First, despite eating far more than their sleep-rested counterparts, the sleep-deprived rats rapidly began losing body mass during the study. Second, they could no longer regulate their core body temperature. The more sleep-deprived the rats were, the colder they became, regressing toward ambient room temperature. This was a perilous state to be in. All mammals, humans included, live on the edge of a thermal cliff. Physiological processes within the mammalian body can only operate within a remarkably narrow temperature range. Dropping below or above these life-defining thermal thresholds is a fast track to death.
It was no coincidence that these metabolic and thermal consequences were jointly occurring. When core body temperature drops, mammals respond by increasing their metabolic rate. Burning energy releases heat to warm the brain and body to get them back above the critical thermal threshold so as to avert death. But it was a futile effort in the rats lacking sleep. Like an old wood-burning stove whose top vent has been left open, no matter how much fuel was being added to the fire, the heat simply flew out the top. The rats were effectively metabolizing themselves from the inside out in response to hypothermia.
The third, and perhaps most telling, consequence of sleep loss was skin deep. The privation of sleep had left these rats literally threadbare. Sores had appeared across the rats’ skin, together with wounds on their paws and tails. Not only was the metabolic system of the rats starting to implode, but so, too, was their immune system.fn3 They could not fend off even the most basic of infections at their epidermis—or below it, as we shall see.
If these outward signs of degrading health were not shocking enough, the internal damage revealed by the final postmortem was equally ghastly. A landscape of utter physiological distress awaited the pathologist. Complications ranged from fluid in the lungs and internal hemorrhaging to ulcers puncturing the stomach lining. Some organs, such as the liver, spleen, and kidneys, had physically decreased in size and weight. Others, like the adrenal glands that respond to infection and stress, were markedly enlarged. Circulating levels of the anxiety-related hormone corticosterone, released by the adrenal glands, had spiked in the sleepless rats.
What, then, was the cause of death? Therein lay the issue: the scientists had no idea. Not all the rats suffered the same pathological signature of demise. The only commonality across the rats was death itself (or the high likelihood of it, at which point the researchers euthanized the animals).
In the years that followed, further experiments—the last of their kind, as scientists felt (rightly, in my personal view) uneasy about the ethics of such experiments based on the outcome—finally resolved the mystery. The fatal final straw turned out to be septicemia—a toxic and systemic (whole organism) bacterial infection that coursed through the rats’ bloodstream and ravaged the entire body until death. Far from a vicious infection that came from the outside, however, it was simple bacteria from the rats’ very own gut that inflicted the mortal blow—one that an otherwise healthy immune system would have easily quelled when fortified by sleep.
The Russian scientist Marie de Manacéïne had in fact reported the same mortal consequences of continuous sleep deprivation in the medical literature a century earlier. She noted that young dogs died within several days if prevented from sleeping (which are difficult studies for me to read, I must confess). Several years after de Manacéïne’s studies, Italian researchers described equally lethal effects of total sleep deprivation in dogs, adding the observation of neural degeneration in the brain and spinal cord at postmortem.
It took another hundred years after the experiments of de Manacéïne, and the advancements in precise experimental laboratory assessments, before the scientists at the University of Chicago finally uncovered why life ends so quickly in the absence of sleep. Perhaps you have seen that small plastic red box on the walls of extremely hazardous work environments that has the following words written on the front: “Break glass in case of emergency.” If you impose a total absence of sleep on an organism, rat or human, it indeed becomes an emergency, and you will find the biological equivalent of this shattered glass strewn throughout the brain and the body, to fatal effect. This we finally understand.
NO, WAIT—YOU ONLY NEED 6.75 HOURS OF SLEEP!
Reflecting on these deathly consequences of long-term/chronic and short-term/acute sleep deprivation allows us to address a recent controversy in the field of sleep research—one that many a newspaper, not to mention some scientists, apprehended incorrectly. The study in question was conducted by researchers at the University of California, Los Angeles, on the sleep habits of specific pre-industrial tribes. Using wristwatch activity devices, the researchers tracked the sleep of three hunter-gatherer tribes that are largely untouched by the ways of industrial modernity: the Tsimané people in South America, and the San and Hadza tribes in Africa, which we have previously discussed. Assessing sleep and wake times day after day across many months, the findings were thus: tribespeople averaged just 6 hours of sleep in the summer, and about 7.2 hours of sleep in the winter.
Well-respected media outlets touted the findings as proof that human beings do not, after all, need a full eight hours of sleep, some suggesting we can survive just fine on six hours or less. For example, the headline of one prominent US newspaper read:
“Sleep Study on Modern-Day Hunter-Gatherers Dispels Notion That We’re Wired to Need 8 Hours a Day.”
Others started out with the already incorrect assumption that modern societies need only seven hours of sleep, and then questioned whether we even need that much: “Do We Really Need to Sleep 7 Hours a Night?”
How can such prestigious and well-respected entities reach these conclusions, especially after the science that I have presented in this chapter? Let us carefully reevaluate the findings, and see if we still arrive at the same conclusion.
First, when you read the paper, you will learn that the tribespeople were actually giving themselves a 7- to 8.5-hour sleep opportunity each night. Moreover, the wristwatch device, which is neither a precise nor gold standard measure of sleep, estimated a range of 6 to 7.5 hours of this time was spent asleep. The sleep opportunity that these tribespeople provide themselves is therefore almost identical to what the National Sleep Foundation and the Centers for Disease Control and Prevention recommend for all adult humans: 7 to 9 hours of time in bed.
The problem is that some people confuse time slept with sleep opportunity time. We know that many individuals in the modern world only give themselves 5 to 6.5 hours of sleep opportunity, which normally means they will only obtain around 4.5 to 6 hours of actual sleep. So no, the finding does not prove that the sleep of hunter-gatherer tribes is similar to ours in the post-industrial era. They, unlike us, give themselves more sleep opportunity than we do.
Second, let us assume that the wristwatch measurements are perfectly accurate, and that these tribes obtain an annual average of just 6.75 hours of sleep. The next erroneous conclusion drawn from the findings was that humans must, therefore, naturally need a mere 6.75 hours of sleep, and no more. Therein lies the rub.
If you refer back to the two newspaper headlines I quoted, you’ll notice they both use the word “need.” But what need are we talking about? The (incorrect) presupposition made was this: whatever sleep the tribespeople were obtaining is all that a human needs. It is flawed reasoning on two counts. Need is not defined by that which is obtained (as the disorder of insomnia teaches us), but rather whether or not that amount of sleep is sufficient to accomplish all that sleep does. The most obvious need, then, would be for life—and healthy life. Now we discover that the average life span of these hunter-gatherers is just fifty-eight years, even though they are far more physically active than we are, rarely obese, and are not plagued by the assault of processed foods that erode our health. Of course, they do not have access to modern medicine and sanitation, both of which are reasons that many of us in industrialized, first-world nations have an expected life span that exceeds theirs by over a decade. But it is telling th
at, based on epidemiological data, any adult sleeping an average of 6.75 hours a night would be predicted to live only into their early sixties: very close to the median life span of these tribespeople.
More prescient, however, is what normally kills people in these tribes. So long as they survive high rates of infant mortality and make it through adolescence, a common cause of death in adulthood is infection. Weak immune systems are a known consequence of insufficient sleep, as we have discussed in great detail. I should also note that one of the most common immune system failures that kills individuals in hunter-gatherer clans are intestinal infections—something that shares an intriguing overlap with the deadly intestinal tract infections that killed the sleep-deprived rats in the above studies.
Recognizing this shorter life span, which fits well with the acclaimed shorter sleep amounts the researchers measured, the next error in logic many made is exposed by asking why these tribes would sleep what appears to be too little, based on all that we know from thousands of research studies.
We do not yet know of all the reasons, but a likely contributing factor lies in the title we apply to these tribes: hunter-gatherers. One of the few universal ways of forcing animals of all kinds to sleep less than normal amounts is to limit food, applying a degree of starvation. When food becomes scarce, sleep becomes scarce, as animals try to stay awake longer to forage. Part of the reason that these hunter-gatherer tribes are not obese is because they are constantly searching for food, which is never abundant for long stretches. They spend much of their waking lives in pursuit and preparation of nutrition. For example, the Hadza will face days where they obtain 1,400 calories or less, and routinely eat 300 to 600 fewer daily calories than those of us in modern Western cultures. A large proportion of their year is therefore spent in a state of lower-level starvation, one that can trigger well-characterized biological pathways that reduce sleep time, even though sleep need remains higher than that obtained if food were abundant. Concluding that humans, modern-living or pre-industrial, need less than seven hours of sleep therefore appears to be a wishful conceit, and a tabloid myth.
IS SLEEPING NINE HOURS A NIGHT TOO MUCH?
Epidemiological evidence suggests that the relationship between sleep and mortality risk is not linear, such that the more and more sleep you get, the lower and lower your death risk (and vice versa). Rather, there is an upward hook in death risk once the average sleep amount passes nine hours, resulting in a tilted backward J shape:
Two points are worthy of mention in this regard. First, should you explore those studies in detail, you learn that the causes of death in individuals sleeping nine hours or longer include infection (e.g., pneumonia) and immune-activating cancers. We know from evidence discussed earlier in the book that sickness, especially sickness that activates a powerful immune response, activates more sleep. Ergo, the sickest individuals should be sleeping longer to battle back against illness using the suite of health tools sleep has on offer. It is simply that some illnesses, such as cancer, can be too powerful even for the mighty force of sleep to overcome, no matter how much sleep is obtained. The illusion created is that too much sleep leads to an early death, rather than the more tenable conclusion that the sickness was just too much despite all efforts to the contrary from the beneficial sleep extension. I say more tenable, rather than equally tenable, because no biological mechanisms that show sleep to be in any way harmful have been discovered.
Second, it is important not to overextend my point. I am not suggesting that sleeping eighteen or twenty-two hours each and every day, should that be physiologically possible, is more optimal than sleeping nine hours a day. Sleep is unlikely to operate in such a linear manner. Keep in mind that food, oxygen, and water are no different, and they, too, have a reverse-J-shape relationship with mortality risk. Eating to excess shortens life. Extreme hydration can lead to fatal increases in blood pressure associated with stroke or heart attack. Too much oxygen in the blood, known as hyperoxia, is toxic to cells, especially those of the brain.
Sleep, like food, water, and oxygen, may share this relationship with mortality risk when taken to extremes. After all, wakefulness in the correct amount is evolutionarily adaptive, as is sleep. Both sleep and wake provide synergistic and critical, though often different, survival advantages. There is an adaptive balance to be struck between wakefulness and sleep. In humans, that appears to be around sixteen hours of total wakefulness, and around eight hours of total sleep, for an average adult.
Chapter 13
iPads, Factory Whistles, and Nightcaps
What’s Stopping You from Sleeping?
Many of us are beyond tired. Why? What, precisely, about modernity has so perverted our otherwise instinctual sleep patterns, eroded our freedom to sleep, and thwarted our ability to do so soundly across the night? For those of us who do not have a sleep disorder, the reasons underlying this state of sleep deficiency can seem hard to pinpoint—or, if seemingly clear, are erroneous.
Beyond longer commute times and “sleep procrastination” caused by late-evening television and digital entertainment—both of which are not unimportant in their top-and-tail snipping of our sleep time and that of our children—five key factors have powerfully changed how much and how well we sleep: (1) constant electric light as well as LED light, (2) regularized temperature, (3) caffeine (discussed in chapter 2), (4) alcohol, and (5) a legacy of punching time cards. It is this set of societally engineered forces that are responsible for many an individual’s mistaken belief that they are suffering from medical insomnia.
THE DARK SIDE OF MODERN LIGHT
At 255–257 Pearl Street, in Lower Manhattan, not far from the Brooklyn Bridge, is the site of arguably the most unassuming yet seismic shift in our human history. Here Thomas Edison built the first power-generating station to support an electrified society. For the first time, the human race had a truly scalable method of unbuckling itself from our planet’s natural twenty-four-hour cycle of light and dark. With a proverbial flick of a switch came a whimsical ability to control our environmental light and, with it, our wake and sleep phases. We, and not the rotating mechanics of planet Earth, would now decide when it was “night” and when it was “day.” We are the only species that has managed to light the night to such dramatic effect.
Humans are predominantly visual creatures. More than a third of our brain is devoted to processing visual information, far exceeding that given over to sounds or smells, or those supporting language and movement. For early Homo sapiens, most of our activities would have ceased after the sun set. They had to, as they were predicated on vision, supported by daylight. The advent of fire, and its limited halo of light, offered an extension to post-dusk activities. But the effect was modest. In the early-evening glow of firelight, nominal social activities such as singing and storytelling have been documented in hunter-gatherer tribes like the Hadza and the San. Yet the practical limitations of firelight nullified any significant influence on the timing of our sleep-wake patterns.
Gas- and oil-burning lamps, and their forerunners, candles, offered a more forceful influence upon sustained nighttime activities. Gaze at a Renoir painting of nineteenth-century Parisian life and you will see the extended reach of artificial light. Spilling out of homes and onto the streets, gas lanterns began bathing entire city districts with illumination. In this moment, the influence of man-made light began its reengineering of human sleep patterns, and it would only escalate. The nocturnal rhythms of whole societies—not just individuals or single families—became quickly subject to light at night, and so began our advancing march toward later bedtimes.
For the suprachiasmatic nucleus—the master twenty-four-hour clock of the brain—the worst was yet to come. Edison’s Manhattan power station enabled the mass adoption of incandescent light. Edison did not create the first incandescent lightbulb—that honor went to the English chemist Humphry Davy in 1802. But in the mid-1870s, Edison Electric Light Company began developing a reliable, mass-marketable lightbul
b. Incandescent light bulbs, and decades later, fluorescent light bulbs, guaranteed that modern humans would no longer spend much of the night in darkness, as we had for millennia past.
One hundred years post-Edison, we now understand the biological mechanisms by which the electric lightbulbs managed to veto our natural timing and quality of sleep. The visible light spectrum—that which our eyes can see—runs the gamut from shorter wavelengths (approximately 380 nanometers) that we perceive as cooler violets and blues, to the longer wavelengths (around 700 nanometers) that we sense as warmer yellows and reds. Sunlight contains a powerful blend of all of these colors, and those in between (as the iconic Pink Floyd album cover of Dark Side of the Moon illuminates [so to speak]).
Before Edison, and before gas and oil lamps, the setting sun would take with it this full stream of daylight from our eyes, sensed by the twenty-four-hour clock within the brain (the suprachiasmatic nucleus, described in chapter 2). The loss of daylight informs our suprachiasmatic nucleus that nighttime is now in session; time to release the brake pedal on our pineal gland, allowing it to unleash vast quantities of melatonin that signal to our brains and bodies that darkness has arrived and it is time for bed. Appropriately scheduled tiredness, followed by sleep, would normally occur several hours after dusk across our human collective.
Electric light put an end to this natural order of things. It redefined the meaning of midnight for generations thereafter. Artificial evening light, even that of modest strength, or lux, will fool your suprachiasmatic nucleus into believing the sun has not yet set. The brake on melatonin, which should otherwise have been released with the timing of dusk, remains forcefully applied within your brain under duress of electric light.
Why We Sleep Page 29