Why Do We Need Sleep? A History
James Goodwin on the Physiological Underpinnings of the Body’s Need to Reset
In 1963, as the Beach Boys were playing on the radio and Christmas was approaching, two California schoolboys threw a coin. They were deciding who would be the guinea pig in a school science project they had designed—to beat the world record for staying awake. The lucky “winner” was Randy Gardner, a 16-year-old from San Diego. When the experiment was over, he had stayed awake for eleven days and twenty five minutes. It yielded some fundamentally important observations, fortunately recorded by William Dement, one of America’s few sleep researchers at the time. Nearly forty years later, Gardner still holds the world record—which is unlikely to be broken, as the Guinness World Records will no longer accept entries. Why? It is much too dangerous for the brain.
There is no more intractable health problem in modern life than sleeplessness. Insomnia, difficulty sleeping and sleep disorders are all prevalent in today’s world. It is as if we are all in some ghastly sleep deprivation experiment. Shift work, long commuting hours, caffeine, stress, social life, travel, technology and, as we get older, age-related changes all influence our sleeping habits.
Lack of sleep won’t kill you, but it could come close. It will seep into every nook and cranny of your being and corrode your health, your well-being, your work and your leisure. Sleep is not an add-on luxury in our lives. It is a “non-negotiable biological necessity,” according to neuroscientist Matthew Walker, author of Why We Sleep. And if we can’t sleep, the reasons why are in our heads. In our brains. But before we go there, history has important lessons to teach us.
In 1907, two French scientists conducted a distressing experiment. Taking two healthy dogs, René Legendre and Henri Piéron tied their collars to a wall so that the animals were unable to sit or lie down. For ten days. A truly terrible experience of sleepless imprisonment for the hapless canines. After ten days of this agony, they were euthanized and their cerebro-spinal fluid (bathing the brain and spinal cord) was extracted, then injected into healthy, active dogs. Within one hour they were asleep. The conclusion? There was a mysterious, sleep-inducing molecule which Piéron and Legendre called “hypnotoxin”—but of which they knew nothing.
These two were not the first to recognize the deadly power of sleep deprivation. Ancient documents reveal that, among others, the early Chinese understood only too well its lethal effects. Forced insomnia was used not only as a form of excruciating torture but also as a brutal means of execution. As Marie de Manaceine, Europe’s first female doctor and a physician at the Military Academy of Moscow, concluded, “the total absence of sleep is more fatal . . . than the total absence of food.”
Legendre and Piéron were pioneers in what became a rich tradition of research into sleep—most of it, fortunately, not so gruesome, but all of it contributing to the gradual revelation of sleep’s secrets. Through it, we have learned much about our human need for sleep, its importance, how it is controlled, and why and how it goes wrong (sleep disorders). In essence, sleep is vital to human health; but we are only just beginning to realize the extent of its impact on our lives—and vice versa. The assertion of this reciprocal association is based on the emergence relatively recently—only fifty years ago—of widespread sleep problems, sparked by a technological revolution which has led to today’s 24-hour society and huge changes in our lifestyles. It’s noteworthy that we Brits are the most sleep-deprived society in the world, with 37 per cent of us feeling we do not get enough.
Despite all the research that has been done over the century and more since 1907, there are still big gaps in our knowledge. I shall highlight one of these, the evolution of sleep, by introducing you to the Mysterious Dolphin. When I was a graduate student, my sleep professor reduced my class to silence by asking a simple question: “How do aquatic mammals sleep, if they need to remain awake to breathe?” No one could answer it. It seemed to be an unresolvable paradox. The answer is as ingenious as it is intriguing. Only one half of the dolphin’s brain sleeps at a time, with the opposite eye closed. The message I drew from this was that sleep was of critical importance.
But the reverse questions from the students were equally difficult to answer: Why did our dolphin need to sleep anyway? What is sleep’s biological purpose? Why do all mammals do it? How did it evolve? My professor had some answers, but could offer no certainty. The biological purpose of sleep (but not its physiology) remains largely a mystery. But the question is of such importance in understanding sleep in humans that we should pursue it in a bit more detail.
Biologically, evolution is about survival—reducing the risks of disease or death at the hands of predators, climate and other adversities, so that the “fittest” survive long enough to reproduce, thereby passing on the most successful adaptations. Sleep appears to be incompatible with survival—it prevents feeding and procreation, and could expose the sleeper to attack by predators. Conversely, it has been argued that sleep may reduce the risk of predation by enforcing behavior modifications such as living in social groups. Some interesting research into the Hadza people in northern Tanzania found frequent night-time waking and widely differing sleep schedules.
Over a three-week period, there were only eighteen minutes when all 33 tribe members were asleep together. The scientists behind the work concluded that fitful sleep could be an ancient survival mechanism designed to guard against nocturnal threats. It seems most likely that sleep evolved to ensure that species are not active when they are most vulnerable to predation and when their food supply is scarce—that is, at night. This principle may well be the driver behind our circadian or 24-hour rhythm, seen increasingly as one of the key factors in human health. We will look at this rhythm later on in this chapter.
One of the major trends of evolution is the development in certain groups of mammals—including us—of bigger and bigger brains with more and more complex structure and function. In the case of humans, this progressive evolution is associated with standing upright, the prehensile grip (opposable thumb), working in social groups, suppression of needless emotion—that is, emotion that does not contribute to primary survival processes such as planning and executing the killing of prey or rivals—and advanced planning and decision-making. The bigger brain means that the infant child has to be nurtured for a long period to allow the brain the time it needs to develop, and this requires intimate bonding and pairing between the parents. Surprising as it may sound, the evolution of our complex advanced brain is the primary reason why humans have sex face to face—pretty much the only primates to do so.
Why should this be? Because the nine months the human infant spends in the womb isn’t a long enough time to allow the brain to mature fully. So this process has to carry on after birth, when the vulnerable child and nursing mother need protection. Eye-to-eye sexual contact promotes bonding, encouraging the male to stay around for far longer than in any other primate.
Given what we now know about the brain and sleep, it is not unreasonable to argue that the bigger brain is responsible for improvements in our survival capacity, and, conversely, that longer, more complex and more dynamic sleep is required for that bigger and more advanced brain to function effectively. Sleep must, after all, confer some pretty substantial benefits to outweigh the risks entailed. As sleep science pioneer Allan Rechtschaffen put it: “If sleep does not serve an absolutely vital function, then it is the biggest mistake the evolutionary process ever made.”
To get to the bottom of why we need sleep, we need more than anecdotal evidence. It is one thing for science to describe what is happening in the brain during sleep. It is quite another to unravel why this mysterious sequence of processes is so important. Thanks to the considerable weight of research currently dedicated to the subject (two hundred sleep labs in the USA, ten world-leading centers in UK universities and over twenty scientific journals), we are beginning to get a much clearer idea of why the brain needs sleep, though we still lack substantial empirical evidence to back up some of the explanations being proposed.
When we quit for the day, as the saying goes, our brain does some serious work. The evidence to date supports the following reasons why we sleep:
- sleep helps to forge new synapses (connections between nerve cells) and solidify memories, experiences and emotions.
- sleep allows the brain to filter out unimportant synapses and thereby to prevent overload in the higher parts of the brain (the cortex).
- sleep de-toxifies the brain, removing unwanted cellular detritus and potentially damaging protein molecules (such as beta amyloid which, as we saw in an earlier chapter, is associated with dementia) through the recently identified “glymphatic system”—a special drainage mechanism which operates twice or three times as fast during sleep as when we are awake.
- sleep may also help the brain repair itself by removing waste products and so-called “free radicals,” which cause “oxidative stress” in the brain cells, thereby acting as the brain’s garbage disposal system.
Sleep affects almost every organ and tissue in the body, including the cardiovascular system, the metabolism and the immune system as well as the brain and nervous system. We know beyond doubt that poor quality sleep increases the risk of a range of disorders including high blood pressure, cardiovascular disease, diabetes, depression and obesity—all associated with shortened lives. We hear a lot about the importance of anti-oxidants, which have cropped up in several chapters of this book already—and sleep is relevant to them too. Lack of sleep reduces the antioxidant defenses of the body, inhibiting the ability to remove molecules that promote cell damage and inflammation, such as free radicals and oxygen-reactive substances.
In summary, sleep is of critical important in re-energizing our body’s cells, clearing waste from the brain, and supporting learning and memory. It also plays vital roles in regulating mood, appetite and libido.
However, the ways in which sleep delivers these effects are largely unknown. What is particularly fascinating is that molecules and metabolic pathways only found outside the brain seem to be influenced by sleep—and then the effects are seen in the brain. We are still at the beginning of a long, long research journey to unravel sleep’s secrets. But we already know enough to make our lives better.
Dozing off at an inopportune moment can be embarrassing or even dangerous. And never more dangerous than on 28 January 1986, when the space shuttle Challenger broke apart seventy-three seconds into its flight, killing all seven crew members. The principal cause of the accident is widely known: the failure of the so-called “O” rings, which allowed pressurized burning gas to escape. Less well known is the role played by operational decisions made during a conference call the day before, when critically important managers had had less than two hours’ sleep and had been on duty since one o’clock that morning. The Presidential Commission on the Space Shuttle Challenger Accident cited human error and poor judgement related to sleep loss as a pivotal contributing factor—so significant, in fact, that a new policy on management decision-making for future launches was implemented.
Sleep loss as a pivotal contributing factor can be laid at the door of other devastating industrial accidents, including those at the nuclear plants at Three Mile Island in 1979 and Chernobyl in 1986. Difficult though it is to gather all the evidence, it is known that in both those cases, human error in the sleepless early morning hours was implicated.
We should be grateful that such incidents do not occur more frequently; and for that we have to thank the merciful intervention of two bodily systems whose interaction reliably governs our pattern of waking and sleeping. They are the circadian rhythm and the homeostatic sleep drive.
Nearly fifty years ago, in a darkened cave in Texas, a perilous experiment took place which would dramatically change what we know about our daily rhythms and sleep. In 1972, French geologist Michel Siffre entered the forbidding Midnight Cave, 100 feet below ground, where he planned to remain for six months. As a researcher, I can only imagine he did it alone because he would never have received ethical permission to involve others (the history of science is littered with examples of such awesomely impressive go-it-alone individuals who have risked their necks for science).
At the time he began his subterranean existence, alone and with no natural light or sound, we already knew that the normal physiology of the body runs on the basis of an internal clock plugged into the external environment. Such rhythms were known to the ancients well over two thousand years ago, though they had little idea, if any, of their significance.
The Greek writer Androsthenes described how the tamarind tree opened its leaves during the day and closed them at night, daily movements conspicuous to the soldiers of Alexander the Great who first reported them during their conquest of Tylos (now Bahrain). Two millennia later, in 1729, the French astronomer Jean-Jacques d’Ortous de Mairan showed that these rhythms actually had an internal origin: he recorded that the leaf movements of the sensitive heliotrope plant (probably a mimosa) persisted throughout the hours of darkness.
In the constant darkness of his self-imposed exile, our modern-day Frenchman recorded his own somewhat different responses, cut off from the natural cycle of day and night. He slept, rose and ate as he wished, and kept a written record of his activities. His records yielded some important findings. Up to this point, scientists had believed that the human natural daily or “circadian” rhythm (from the Latin circa diem—about a day) was a 24-hour cycle, tied into day length. But Siffre disproved this.
Separated from the natural cues of night and day, his circadian biorhythm free-ran, averaging between twenty-four and twenty-five hours, so that his periods of waking and sleeping became distorted in his light- and sound-deprived environment. Because the rhythm does not work on a cycle of precisely twenty-four hours, it has to be reset each day; this is done by means of daylight entering the brain via the eye (a point we shall revisit later in this chapter). Second, Siffre found that his sense of time evaporated. Without natural cycles of daylight and darkness, it became impossible for him to discern the passage of time accurately.
Such environments as that to which Siffre subjected himself are completely unnatural and, as it turned out, after as long as six months, immensely damaging to health. As he emerged from his underground ordeal, Siffre was found to be suffering from impaired movement and coordination and from memory problems—all evidence of the ill-effects of detachment from our brain’s natural rhythms. From jet-lag to shift work to daylight saving, many of us will be familiar with disturbances in how we feel, how we perform and above all how we sleep when our internal clock becomes disturbed. Similar problems are encountered by astronauts living for long periods in space.
Adapted from Supercharge Your Brain: How to Maintain a Healthy Brain Throughout Your Life by James Goodwin, available via Pegasus Books.