Why We Sleep

To Sleep . . .

Do you think you got enough sleep this past week? Can you recall the last time you woke up without an alarm clock feeling refreshed, not needing caffeine? If the answer to either of these questions is “no,” you are not alone. Two-thirds of adults throughout all developed nations fail to obtain the recommended eight hours of nightly sleep.I

I doubt you are surprised by this fact, but you may be surprised by the consequences. Routinely sleeping less than six or seven hours a night demolishes your immune system, more than doubling your risk of cancer. Insufficient sleep is a key lifestyle factor determining whether or not you will develop Alzheimer’s disease. Inadequate sleep—even moderate reductions for just one week—disrupts blood sugar levels so profoundly that you would be classified as pre-diabetic. Short sleeping increases the likelihood of your coronary arteries becoming blocked and brittle, setting you on a path toward cardiovascular disease, stroke, and congestive heart failure. Fitting Charlotte Brontë’s prophetic wisdom that “a ruffled mind makes a restless pillow,” sleep disruption further contributes to all major psychiatric conditions, including depression, anxiety, and suicidality.

Perhaps you have also noticed a desire to eat more when you’re tired? This is no coincidence. Too little sleep swells concentrations of a hormone that makes you feel hungry while suppressing a companion hormone that otherwise signals food satisfaction. Despite being full, you still want to eat more. It’s a proven recipe for weight gain in sleep-deficient adults and children alike. Worse, should you attempt to diet but don’t get enough sleep while doing so, it is futile, since most of  the weight you lose will come from lean body mass, not fat.

Add the above health consequences up, and a proven link becomes easier to accept: the shorter your sleep, the shorter your life span. The old maxim “I’ll sleep when I’m dead” is therefore unfortunate. Adopt this mind-set, and you will be dead sooner and the quality of that (shorter) life will be worse. The elastic band of sleep deprivation can stretch only so far before it snaps. Sadly, human beings are in fact the only species that will deliberately deprive themselves of sleep without legitimate gain. Every component of wellness, and countless seams of societal fabric, are being eroded by our costly state of sleep neglect: human and financial alike. So much so that the World Health Organization (WHO) has now declared a  sleep loss epidemic throughout industrialized nations.II It is no coincidence that countries where sleep time has declined most dramatically over the past century, such as the US, the UK, Japan, and South Korea, and several in western Europe, are also those suffering the greatest increase in rates of the aforementioned physical diseases and mental disorders.

Scientists such as myself have even started lobbying doctors to start “prescribing” sleep. As medical advice goes, it’s perhaps the most painless and enjoyable to follow. Do not, however, mistake this as a plea to doctors to start prescribing more sleeping pills—quite the opposite, in fact, considering the alarming evidence surrounding the deleterious health consequences of these drugs.

But can we go so far as to say that a lack of sleep can kill you outright?
Actually, yes—on at least two counts. First, there is a very rare genetic disorder
that starts with a progressive insomnia, emerging in midlife. Several months into
the disease course, the patient stops sleeping altogether. By this stage, they have
started to lose many basic brain and body functions. No drugs that we currently
have will help the patient sleep. After twelve to eighteen months of no sleep, the
patient will die. Though exceedingly rare, this disorder asserts that a lack of sleep
can kill a human being.
Second is the deadly circumstance of getting behind the wheel of a motor
vehicle without having had sufficient sleep. Drowsy driving is the cause of
hundreds of thousands of traffic accidents and fatalities each year. And here, it is
not only the life of the sleep-deprived individuals that is at risk, but the lives of
those around them. Tragically, one person dies in a traffic accident every hour in
the United States due to a fatigue-related error. It is disquieting to learn that
vehicular accidents caused by drowsy driving exceed those caused by alcohol and
drugs combined.
Society’s apathy toward sleep has, in part, been caused by the historic failure of
science to explain why we need it. Sleep remained one of the last great biological
mysteries. All of the mighty problem-solving methods in science—genetics,
molecular biology, and high-powered digital technology—have been unable to
unlock the stubborn vault of sleep. Minds of the most stringent kind, including
Nobel Prize–winner Francis Crick, who deduced the twisted-ladder structure of
DNA, famed Roman educator and rhetorician Quintilian, and even Sigmund
Freud had all tried their hand at deciphering sleep’s enigmatic code, all in vain.
To better frame this state of prior scientific ignorance, imagine the birth of
your first child. At the hospital, the doctor enters the room and says,
“Congratulations, it’s a healthy baby boy. We’ve completed all of the preliminary
tests and everything looks good.” She smiles reassuringly and starts walking
toward the door. However, before exiting the room she turns around and says,
“There is just one thing. From this moment forth, and for the rest of your child’s
entire life, he will repeatedly and routinely lapse into a state of apparent coma. It
might even resemble death at times. And while his body lies still his mind will
often be filled with stunning, bizarre hallucinations. This state will consume onethird
of his life and I have absolutely no idea why he’ll do it, or what it is for. Good
luck!”
Astonishing, but until very recently, this was reality: doctors and scientists
could not give you a consistent or complete answer as to why we sleep. Consider
that we have known the functions of the three other basic drives in life—to eat, to
drink, and to reproduce—for many tens if not hundreds of years now. Yet the
fourth main biological drive, common across the entire animal kingdom—the
drive to sleep—has continued to elude science for millennia.
Addressing the question of why we sleep from an evolutionary perspective only
compounds the mystery. No matter what vantage point you take, sleep would
appear to be the most foolish of biological phenomena. When you are asleep, you
cannot gather food. You cannot socialize. You cannot find a mate and reproduce.
You cannot nurture or protect your offspring. Worse still, sleep leaves you
vulnerable to predation. Sleep is surely one of the most puzzling of all human
behaviors.
On any one of these grounds—never mind all of them in combination—there
ought to have been a strong evolutionary pressure to prevent the emergence of
sleep or anything remotely like it. As one sleep scientist has said, “If sleep does not
serve an absolutely vital function, then it is the biggest mistake the evolutionary
process has ever made.”III
Yet sleep has persisted. Heroically so. Indeed, every species studied to date
sleeps.IV This simple fact establishes that sleep evolved with—or very soon after—
life itself on our planet. Moreover, the subsequent perseverance of sleep
throughout evolution means there must be tremendous benefits that far outweigh
all of the obvious hazards and detriments.
Ultimately, asking “Why do we sleep?” was the wrong question. It implied there
was a single function, one holy grail of a reason that we slept, and we went in
search of it. Theories ranged from the logical (a time for conserving energy), to the
peculiar (an opportunity for eyeball oxygenation), to the psychoanalytic (a nonconscious
state in which we fulfill repressed wishes).
This book will reveal a very different truth: sleep is infinitely more complex,
profoundly more interesting, and alarmingly more health-relevant. We sleep for a
rich litany of functions, plural—an abundant constellation of nighttime benefits
that service both our brains and our bodies. There does not seem to be one major
organ within the body, or process within the brain, that isn’t optimally enhanced
by sleep (and detrimentally impaired when we don’t get enough). That we receive
such a bounty of health benefits each night should not be surprising. After all, we
are awake for two-thirds of our lives, and we don’t just achieve one useful thing
during that stretch of time. We accomplish myriad undertakings that promote
our own well-being and survival. Why, then, would we expect sleep—and the
twenty-five to thirty years, on average, it takes from our lives—to offer one
function only?
Through an explosion of discoveries over the past twenty years, we have come
to realize that evolution did not make a spectacular blunder in conceiving of
sleep. Sleep dispenses a multitude of health-ensuring benefits, yours to pick up in
repeat prescription every twenty-four hours, should you choose. (Many don’t.)
Within the brain, sleep enriches a diversity of functions, including our ability to
learn, memorize, and make logical decisions and choices. Benevolently servicing
our psychological health, sleep recalibrates our emotional brain circuits, allowing
us to navigate next-day social and psychological challenges with cool-headed
composure. We are even beginning to understand the most impervious and
controversial of all conscious experiences: the dream. Dreaming provides a unique
suite of benefits to all species fortunate enough to experience it, humans included.
Among these gifts are a consoling neurochemical bath that mollifies painful
memories and a virtual reality space in which the brain melds past and present
knowledge, inspiring creativity.

Downstairs in the body, sleep restocks the armory of our immune system, helping fight malignancy, preventing infection, and warding off all manner of sickness. Sleep reforms the body’s metabolic state by fine-tuning the balance of insulin and circulating glucose. Sleep further regulates our appetite, helping control body weight through healthy food selection rather than rash impulsivity.   Plentiful sleep maintains a flourishing microbiome within your gut from which we know so much of our nutritional health begins. Adequate sleep is intimately tied to the fitness of our cardiovascular system, lowering blood pressure while keeping our hearts in fine condition.

A balanced diet and exercise are of vital importance, yes. But we now see sleep
as the preeminent force in this health trinity. The physical and mental
impairments caused by one night of bad sleep dwarf those caused by an
equivalent absence of food or exercise. It is difficult to imagine any other state—
natural or medically manipulated—that affords a more powerful redressing of
physical and mental health at every level of analysis.