Why We Sleep

by Matthew Walker

  Figure 12: Sleep Loss and Car Crashes

  Drunk driving and drowsy driving are deadly propositions in their own right, but what happens when someone combines them? It is a relevant question, since most individuals are driving drunk in the early-morning hours rather than in the middle of the day, meaning that most drunk drivers are also sleep-deprived.

  We can now monitor driver error in a realistic but safe way using driving simulators. With such a virtual machine, a group of researchers examined the number of complete off-road deviations in participants placed under four different experimental conditions: (1) eight hours of sleep, (2) four hours of sleep, (3) eight hours of sleep plus alcohol to the point of being legally drunk, and (4) four hours of sleep plus alcohol to the point of being legally drunk.

  Those in the eight-hour sleep group had few, if any, off-road errors. Those in the four-hour sleep condition (the second group) had six times more off-road deviations than the sober, well-rested individuals. The same degree of driving impairment was true of the third group, who had eight hours of sleep but were legally drunk. Driving drunk or driving drowsy were both dangerous, and equally dangerous.

  A reasonable expectation was that performance in the fourth group of participants would reflect the additive impact of these two groups: four hours of sleep plus the effect of alcohol (i.e., twelve times more off-road deviations). It was far worse. This group of participants drove off the road almost thirty times more than the well-rested, sober group. The heady cocktail of sleep loss and alcohol was not additive, but instead multiplicative. They magnified each other, like two drugs whose effects are harmful by themselves but, when taken together, interact to produce truly dire consequences.

  After thirty years of intensive research, we can now answer many of the questions posed earlier. The recycle rate of a human being is around sixteen hours. After sixteen hours of being awake, the brain begins to fail. Humans need more than seven hours of sleep each night to maintain cognitive performance. After ten days of just seven hours of sleep, the brain is as dysfunctional as it would be after going without sleep for twenty-four hours. Three full nights of recovery sleep (i.e., more nights than a weekend) are insufficient to restore performance back to normal levels after a week of short sleeping. Finally, the human mind cannot accurately sense how sleep-deprived it is when sleep-deprived.

  We shall return to the ramifications of these results in the remaining chapters, but the real-life consequences of drowsy driving deserve special mention. This coming week, more than 2 million people in the US will fall asleep while driving their motor vehicle. That’s more than 250,000 every day, with more such events during the week than weekends for obvious reasons. More than 56 million Americans admit to struggling to stay awake at the wheel of a car each month.

  As a result, 1.2 million accidents are caused by sleepiness each year in the United States. Said another way: for every thirty seconds you’ve been reading this book, there has been a car accident somewhere in the US caused by sleeplessness. It is more than probable that someone has lost their life in a fatigue-related car accident during the time you have been reading this chapter.

  You may find it surprising to learn that vehicle accidents caused by drowsy driving exceed those caused by alcohol and drugs combined. Drowsy driving alone is worse than driving drunk. That may seem like a controversial or irresponsible thing to say, and I do not wish to trivialize the lamentable act of drunk driving by any means. Yet my statement is true for the following simple reason: drunk drivers are often late in braking, and late in making evasive maneuvers. But when you fall asleep, or have a microsleep, you stop reacting altogether. A person who experiences a microsleep or who has fallen asleep at the wheel does not brake at all, nor do they make any attempt to avoid the accident. As a result, car crashes caused by drowsiness tend to be far more deadly than those caused by alcohol or drugs. Said crassly, when you fall asleep at the wheel of your car on a freeway, there is now a one-ton missile traveling at 65 miles per hour, and no one is in control.

  Drivers of cars are not the only threats. More dangerous are drowsy truckers. Approximately 80 percent of truck drivers in the US are overweight, and 50 percent are clinically obese. This places truck drivers at a far, far higher risk of a disorder called sleep apnea, commonly associated with heavy snoring, which causes chronic, severe sleep deprivation. As a result, these truck drivers are 200 to 500 percent more likely to be involved in a traffic accident. And when a truck driver loses his or her life in a drowsy-driving crash, they will, on average, take 4.5 other lives with them.

  In actual fact, I would like to argue that there are no accidents caused by fatigue, microsleeps, or falling asleep. None whatsoever. They are crashes. The Oxford English Dictionary defines accidents as unexpected events that happen by chance or without apparent cause. Drowsy-driving deaths are neither chance, nor without cause. They are predictable and the direct result of not obtaining sufficient sleep. As such, they are unnecessary and preventable. Shamefully, governments of most developed countries spend less than 1 percent of their budget educating the public on the dangers of drowsy driving relative to what they invest in combating drunk driving.

  Even well-meaning public health messages can get lost in a barrage of statistics. It often takes the tragic recounting of personal stories to make the message real. There are thousands of such events that I could describe. Let me offer just one in the hopes of saving you from the harms of driving drowsy.

  Union County, Florida, January 2006: a school bus transporting nine children came to a halt at a stop sign. A Pontiac Bonneville car carrying seven occupants pulled up behind the bus and also came to a stop. At this moment, an eighteen-wheel truck came barreling down the road behind both vehicles. It didn’t stop. The truck struck the Pontiac, riding up over it and, with the car concertinaed underneath, then hit the bus. All three vehicles traveled through a ditch and continued moving, at which point the imploded Pontiac became engulfed in flames. The school bus rotated counterclockwise and kept traveling, now on the opposite side of the road, back-to-front. It did so for 328 feet until it went off the road and collided with a thick grove of trees. Three of the nine children in the bus were ejected through the windows upon impact. All seven passengers in the Pontiac were killed, as was the bus driver. The truck driver and all nine children in the bus sustained serious injuries.

  The trucker was a qualified and legally licensed driver. All toxicology tests performed on his blood were negative. However, it later emerged that he had been awake for thirty-four hours straight and had fallen asleep at the wheel. All of the Pontiac’s seven occupants who died were children or adolescents. Five of the seven were children in the Pontiac car were from a single family. The oldest occupant was a teenager, who had been legally driving the car. The youngest occupant was a baby of just twenty months old.

  There are many things that I hope readers take away from this book. This is one of the most important: if you are drowsy while driving, please, please stop. It is lethal. To carry the burden of another’s death on your shoulders is a terrible thing. Don’t be misled by the many ineffective tactics people will tell you can battle back against drowsiness while driving.fn2 Many of us think we can overcome drowsiness through sheer force of will, but, sadly, this is not true. To assume otherwise can jeopardize your life, the lives of your family or friends in the car with you, and the lives of other road users. Some people only get one chance to fall asleep at the wheel before losing their life.

  If you notice yourself feeling drowsy while driving, or actually falling asleep at the wheel, stop for the night. If you really must keep going—and you have made that judgment in the life-threatening context it genuinely poses—then pull off the road into a safe layby for a short time. Take a brief nap (twenty to thirty minutes). When you wake up, do not start driving. You will be suffering from sleep inertia—the carryover effects of sleep into wakefulness. Wait for another twenty to thirty minutes, perhaps after having a cup of coffee if you really must
, and only then start driving again. This, however, will only get you so far down the road before you need another such recharge, and the returns are diminishing. Ultimately, it is just not worth the (life) cost.

  CAN NAPS HELP?

  In the 1980s and ’90s, David Dinges, together with his astute collaborator (and recent administrator of the National Highway Traffic Safety Administration) Dr. Mark Rosekind, conducted another series of groundbreaking studies, this time examining the upsides and downsides of napping in the face of unavoidable sleep deprivation. They coined the term “power naps”—or, should I say, ceded to it. Much of their work was with the aviation industry, examining pilots on long-haul travel.

  The most dangerous time of flight is landing, which arrives at the end of a journey, when the greatest amount of sleep deprivation has often accrued. Recall how tired and sleepy you are at the end of an overnight, transatlantic flight, having been on the go for more than twenty-four hours. Would you feel at peak performance, ready to land a Boeing 747 with 467 passengers on board, should you have the skill to do so? It is during this end phase of flight, known in the aviation industry as “top of descent to landing,” that 68 percent of all hull losses—a euphemism for a catastrophic plane crash—occur.

  The researchers set to work answering the following question, posed by the US Federal Aviation Authority (FAA): If a pilot can only obtain a short nap opportunity (40–120 minutes) within a thirty-six-hour period, when should it occur so as to minimize cognitive fatigue and attention lapses: at the start of the first evening, in the middle of the night, or late the following morning?

  It first appeared to be counterintuitive, but Dinges and Rosekind made a clever, biology-based prediction. They believed that by inserting a nap at the front end of an incoming bout of sleep deprivation, you could insert a buffer, albeit temporary and partial, that would protect the brain from suffering catastrophic lapses in concentration. They were right. Pilots suffered fewer microsleeps at the end stages of the flight if the naps were taken early that prior evening, versus if those same nap periods were taken in the middle of the night or later that next morning, when the attack of sleep deprivation was already well under way.

  They had discovered the sleep equivalent of the medical paradigm of prevention versus treatment. The former tries to avert an issue prior to occurrence, the latter tries to remedy the issue after it has happened. And so it was with naps. Indeed, these short sleep bouts, taken early, also reduced the number of times the pilots drifted into light sleep during the critical, final ninety minutes of flight. There were fewer of these sleep intrusions, measured with EEG electrodes on the head.

  When Dinges and Rosekind reported their findings to the FAA, they recommended that “prophylactic naps”—naps taken early during long-haul flights—should be instituted as policy among pilots, as many other aviation authorities around the world now permit. The FAA, while believing the findings, was not convinced by the nomenclature. They believed the term “prophylactic” was ripe for many a snide joke among pilots. Dinges suggested the alternative of “planned napping.” The FAA didn’t like this, either, feeling it to be too “management-like.” Their suggestion was “power napping,” which they believed was more fitting with leadership- or dominance-based job positions, others being CEOs or military executives. And so the “power nap” was born.

  The problem, however, is that people, especially those in such positions, came to erroneously believe that a twenty-minute power nap was all you needed to survive and function with perfect, or even acceptable, acumen. Brief power naps have become synonymous with the inaccurate assumption that they allow an individual to forgo sufficient sleep, night after night, especially when combined with the liberal use of caffeine.

  No matter what you may have heard or read in the popular media, there is no scientific evidence we have suggesting that a drug, a device, or any amount of psychological willpower can replace sleep. Power naps may momentarily increase basic concentration under conditions of sleep deprivation, as can caffeine up to a certain dose. But in the subsequent studies that Dinges and many other researchers (myself included) have performed, neither naps nor caffeine can salvage more complex functions of the brain, including learning, memory, emotional stability, complex reasoning, or decision-making.

  One day we may discover such a counteractive method. Currently, however, there is no drug that has the proven ability to replace those benefits that a full night of sleep infuses into the brain and body. David Dinges has extended an open invitation to anyone suggesting that they can survive on short sleep to come to his lab for a ten-day stay. He will place that individual on their proclaimed regiment of short sleep and measure their cognitive function. Dinges is rightly confident he’ll show, categorically, a degradation of brain and body function. To date, no volunteers have matched up to their claim.

  We have, however, discovered a very rare collection of individuals who appear to be able to survive on six hours of sleep, and show minimal impairment—a sleepless elite, as it were. Give them hours and hours of sleep opportunity in the laboratory, with no alarms or wake-up calls, and still they naturally sleep this short amount and no more. Part of the explanation appears to lie in their genetics, specifically a sub-variant of a gene called BHLHE41.fn3 Scientists are now trying to understand what this gene does, and how it confers resilience to such little sleep.

  Having learned this, I imagine that some readers now believe that they are one of these individuals. That is very, very unlikely. The gene is remarkably rare, with but a soupçon of individuals in the world estimated to carry this anomaly. To impress this fact further, I quote one of my research colleagues, Dr. Thomas Roth at the Henry Ford Hospital in Detroit, who once said, “The number of people who can survive on five hours of sleep or less without any impairment, expressed as a percent of the population, and rounded to a whole number, is zero.”

  There is but a fraction of 1 percent of the population who are truly resilient to the effects of chronic sleep restriction at all levels of brain function. It is far, far more likely that you will be struck by lightning (the lifetime odds being 1 in 12,000) than being truly capable of surviving on insufficient sleep thanks to a rare gene.

  EMOTIONAL IRRATIONALITY

  “I just snapped, and …” Those words are often part of an unfolding tragedy as a soldier irrationally responds to a provocative civilian, a physician to an entitled patient, or a parent to a misbehaving child. All of these situations are ones in which inappropriate anger and hostility are dealt out by tired, sleep-deprived individuals.

  Many of us know that inadequate sleep plays havoc with our emotions. We even recognize it in others. Consider another common scenario of a parent holding a young child who is screaming or crying and, in the midst of the turmoil, turns to you and says, “Well, Steven just didn’t get enough sleep last night.” Universal parental wisdom knows that bad sleep the night before leads to a bad mood and emotional reactivity the next day.

  While the phenomenon of emotional irrationality following sleep loss is subjectively and anecdotally common, until recently we did not know how sleep deprivation influenced the emotional brain at a neural level, despite the professional, psychiatric, and societal ramifications. Several years ago, my team and I conducted a study using MRI brain scanning to address the question.

  We studied two groups of healthy young adults. One group stayed awake all night, monitored under full supervision in my laboratory, while the other group slept normally that night. During the brain scanning session the next day, participants in both groups were shown the same one hundred pictures that ranged from neutral in emotional content (e.g., a basket, a piece of driftwood) to emotionally negative (e.g., a burning house, a venomous snake about to strike). Using this emotional gradient of pictures, we were able to compare the increase in brain response to the increasingly negative emotional triggers.

  Analysis of the brain scans revealed the largest effects I have measured in my research to date. A structure
located in the left and right sides of the brain, called the amygdala—a key hot spot for triggering strong emotions such as anger and rage, and linked to the fight-or-flight response—showed well over a 60 percent amplification in emotional reactivity in the participants who were sleep-deprived. In contrast, the brain scans of those individuals who were given a full night’s sleep evinced a controlled, modest degree of reactivity in the amygdala, despite viewing the very same images. It was as though, without sleep, our brain reverts to a primitive pattern of uncontrolled reactivity. We produce unmetered, inappropriate emotional reactions, and are unable to place events into a broader or considered context.

  This answer raised another question: Why were the emotion centers of the brain so excessively reactive without sleep? Further MRI studies using more refined analyses allowed us to identify the root cause. After a full night of sleep, the prefrontal cortex—the region of the brain that sits just above your eyeballs; is most developed in humans, relative to other primates; and is associated with rational, logical thought and decision-making—was strongly coupled to the amygdala, regulating this deep emotional brain center with inhibitory control. With a full night of plentiful sleep, we have a balanced mix between our emotional gas pedal (amygdala) and brake (prefrontal cortex). Without sleep, however, the strong coupling between these two brain regions is lost. We cannot rein in our atavistic impulses—too much emotional gas pedal (amygdala) and not enough regulatory brake (prefrontal cortex). Without the rational control given to us each night by sleep, we’re not on a neurological—and hence emotional—even keel.