Why We Sleep

Walker immediately presents the reader with a sobering statistic: “two thirds of adults throughout all developed nations fail to obtain the recommended eight hours of nightly sleep” (3). Even the World Health Organization (WHO) considers sleep loss an epidemic.

Sleep deprivation is unnatural. In fact, humans are the only animal known to intentionally deny themselves a full night of sleep. Walker provides several examples of sleep behaviors from the animal world to reinforce the unnaturalness of sleep deprivation. Even during transoceanic migrations, where birds “fly for many more hours than is normal” (67), their brain has found an ingenious way to still get much needed sleep through “ultra-power naps” (67). These naps last only seconds in length but ensure that the birds do not suffer deadly brain and body health consequences related to sleep loss.

After situating the importance of sleep in an evolutionary context, Walker shifts his focus to how inadequate sleep impacts the brain and body, including: playing havoc with human emotions (e.g., inappropriate anger, hostility, and irrationality), weaker memory retention, higher depression rates, greater chances of developing Alzheimer’s disease and cancer, high blood pressure, loss of hunger control, inability to manage concentrations of sugar in the blood, sub-fertility for both men and women, and a weaker immune system. Moreover, sleep deprivation leads to shorter lifespans and even death.

Many of the key factors that prevent individuals from sleeping are products of our modern environment. Consumption of caffeine is one example. Caffeine is a psychoactive stimulant that artificially mutes adenosine. Caffeine hijacks and occupies the same receptors that adenosine does. Because it takes the body 10-14 hours to remove a single dose of caffeine, time of caffeine consumption matters. As a result, caffeine is the most common reason why individuals have a hard time falling asleep.

Walker expands his argument from impacts of sleep loss on the individual to society. Sleep loss costs nations a portion of their GDP each year. Sleep-deprived employees cost companies money due to lost productivity, less creativity, less happiness, less motivation, and greater unethical behavior. Children, especially those from lower socioeconomic backgrounds, also cannot reap the full learning benefits of schooling due to sleep loss. This prevents these children from reaching their full potential, which will have national impacts because they represent the next generation. Sleep-deprived doctors also result in sub-par (and even deadly) healthcare for a populace.

These sleep loss statistics related to the individual and society drive home Walker’s central claim that this sleep loss epidemic is one of the most serious public health challenges the global community currently faces. Despite the enormity of the public health issue, Walker also provides opportunities for interventions. If individuals, industry and education leaders, and policymakers and government officials adopt some of these suggestions, we will start combating this epidemic. 

Homo sapiens evolved under particular environmental conditions in eastern Africa, which, in turn, shaped our sleep patterns. As one example, most of our early ancestors’ activities stopped after sunset, due to visual impairments without daylight. The advent of fire likely extended some activities into post-dusk, but this effect was modest. Research with hunter-gatherer tribes, such as the Hadza and the San, document “nominal social activities such as singing and storytelling” (266) occurring around firelight. Our early ancestors and these modern hunter-gatherer tribes have healthy sleep quantity and quality because of lack of disruptions in their sleep-wake cycle. In stark contrast, the ubiquity of smartphones, tablets, and computers has radically impacted sleep transition abilities in industrialized societies. These technological devices emit light wavelengths that signal the brain that it is still daytime, even when it is nighttime. This signal suppresses the release of melatonin, and thus there is a delay in the brain and body preparing for sleep.

There are also biologically determined variations in the circadian rhythm across individuals. Night owls and morning larks are not owls and larks by choice, but rather this is “their genetic fate” (21). Despite this genetic basis, society still expects all individuals to adhere to the same work schedule, putting owls into “an unnatural sleep-wake rhythm” (21). Poorer work performance and greater ill health therefore befall owls. These consequences are one of the reasons that Walker argues for flexible work shifts to accommodate individual circadian rhythms.

Humans are also genetically hardwired to practice biphasic sleep. This pattern is seen in modern hunter-gatherer populations, particularly in the hot summer months, where they take a nap at high noon. These populations then switch to monophasic sleep patterns during the cooler winter months. According to Walker, “all humans, irrespective of culture or geographic location, have a genetically hardwired dip in alertness that occurs in the midafternoon hours” (69). The insatiable demands of current work schedules prevent many from appeasing this inner drive and taking a nap. Even siesta cultures around the world, such as regions of Mediterranean Europe and South America, are abandoning this nap practice. Walker points to one study that found Greek adults who stopped daytime naps had a 37% increase in risk of death from heart disease compared to those who maintained siestas. Thus, by divorcing ourselves from the “innate practice of biphasic sleep” (71), we are literally shortening our lives.