The Mystery of Sleep: Why a Good Night's Rest Is Vital to a Better, Healthier Life


Why Do We Sleep?


THE MYSTERY. Why do all forms of life, from plants, insects, sea creatures, amphibians, and birds to mammals, need rest or sleep?

Our Sleepy Lives

It is 3 A.M. You awaken sweating, your heart pounding. Your mind is racing, reminding you that you have to get up early to drop the kids off at daycare, then dash to work for an important meeting. Obsessed with your personal and work issues, you eventually fall back asleep, only to be dragged awake by the alarm clock. You are cranky and yell at the kids, trying to rush everyone out of the house. By the time you get to the meeting, you have trouble paying attention; you can neither concentrate nor listen to others. You gulp down a second cup of coffee.

As you drive down the highway you feel an uncontrollable urge to shut your eyes. You turn on the radio, pop a candy into your mouth, and start to sing along with the radio. You roll down the window. Maybe you slap your face. But nothing seems to be working. You cannot overcome the urge to fall asleep.

If you have ever been affected by severe sleepiness, you are not alone. Every year, U.S. businesses lose an estimated $18 billion in productivity and injuries because of daytime sleepiness, while sleepy drivers cause an estimated 20 percent of car accidents, as high as 1.2 million crashes, resulting in thousands of deaths and injuries and billions of dollars in property damage. Research has also shown important links between the amount of sleep an individual gets and risk of health problems: reduced sleep may lead to obesity; women who sleep much more or much less than the average are at increased risk of disease. A study of 71,000 nurses published in 2003 showed that those sleeping five hours or less had a 45 percent greater risk of developing heart disease after ten years than those sleeping eight hours. Those sleeping nine to eleven hours increased their risk by 38 percent. Sleep disorders, ranging from mild insomnia to sleep apnea to narcolepsy, affect an estimated 50 to 70 million Americans.

Why do our bodies need sleep? How does sleep deprivation affect us, and how can we recognize its signs? In this book I address these questions and others. Sleep medicine is such a new field that even sleep specialists are still learning about it, and many mysteries remain about the nature of sleep and its disorders. I tell readers how to recognize the symptoms of sleep problems they or their family might be suffering from and how they can manage these problems so that they get enough of the sleep they need.

What Is Sleep?

All forms of life have periods of activity alternating with periods of inactivity, taking the form of either rest or sleep. The amount of sleep each lifeform needs, and its sleep schedule, is controlled by its genetic blueprint—and we humans share some of our sleep-controlling genes with flies!


Yet despite the thousands of experiments scientists have performed to study sleep, no one has been able to declare with certainty why all lifeforms need sleep; we know only that when animals are prevented from sleeping they eventually die. It is possible that sleep serves different functions for different species. Most animals, for instance, spend their waking time looking for food, then retreat to a safe place to sleep and hide from predators. (Animals near the top of their food chain, such as lions, seem to sleep whenever and wherever they want.)

Some of the reasons suggested for why humans sleep include removal from the brain of waste material produced by brain cells, conservation of energy, the restoration of important bodily functions, and the repair of damaged tissues. Certain hormones, for example, are secreted mainly during sleep. The one thing scientists are certain of is that if people do not get enough sleep, their brains will not work properly; they will feel rotten, and they might find themselves unable to perform complex tasks. No one wants a sleep-deprived pilot flying a commercial jet across the Pacific. And even the medical profession now admits that doctors who are sleep deprived (a condition that used to be the norm among residents, who were on continuous duty for up to thirty-six hours and a hundred hours a week) are a hazard to their patients and their own health.

We can conclude that sleep serves many important functions, and different types of sleep serve different needs. For instance, scientists theorize that slow-wave sleep makes us feel refreshed upon awakening, while rapid eye movement (REM) sleep is connected to our ability to store memories in our nervous system. The right amount of sleep contributes not only to our ability to function but to how we feel. Sleep may in fact be, as Macbeth describes it, the “chief nourisher in life’s feast.”

Since ancient times scientists, philosophers, artists, and writers have been fascinated by sleep and dreams. Up to the nineteenth century, many believed that sleep was a form of death: as Dr. Robert MacNish wrote in Philosophy of Sleep (1830): “Sleep is the intermediate state between wakefulness and death: wakefulness is regarded as the active state of all the animal and intellectual functions and death as that of their total suspension.”

In 1875 an English doctor, Richard Caton, reported that he could measure electrical activity in the brains of animals. It was not until the twentieth century that scientists discovered that the human brain was active during sleep and that they could measure its electrical activity. In 1928 a German doctor, Hans Berger, successfully recorded the electrical activity of a sleeping brain using electrodes placed on the scalp. This was the first EEG (electroencephalogram). Soon techniques were devised to measure the millionths of volts of electrical energy put out by the human brain during sleep. In 1953 Nathaniel Kleitman of the University of Chicago and his student Eugene Aserinsky used EEGs to measure electrical activity in sleeping infants, while also measuring their eye movements, becoming the first to describe REM sleep. Scientists then realized that there are three states of consciousness in all mammals: non-REM sleep, REM sleep, and wakefulness.


It was soon discovered that REM sleep is the time when sleepers are most likely to experience vivid dreams. Scientists then learned that there were several types or stages of sleep. In 1968, at a meeting of an international group of sleep scientists, a more detailed picture of the brain’s electrical activity during sleep emerged. Using brain waves and other measures, they initially segregated non-REM sleep into four stages. Typically, as a sleeper went from stage 1 to stage 4, the brain waves were shown to move progressively more slowly and their size became increasingly bigger as the sleep became deeper. Stages 3 and 4 are now considered together as slow wave or deep sleep.

Paradoxically, a normal feature of sleep is that we experience brief awakenings (also called arousals), each lasting only seconds, while we sleep. These arousals occur from the time we are born, and healthy sleepers might experience about five of these awakenings an hour, although they will not remember them. Scientists believe that arousals represent a response to a sensation or a protection from danger: a newborn, for instance, must be able to arouse if its breathing passage is blocked by bedding, so that it can move into a position where its breathing can start again. An adult who stops breathing during sleep must also arouse to start breathing again. People with some of the diseases described in this book wake up ten times more often than healthy sleepers. The many awakenings reduce the amount of time these sleepers spend in the various sleep stages and prevent them from experiencing the amount of continuous high-quality sleep their bodies and brains need. The result is often daytime sleepiness and difficulty performing daily activities.

REM Sleep

Although scientists have uncovered a great deal about rapid eye movement sleep, there is still much to learn. As illustrated in the figure above, brain activity during REM sleep is similar to that seen in wakefulness. Furthermore, the brain cells use a great deal of energy, another indication of brain activity, which has led some scientists to theorize that REM sleep might play an important role in learning, laying down the networks in the brain that allow us both to learn new things and to remember what we’ve learned.

Curiously, at the same time that our brains are experiencing this activity during REM sleep, almost all of our muscles are paralyzed, with the exception of the major breathing muscle, the diaphragm, and certain sphincters at the top and bottom of our gastrointestinal tract.

In spite of this general paralysis, cells in the pons, a vital part of the brain that is also involved in controlling breathing, become active and create electrical storms, with electrical impulses that work their way through the central nervous system until they reach the part of the brain that controls eye movements, setting off the rapid eye movements characteristic of this state of sleep. When these impulses pass through the parts of the nervous system that control breathing and the cardiovascular system, they can create dangerous irregularities in the pattern of breathing as well as in heart rate and rhythm and blood pressure. Sleep apnea in women, for example, might only occur during REM sleep.

Perhaps the most intriguing—and mysterious—feature of REM sleep is that most vivid dreaming occurs during this enigmatic state. All humans with intact brains dream, usually three to five times a night. Although scholars and thinkers ranging from the indigenous Australians to the seventh-century B.C.E. Assyrian king Ashurbanipal to Sigmund Freud have ascribed meanings to dreams, we still know very little about their origins or function.

In about 400 C.E., a Roman by the name of Macrobius wrote a treatise in which he characterized five different types of dreams: the mysterious dream that requires interpretation (as Freud later did); the prophetic dream that comes true (as happened in the biblical story of Jacob’s ladder dream); dreams in which authority figures direct the dreamer (such as the saints directing Joan of Arc in her visions); dreams related to nightmares (as are found with post-traumatic stress); and nightmares in which there is contact with an apparition (such as Scrooge’s encounters in Charles Dickens’s Christmas Carol).

We know, for example, that all animals apparently experience REM sleep, but do they dream? When a puppy thrashes about or barks or appears to run during sleep, is it dreaming? Newborns spend half their sleep time in REM sleep. Are they dreaming?

Every time a male dreams, he has an erection, and every time a female dreams, the blood vessels of her vagina become engorged. Yet it appears that sexual thoughts before sleeping or explicitly sexual dreams do not cause these events; they are the result of the state of dreaming itself.

Some people react physically to what they are dreaming, instead of being paralyzed.

We do not know why some people who have suffered psychological trauma awaken for years with dreams that replay that trauma.

How Much Sleep Do We Need?

How much we sleep, when we sleep, and how much deep sleep and dreaming sleep we need vary with age. Seven to nine hours of sleep, which is adequate for most adults, would not be enough for the average nine-year-old. The table below shows the range of sleep required at different ages. For all age groups, however, the amount of sleep a person needs is an individual characteristic just as his or her height is.

The amount of sleep an individual needs each night is however much will enable him or her to remain wide awake and alert the next day, and it varies over the individual’s lifespan, decreasing with age. Babies spend most of their time sleeping (although it may not feel that way to their sleep-deprived parents), and they do not follow a pattern: for the first few months of life, infants might sleep at any time during the twenty-four-hour day. Mercifully for the parents, they soon start to spend periods in sleep mostly at night. Infants and toddlers nap. By the time children go to school, most will no longer nap.


The amount of REM sleep also decreases with age. Newborns spend roughly half their sleep time in REM sleep. In adults the amount of time spent in REM sleep decreases to 20–25 percent. The amount of slow-wave sleep is also much higher in children, since this is the sleep state during which most of the human growth hormone is secreted; the time most people spend in slow-wave sleep decreases with aging. Some elderly people have no slow-wave sleep.

As will be detailed later in this book, few of us get the amount of sleep we need. Today’s world and its demands on our time have eaten away at our sleep time. In teenagers the deficit can be more than two hours every night. The average American sleeps fewer than seven hours a night.

Teenagers, in particular, tend to develop bad sleep habits. They go to sleep later and awaken later than they did during childhood years. On school nights, it may take them several hours to fall asleep, and they are unable to get the eight to ten hours of sleep they need to be alert throughout the day. Parents may find themselves dragging an unwilling and unresponsive teenager out of bed, with the result that for the first few hours of the school day, the teenager might seem to be in a daze or might actually be asleep and consequently is likely to perform poorly. On weekends, these teenagers generally sleep until noon or later. For them to try to be alert at 9:00 A.M. would be like the average adult trying to be alert at 2:00 or 3:00 in the morning. It simply does not work. In the afternoon and evening, these adolescents frequently get a second wind.

Older persons, especially after retirement, might start to nap again. Whether reduced nighttime sleep in the elderly is a consequence of the daytime naps is not clear. Many elderly people sleep poorly not just because of their age but also because of medical conditions, medications, pain, sensitivity to their environment, or disruptions and changes in their sleep pattern. A survey of sleep in the older population, released in 2003, showed that older people without medical problems usually sleep the normal amount of time for their age group.

How the Brain Controls Our Sleep

Research has been done to find the chemicals and brain cells and pathways that control when humans sleep and when they wake up. Many brain structures are involved, as can be seen in the diagram of the brain centers (below) that control sleeping and waking. The darker-shaded boxes indicate structures involved in sleep, the lighter-shaded boxes, structures involved in wakefulness. However, to understand what starts and stops sleep we need to understand two concepts: the wake gauge and the body clock.


The fuel gauge in a car indicates when the driver needs to refuel the car. The wake gauge indicates when it is time to sleep. Once the average adult has been awake for about fourteen hours he or she starts to become sleepy, and the sleepiness becomes greater at sixteen hours, and much greater at eighteen hours, when it becomes difficult to stay awake. The wake gauge in the brain measures the amount of a chemical called adenosine, which is involved in the transfer of energy in the body. The longer a brain is active and using energy, the greater the concentration of adenosine. Adenosine acts to promote sleep and suppress wakefulness. Caffeine counteracts the effect of adenosine, which is how it keeps people awake.



In addition to influencing the amount of sleep the body gets, the brain helps control the time when people sleep. How does the body know when it is time to go to sleep and when to wake up? A collection of cells in the suprachiasmatic nucleus (SCN) of the brain has the ability to keep time and monitor ithe sleep-wake cycle. A hormone called melatonin is secreted by a tiny gland in the brain called the pineal gland when it starts to become dark, at dusk. The SCN cells control not only the times when individuals are sleepy or alert; they also control the function of many other systems in the body. The word many,in fact, may be an understatement. Most of the systems in the body have a pattern that varies over a twenty-four-hour period. This is true of the secretion of a variety of hormones, of blood pressure, of heart rate, and of some other functions in the body.

This natural, internal rhythm is called the circadian rhythm. The word derives from circa, “about,” and diem, “day.” The circadian rhythm changes the way many systems in the body work over the twenty-four-hour day so that the function of the systems matches what the body needs. Indeed, it has been shown that tissues in the body far away from the brain (such as the liver and kidneys) also have circadian clocks that are synchronized to the master clock in the brain. As a result, humans generally do not experience hunger or a need to go to the bathroom at night. People who have traveled across time zones know how discombobulated or out of sync they can feel because their body clock is not on the same time as the place they happen to be.

For many years, scientists wondered how the brain knew what time it was. How could it tell whether it was morning and time to wake up? Suppose we were living on a planet that was rotating around a sun every thirty hours instead of twenty-four hours—how would the body clock adjust? Studies conducted at Harvard University, the University of Pittsburgh, and other academic centers revealed that the light could reset the body clock. Exposure to morning sunlight was responsible for synchronizing the body’s circadian clock, in humans and other animals (and would do so for a thirty-hour day on another planet). Light enters the eye and hits the retina, stimulating specialized cells. The visual information travels from these cells along nerves to the suprachiasmatic nucleus, where the cells that control circadian rhythms reside. These cells are located above the optic chiasm, where visual information crosses over from one side of the nervous system to the other on its way to the part of the brain that processes vision. The SCN uses the information from the retina to tell the brain that it is morning; this in turn synchronizes the SCN cells. People who are blind because of problems between the eye and the optic chiasm can have serious difficulty in synchronizing their body clocks, and often have severe sleep problems as a result. On the other hand, people whose blindness results from problems in the visual cortex, the part of the brain that processes the information, might have a normal circadian system.

Circadian systems appear to be present not only in higher life forms, but also, for example, in plants. The chemical melatonin that is produced in the brain at dusk is also found in forms of life ranging from animals to insects to jellyfish to bacteria and even to plants. The first demonstration of a circadian rhythm was given by Jean-Jacques d’Ortous de Mairan, a Swiss scientist who set up an experiment using a mimosa plant that always opened its leaves at a certain time when it was sunny. He put the plant into a box, where it received no exposure to light, and the leaves still opened at the same time. This plant was able to keep track of time.

Life forms have existed on earth for more than a billion years. Mammals have inhabited the planet for about 200 million years. The dinosaurs disappeared about 65 million years ago. The earliest ancestors of humans appeared about 7 million years ago, and humans that resembled us arrived only two hundred thousand years ago. Humans evolved as diurnal beings: active during the day and sleeping when it is dark. Many other animals are nocturnal, sleeping during the day and active at night. It is likely that until about 150 years ago, with the arrival of artificial light, humans’ sleeping habits did not change much. Prehistoric people living in caves probably slept about two hours more than the average adult sleeps now. But people have not always slept in one continuous period at night. Long before the invention of the lightbulb, many people had two sleep periods a night. The first sleep started about two hours after sunset and would last three to four hours. The second sleep, also about four hours long, would start one to three hours later. Between the two sleeps the person might pray, read, or participate in sexual activity. The middle of the night was a literal “midnight” between these periods. In the past 150 years there have been dramatic changes in the quantity and pattern of human sleep. People sleep less, and they no longer sleep exclusively in the hours between dusk and dawn.

(A fascinating side note on our forebears: Although most probably slept on flat surfaces, ranging from the hard ground to soft mattresses, at various times and places sleeping sitting up was the norm. You can still see ancient buildings in the city of Bergen, Norway, in which the occupants slept in enclosed cubby holes that forced them to sleep sitting up. In Antwerp, one of the highlights of a visit to the home of Peter Paul Rubens is the short bed in which he slept sitting up.)

There are many time-related rhythms in biology, not all of which are understood. Some biological rhythms can be measured in seconds (for example, breathing and heartbeat), while others have much longer cycles. Although the cycle we know the most about is the circadian rhythm controlled by the body clock in our brains, seasons of the year seem to play an important role in when mammals can become sexually active, when they become pregnant, and when they give birth. One of the most mysterious cycles in biology is the menstrual cycle in women, which is reviewed in Chapter 3. This cycle is about the same length as the lunar cycle, and the timing system that results in the cycle, which averages twenty-eight days, is unknown.

How to Recognize When You Have Slept Well

No matter what your circadian rhythm or body clock is, or what position you sleep in, you should be able to recognize a healthy sleep pattern. After a good night of sleep you should feel wide awake and alert shortly after waking up, and remain so for the rest of the day. Your mood is generally good, and you feel no need for a nap. A good night of sleep is a matter of both quantity (it should be the right amount for your age group) and quality (it should be uninterrupted and consist of the right amount of each stage of sleep).

You should not wake up feeling as though you have not slept. You should not feel as though you won’t be able to function until you have had one or more cups of coffee. Struggling to stay awake while driving or falling asleep or feeling uncontrollably fidgety at movies, public meetings, or even in front of the television or computer screen are all signs that you may be sleep deprived. You should not feel as though you are about to fall asleep when reading.

If you do experience these symptoms, it indicates that the amount or the quality of your sleep is inadequate to keep you optimally awake and alert. You probably have a sleep problem if you are sleepy in the morning, feel tired all day, fall asleep when you don’t want to, need to nap, and are irritable and moody when you awaken. It is important to note that boredom does not cause sleepiness. Boredom simply gives the sleepy person the excuse to nod off.

Other symptoms, which will be discussed later in the book, might indicate that you have a medical problem. These include waking up with heart-burn, chest pain, shortness of breath, or an unusually fast or slow heartbeat. Waking up with a headache more than just occasionally or having to make frequent trips to the bathroom at night could also be signs of a medical problem. You should not wake up unable to move or with severe sweating. Nor should you be thrashing around in a way that could injure yourself or others while you are sleeping. Bed partners should not be telling you that you stop breathing during sleep and that it is scary to watch you sleep. If you have any of these symptoms, they probably relate to a medical problem that should be investigated.

You should also see a doctor if you are an adult who sleeps more than ten or less than five hours a night. Research has shown repeatedly that people who consistently sleep too much (more than ten hours a night) or too little (fewer than five hours a night) have a higher death rate than those who sleep the appropriate length of time. However, the key issue is not the length of sleep. Such abnormal amounts of sleep are rather a symptom of a sleep or medical disorder that may cause or result in death.

The bottom line is that if you are not wide awake and alert throughout the day, if you experience daytime sleepiness, or if you have any of the symptoms I just described, you may have a problem. Your sleep problem could affect you, your family, or the entire world. Consider the cases of some recent U.S. presidents.

They have perhaps the most grueling job in the world. No days off. Stress. Travel. Always on call. This is a recipe for sleep deprivation. Sleep deprivation takes its toll. Did lack of sleep almost change U.S. history?

There is widespread speculation that a sleep-deprived Barack Obama was falling asleep during his first debate with Mitt Romney in Denver during the 2012 election. There are several possible explanations for Obama’s sleep problem. The debate preparation resulted in sleepless nights. In addition, the city where the debate took place, Denver, is a mile above sea level. Some people at this altitude develop an abnormal sleep breathing pattern (a variant of sleep apnea) which causes them to have short awakenings during the night, resulting in restless or non-refreshing sleep. Despite reports that suggest Obama is a night owl who goes to bed late and gets up early, apparently getting only four to five hours sleep and sometimes not even that, such sleep deprivation might have accounted for his poor showing in Denver. He needs to sleep more. He mentioned at a prayer meeting that after he leaves office, “I am going to take three, four months where I just sleep.”

Ronald Reagan fell asleep during an audience with the pope. Bill Clinton had meetings that went through much of the night, as reported in Newsweek: “Over the years Clinton had tried to convince himself he could get by just fine on a few hours of sleep a night. Time and again, he proved himself wrong. Struggling to extricate himself from a previous scandal, Clinton once told a friend, ‘Every important mistake I’ve made in my life, I’ve made because I was too tired.’” William Taft, who weighed as much as 350 pounds (there is to this day an extra-wide “Taft chair” in Woolsey Hall at Yale), had sleep apnea and was sleepy while in office. The most powerful people in the world are sometimes almost incapacitated by their sleep problems.

We Are Not All the Same, but We All Need a Good Night’s Sleep

With all that said, some very high-functioning people sleep more than ten hours a night, while some high-functioning people sleep less than four. It is important to remember that data obtained from a population may not apply to an individual.

The importance, control, and complexity of sleep is something we have only recently begun to understand. A good night’s sleep can make a person feel wonderful and be highly productive. A poor night’s sleep or insufficient sleep leaves a person feeling exhausted and nonproductive; he or she may even pose a danger to others. Women are much more likely than men to have sleep problems. This is not just because some sleep disorders are more common in women but also because family responsibilities, menstruation, pregnancy, and menopause can disrupt normal sleep.

Often in a household the main caregiver does double duty. She or he often both works outside the home and acts as the primary caregiver, and this person is the first up in the morning and the last to go to bed at night. The primary caregiver often organizes and runs the household, prepares meals, and does the housecleaning. These caregivers need to be emotionally alert to the needs of spouses and children, to problems and issues in the family, and so on. If they lose sleep, not only they but the entire family can suffer. They can be cranky, irritable, and short-tempered.

For those performing these roles, a good night’s sleep is essential. After a good night of sleep people start the day fresh, with enough energy to take them through the necessary tasks. For a healthy lifestyle, a good night’s sleep is as important as healthy eating and adequate exercise. But not everyone recognizes sleep problems when they arise. In the following chapters I offer the information readers need to recognize when they have a problem, and what they can do about it.