47 pages • 1 hour read
Why We Sleep
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Part 1, Chapters 1-3Chapter Summaries & Analyses
Part 1: “This Thing Called Sleep”
Chapter 1 Summary: “To Sleep…”
Walker presents his primary argument: Sleep loss is an epidemic, and the greatest public health challenge faced by modern society in the past century. Researchers considered sleep to be “one of the last great biological mysteries” (5), but the advent of brain-imagining machines in the early-21st century led to major scientific breakthroughs. These recent discoveries in the field of sleep health illustrate that sleep deprivation (or less than eight hours of sleep a night) has catastrophic health and well-being consequences.
Despite the overwhelming evidence that supports adequate nightly sleep, adults in developed nations do not obtain the “recommended eight hours of nightly sleep” (3). To Walker, society’s indifference towards sleep is due to scientists, government officials, and policymakers failing to explain the reasons why we need it and prioritizing policy around it. Walker hopes that readers gain an appreciation of sleep and reverse their own and their families’ neglect of it.
Chapter 2 Summary: “Caffeine, Jet Lag, and Melatonin: Losing and Gaining Control of Your Sleep Rhythm”
Walker explains in detail the “two main factors that determine when you want to sleep and when you want to be awake” (13). The first factor is known as the circadian rhythm, which is a natural process generated by the suprachiasmatic nucleus in the brain that regulates the sleep-wake cycle and repeats approximately every 24 hours. The suprachiasmatic nucleus uses melatonin to circulate throughout the brain and body its signal of night and day. While sunlight does not dictate the circadian rhythm, the brain uses it, along with temperature fluctuations and other regularly scheduled activities (e.g., eating, exercise, and social gatherings), to help reset the biological clock every 24 hours. There are variations in the circadian rhythm across individuals, with morning types (prefer to wake up early and go to bed early), evening types (prefer to go to bed late and wake up late), and between morning and evening types.
The second factor that determines the sleep-wake cycle is sleep pressure, which a chemical called adenosine mediates. Adenosine accumulates during waking hours. As it reaches its peak in the evening, it signals to the brain that it is time to sleep. The concentrations begin to build once more upon awakening.
Despite the circadian rhythm and sleep-pressure signal of adenosine both regulating sleep, these processes are independent systems that do not communicate with one another. Pulling an all-nighter illustrates this biological phenomenon. The sleep pressure of adenosine continues to rise in an individual throughout the night because sleep is absent. Individuals will catch a second wind in the morning due to an individual’s circadian rhythm. Its rhythm falls and rises depending on time of day and not the adenosine concentration levels in the brain. Thus, the rise of the circadian rhythm in the morning helps offset the rising levels of adenosine sleep pressure. However, as the circadian rhythm begins to decline in the afternoon, individuals lose the temporary alertness boost and feel the immense sleep pressure.
Chapter 3 Summary: “Defining and Generating Sleep: Time Dilation and What We Learned from a Baby in 1952”
Walker expands his discussion on sleep from the factors that determine the sleep-wake cycle to defining and generating sleep. There are five observable features that indicate sleep in other humans: (1) the individual is horizontal, (2) the individual has lowered muscle tone and is draped over whatever element supports them underneath, (3) the individual is not responsive or communicative, (4) sleep, in contrast to a coma or death, is easily reversible, and (5) sleep adheres to the circadian rhythm, and humans typically are asleep at night.
In addition to these observable features, there are two phenomenological indicators that help an individual determine sleep in themselves. The first is “loss of external awareness” (39), meaning an individual no longer perceives their external surroundings. The thalamus, or the sensory gate of the brain, imposes a “sensory blackout” (40) at the onset of sleep, which helps the human brain lose waking contact with the outside world. The second indicator is the sense of time distortion, meaning you cannot consciously keep track of time while asleep.
Scientific verification of sleep requires the recording of signals via electrodes from three areas of the human body: “(1) brainwave activity, (2) eye movement activity, and (3) muscle activity” (41). These signals are grouped collectively under the term “‘polysomnography’ (PSG)” (41), which is a readout of these signals that comprise sleep. Using this collection of measures on infants, Kleitman and his graduate student Eugene Aserinsky discovered in 1952 that humans have two different phases of sleep. These sleep stages are: NREM (non-rapid eye movement) sleep and REM (rapid eye movement) or dream sleep. Humans move back and forth between NREM and REM sleep every 90-minutes throughout the night. The ratio of these two stages changes during these 90-minute cycles as the night progresses, with NREM and REM sleep dominating the first and second half of the night, respectively.
Part 1, Chapters 1-3 Analysis
As Walker lays the groundwork for his argument, he uses a combination of cited—his and other scientists—research, statistics, and analogies. The combination of these three results in a narrative that, while packed with data and technical terminology, is still easy to understand as a layperson. As one example of an analogy, Walker uses the Olympic 100-meter race to explain the role melatonin plays in the sleep-wake cycle. Melatonin is akin to the timing official in the Olympic race because it “governs when the race (sleep) begins, but does not participate in the race” (23). Rather, the other brain regions and processes (the sprinters) actively participate in generating sleep (the race itself). The use of Walker’s own research also helps him build credibility with readers. Readers are thus more apt to believe his central claims, even when they might otherwise not find them as dire as Walker describes (e.g., sleep loss is an epidemic).
Walker “demystifies this beguiling thing called sleep” (10). His reason for doing so is that despite the scientific evidence making clear the dangers of sleep deprivation for both individuals and society, most lay people do not understand what sleep is and is not, how our sleep patterns are similar and different to other animals, and how sleep changes across the human lifetime. In fact, he considers this “the most glaring omission in the contemporary health conversation” (8). Walker notes that the question “Why do we sleep?” (6) was the wrong one to ask because it implied that sleep was only important for a single function. As his narrative details, sleep is critical to a diversity of functions within the human body and brain, which the title of his book “Why We Sleep” reflects.
One of the central claims Walker presents is that modern society divorces humans from the way nature/evolution intended sleep. This is evident with “night owls” (21), who naturally go to bed late and subsequently wake up late (larks are the opposite of night owls). Genetics strongly determine an adult’s owlness rather than choice. Unfortunately, modern society’s work schedule favors early start times, which makes sense for larks but punishes owls. Compared to larks, owls are more chronically sleep deprived because the standard employment schedule forces them into an unnatural sleep-wake rhythm. Because of this sleep deprivation, they face greater ill health, including higher rates of diabetes, cancer, anxiety, and depression.
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