47 pages • 1 hour read
Matthew WalkerWhy We Sleep
Nonfiction | Book | Adult | Published in 2017A modern alternative to SparkNotes and CliffsNotes, SuperSummary offers high-quality Study Guides with detailed chapter summaries and analysis of major themes, characters, and more.
Part 1, Chapters 4-5Chapter Summaries & Analyses
Part 1: “This Thing Called Sleep”
Chapter 4 Summary: “Ape Beds, Dinosaurs, and Napping with Half a Brain: Who Sleeps, How Do We Sleep, and How Much?”
Sleep is universal across animal species and emerged with the earliest forms of life on Earth. There are four differences in sleep found within and between species. The first is diversity in amount of sleep time. For example, animal species within the same family group might have remarkably different sleep needs, yet there are also species from different family groups that have similar needs. Evolution shaped these needs as part of a balancing act between waking survival demands, restoring physiological needs, and meeting community needs.
The second difference is composition of sleep across species, meaning “not all species experience all stages of sleep” (60). NREM sleep appears universal across species, whereas mammals, except possibly for aquatic mammals, and birds are the only species in the animal kingdom that experience REM sleep. Given that mammals and birds evolved separately, the fact that REM sleep evolved in both lineages indicates its fundamental need. While NREM sleep is common across all species, this does not mean this sleep stage is more important than REM sleep. Sleep studies illustrate that after sleep deprivation, the brain tries to salvage both over several recovery nights. Unfortunately, no species can gain back all the sleep lost.
The form of sleep also differs. Species such as aquatic mammals and birds are unihemispheric, meaning one half of the brain sleeps and the other half remains awake, whereas the whole brain sleeps in other species. Walker notes that certain external triggers, such as sleeping in an unfamiliar environment, might cause humans to have a mild version of unihemispheric sleep. In this unfamiliar sleep environment, one half of the human brain sleeps lighter than the other half, maintaining some degree of vigilance.
Finally, sleep patterns also differ among species. There are three sleep patterns: monophasic (one period of sleep within 24 hours), biphasic (sleeping during two periods over the course of 24 hours), and polyphasic (sleeping multiple times over the course of 24 hours). Typically, species follow one sleep pattern. However, there are rare circumstances where the sleep pattern changes due to extreme environmental challenges or pressures (e.g., starvation).
Chapter 5 Summary: “Changes in Sleep Across the Life Span”
Walker begins this discussion with the sleep duration of babies in the womb. Prior to birth, babies spend most of their time in a “sleep-like state, much of which resembles the REM-sleep stage” (78). Total sleep time only begins to decrease in the final trimester, but REM-sleep time hits a lifetime high. The skyrocketing of REM sleep coincides with brain development and maturation, specifically the wiring of the brain’s neural connections. The disturbance or impairment of REM sleep during this stage of the infant’s development has serious consequences, including incomplete wiring of the brain’s neural connections, which might lead to autism spectrum disorder (ASD).
Similar to babies in the womb, infants also spend a good portion of the day sleeping. Because it takes one year after birth for the suprachiasmatic nucleus to fully develop, the circadian rhythm does not govern infants’ sleep patterns. In a newborn’s first few months, there is an even split between NREM and REM sleep. Total sleep duration begins to decrease in early childhood, with NREM and REM sleep increasing and decreasing, respectively. By the late adolescent years, the proportion of NREM and REM sleep stabilizes and remains 80/20 NREM/REM sleep through mid-adulthood.
Brains reach close to adult size by age six but still need remodeling before they function as an adult brain. This remodeling takes place during adolescence where NREM sleep dominates. During this remodeling stage, pruning of unused neural connections occurs in conjunction with the strengthening of other connections. The remodeling focuses first on visual and spatial perceptions in the back of the brain and gradually moves towards the front. The remodeling of the prefrontal cortex, “which enables rational thinking and critical decision-making” (90), occurs last, explaining the high degree of irrationality in teenagers. A reduction in NREM sleep leads to faulty pruning, which might lead to the development of psychiatric disorders, including attention deficit hyperactivity disorder (ADHD), schizophrenia, bipolar disorder, and depression.
While many believe that older adults need less sleep than younger adults, this is a myth. Aging causes three impairments of sleep: (1) a reduction in the quantity and quality, attributed to the decline in deep NREM sleep; (2) greater fragmentation of sleep, meaning older adults wake up more frequently throughout the night; and (3) changes to the circadian rhythm, which disrupts the onset/offset of sleep timing. Poor sleep in older adults contributes to cognitive (e.g., Alzheimer’s disease and depression) and medical (e.g., diabetes, cardiovascular disease, chronic pain, and stroke) ill health.
Part 1, Chapters 4-5 Analysis
Chapters 4 and 5 expand the discussion on sleep from its underlying physiological mechanisms to its evolutionary and developmental contexts. Sleep is universal across species. Even aquatic mammals, such as dolphins, must sleep despite the “unyielding need to swim in perpetuum from birth to death” (65). Their sleep evolved to be unihemispheric, meaning they sleep with only half a brain at a time. The other half of the brain must remain awake to maintain “life-necessary movement” (64) in the water. The fact that animals develop physiological and behavioral patterns to ensure they sleep when they need to, even in the face of pressures that constantly threaten their existence, illustrates the irony of humans intentionally denying themselves sleep.
While sleep is a unifying feature across the animal kingdom, “humans are special when it comes to sleep” (72). Compared to other primates, humans spend less time asleep, yet have more than double the amount of REM sleep. Humans are also exclusively terrestrial sleepers. Walker posits that it was sleeping on the ground that enabled humans to develop “enriched and enhanced amounts of REM sleep” (73), while shortening the duration of sleep time. Prolonged periods of REM sleep while tree sleeping would have been dangerous, especially considering that the brain paralyzes voluntary muscles during this sleep state. Sleeping on the ground, however, would have also been tricky for our early ancestors due to the plethora of predators (e.g., hyenas and saber-toothed tigers) that hunt at night. While the invention of fire helped our Homo erectus ancestors to safely sleep on the ground, it did not solve all the issues associated with ground sleeping. Walker suggests that the evolution favored any Homo erectus that accomplished more efficient sleep in less time. Evolution drove our ancient form of sleep so that it “became somewhat shorter in duration, yet increased in intensity, especially by enriching the amount of REM sleep we packed int the night” (73).
In turn, REM sleep propelled “Homo sapiens to the top of evolution’s lofty pyramid” (74) by shaping our degree of complex emotional processing and our cognitive intelligence. Our superb ability to decode emotions—which REM sleep facilitates—enabled the creation of social communities of other humans. The second benefit of REM sleep, creativity, paved the way for a uniquely human feature: our degree of ingenuity. Yet, it was the emotionally rich social bonds found within communities that enabled humans to share these creative solutions with each other. For this reason, humans became “a new (sleep-fueled), globally dominant social superclass” (77).
Sleep is also critical for human development, although Walker notes we are still learning about this relationship. Walker reiterates that sleep disruption has severe consequences, but this is especially the case during phases of brain development. For example, alcohol consumption by mothers significantly reduces the time unborn babies spend in REM sleep compared to those whose mothers did not drink during pregnancy. Babies born of mothers who drank during pregnancy display different electrical activity during REM sleep compared to those in the non-alcohol condition. Because of the importance of REM sleep in developing neural connections in the fetus, some researchers hypothesize that the disruption of REM sleep due to alcohol might contribute to the development of autism.
Walker also suggests that the shifting of the circadian rhythm forward in adolescent teenagers is critical to their development. Shifting their wake time several hours after adults enables teenagers to begin to become more independent and learn to navigate the complexities of peer-group relationships. Development of these skills enables them to successfully transition from the teenager to adult phase over time.
