-term memories into long-term storage is one benefit of NREM sleep – Walker describes experiments which show that sufficient NREM sleep both before and after learning are important to consolidate memories. An experiment in which participants had electrodes placed on their head and a small electrical current applied transcranially during NREM sleep, is also described. The participants showed impressive memory improvements in the study, as the size of their slow-wave brainwaves and number of sleep spindles were increased by the stimulation. Increased amount of sleep spindles have been shown to make it easier to encode memories, even during a short nap.
More fascinating information is expounded when Walker talks about the ‘glymphamtic system’, the brain’s cleansing system which goes to work during NREM sleep. This system is always working at a low level but during NREM sleep it ramps up to ‘ten-to-twenty times its usual effluent expulsion from the brain’. Cerebrospinal fluid bathes the brain as glial cells shrink to allow the area around neurons to be cleaned – which Walker compares to building in a city ‘physically shrinking at night to allow municipal cleaning crews easy access’. This has implications for diseases such as Alzheimer’s as the waste that this process clears out includes some of the proteins linked to the disease including tau and amyloid proteins. This can lead to a feedback loop, for example, where amyloid protein build up causes problems in deep sleep generation, creating a vicious cycle as the disease progresses.
Dreaming is one of the most well-known phenomena related to the other major type of sleep – REM sleep. Walker describes how modern neuroscience has revolutionized the understanding of dreams, which had previously been less scientific. Scans have shown there is activation of several major areas in the brain during dreaming, such as the visuospatial areas, movement centres, areas related to autobiographical memory and the deep emotional centres. There is also deactivation of the prefrontal cortex, an area known to act as the CEO of the brain and deal with rational decision making.
Dreams are often dismissed as having no function in the modern word, but Walker explains how one benefit of dreaming during REM sleep can be to soothe harmful emotional memories – although with the proviso that as well as being in the REM sleep state, the person also needs to be dreaming about the emotional circumstances in question. Because noradrenaline, a chemical related to stress and anxiety, is shut off during REM sleep, this allows a safe environment for the memories to be processed and separate the ‘bitter emotional rind from the information-rich fruit.’
Walker reports how this knowledge has had an impact on the treatment of post-traumatic stress disorder (PTSD), when evidence showed higher than normal levels of noradrenaline in the brains of people with PTSD. Although in the early-stages, Walker describes how the ‘overnight therapy’ of REM sleep when combined with a medication, prazosin, which has a side-effect of lowering levels of noradrenaline, has shown promising evidence for future treatment of PTSD.
Creativity is another benefit of REM sleep Walker says. “NREM sleep helps transfer and make safe newly learned information into long-term storage sites of the brain. But it is REM sleep that takes these freshly minted memories and begins colliding them with the entire back catalog of your life’s autobiography.” With these creative collisions, he reports, “We can awake the next morning with new solutions to previously intractable problems or even be infused with radically new and original ideas.” The seemingly divine inspiration which dreams can provide is also discussed with the examples of Dmitri Mendeleev formulating the periodic table and Paul McCartney writing ‘Yesterday’ following dreams.
Walker also describes the importance of REM sleep for decoding the social world and shows how a lack of quality REM sleep can leave a person coming up short in this area. He posits an interesting evolutionary theory to related to REM sleep. Humans have a relatively short sleep schedule and high percentage of REM sleep when compared to other primates. When humans evolved and moved from the trees, the necessity to stay alive with ground predators roaming about (compared to most primates who sleep in daily-built tree platforms) forced the evolution of shorter sleep hours with a higher percentage of REM sleep packed in. This, Walker says, facilitated a cognitive and sociocultural revolution due to the cumulative effect over milennia of REM sleep’s benefits to the individual – such as processing of emotions and navigating the social landscape of recognizing body language and facial expressions.
Not only does Walker go into detail about the stages of sleep, their functions and their benefits, but he also describes what happens when we do not get enough sleep – with constant reminders that many people, especially in the western world, typically report sleeping less than the recommended seven to nine hours per night.
Obesity and diabetes are two closely linked conditions which lack of sleep can have an affect on. The hormone which stimulates hunger (ghrelin) increases when we don’t sleep enough and at the same time as the hormone which makes us feel full (leptin) decreases, causing us to eat more during the day. Over time this often leads to obesity, which is a large risk factor for type 2 diabetes. Walker goes on to describe, with the aid of current scientific studies, how not getting our eight hours can increase risk factors for many other diseases and conditions such as hypertension, cardiovascular disease, dementia, immune dysfunction and even cancer. Interesting factoids constantly show up, as with the rest of the book, to astound the reader with ‘surely not?’ data such as when the clocks go back and people get an extra hour of sleep, both heart attacks and traffic accidents decrease across populations, with the inverse being true when the clocks go forward.
Traffic accidents are another subject Walker goes into detail about – sleep deprivation of course causing the lapses of attention which can result in car crashes. Another subject which links to this is the fact that taking sleep pills is correlated with increased risk of death in several studies, during the study period. Although explanations of why the mortality risk increased were tentative, Walker shows evidence for several possible causes, including drowsy driving, lowered immune function and even higher risk of cancer – these effects all seem to stem from the research that shows sleeping pills seem to sedate people more than providing the deep restorative effects of natural sleep. There is no magic bullet for finding sleep it seems, even hormones such as melatonin known to be linked to sleep only have the effect of corralling ‘sleep-generating regions of the brain to the starting line of bedtime’, rather than generating sleep itself.
Motor function is another factor which sleep improves. Walker reports a conversation with a pianist, who cannot master a sequence until the next morning when he says “I can just play”. Walker’s research showed that implications for motor learning, whether for musicians or athletes, came specifically in a the last two hours of an eight-hour sleep rich in sleep spindles and in a location of the scalp about the motor cortex. Usain Bolt the superstar sprinter, is named as particular example of an athlete who took advantage of sleep spindles effect by taking naps prior to his races – naps with significant sleep spindles have been shown in experiments to provide increases in motor memory learning and reduced muscle fatigue. Naps were also shown to be advantageous for long-haul pilots by decreasing the catastrophic mistakes which can come with concentration lapses, although Walker reminds us, that naps cannot replace the rich restorative elements of a full nights sleep.
Our substance use is also brought under Walker’s lens as he describes the effects of the most widely used drugs, caffeine and alcohol. Caffeine, which blocks adenosine receptors effectively mutes the signals to sleep, while caffeine is still working with obvious implications for getting a good nights sleep. You may however, Walker says, be one of the small percentage of people who have a more efficient enzyme which degrades caffeine, allowing you to drink coffee late in the day and still find sleep at a reasonable hour.
Alcohol is described as ‘one of the most powerful suppressors of REM sleep that we know of’, which of course leads to the cancelling out of most of the effects described above such as learning and the benefits of dreaming. The depths this deprivation of REM sleep can sink to is the psychotic state called ‘delirium tremens’ which heavy drinkers can experience when suddenly abstinent and the build-up of demand for REM sleep becomes overwhelming to the body, although this is of course rare in casual drinkers. Even so, Walker says, that because the liver takes so long to process alcohol, abstinence is the best way to ensure we have decent REM sleep levels – he cites a study which showed effects on learning lasting days and even weeks.
School start times are also examined. In the US, Walker says, more than 80 percent of public high school start before 08:15 and almost 50 percent of those start before 07:20. Factoring in the circadian rhythm shift that occurs in teenagers of one to three hours forward, he points out the ridiculous nature of these children waking up at the equivalent of 03:15 to catch their school bus and be expected to be able to learn in an efficient manner. Although this an extreme example, it demonstrates the need for reexamination of starting times based on sleep science. Walker also points out that the societal trend of poorer families having jobs which require them to leave earlier in the morning, necessitating their children to catch the school bus rather than being driven to school, meaning they have to wake up earlier, further compounding the effects of consistent early starts for the economically disadvantaged.
There is so much more in book, from explanations of why almost all of infants’ sleep is comprised of REM sleep and why old people’s sleep becomes fragmented to whether dolphins dream and why cultures that have an afternoon siesta are giving themselves benefits not afforded to those who sleep only once nightly. How much does sleep deprivation compromise the care you will receive from a doctor and increase the risk of going under the knife with a drowsy surgeon? Why should you sleep in a room with a low temperature? This book has such a bounty of information.
I have always been a person who prioritized sleep and ‘Why We Sleep’ has been a bit of vindication against those who called me lazy, yet due to societal expectations, even in my own mind the qualifier emerges that I am really just making excuses for indolence. Walker has shown however, by trumpeting the positives of ample sleep such as increased learning, in terms of both facts and motor function and at the same time firmly imprinting the negatives in terms of physical disease, mental health and productivity that sleep should have a much higher significance in society.
Although the current 9-5 schedule of society, the blueprint of which seems to have evolved from the industrial revolution, is being subverted somewhat by the flexible working hours of the internet age, the majority of lives are still subject to this straitjacket and this makes it a lot harder to prioritize sleep to the extent Walker calls for. Before reading this book, I had no idea of the far-reaching implications of sleep and I imagine most other people are in the same boat. By combining such a wealth of scientific evidence on the impact of sleep deprivation with persuasive ideas about how we can improve our lives by sleeping adequately, I believe Walker has created something potentially life-changing.