Counting Sheep: The Science and Pleasures of Sleep and Dreams

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Sir Thomas Browne, Religio Medici (1642)

What happens when sleep deprivation is taken to the extreme? If a slight insufficiency of sleep makes us feel unwell, would a prolonged absence kill us? Setting aside purely anecdotal accounts, science has unsurprisingly not investigated whether forcibly depriving humans of sleep is fatal. Ethical committees would tend to frown upon applications from scientists proposing to test this experimentally. But the evidence from other species is clear. Animals that have been experimentally deprived of sleep for long enough invariably die. There is no reason to suppose that humans are fundamentally different.

Some of the earliest experiments on extreme sleep deprivation were performed in the late nineteenth century by a Russian scientist called Marie de Manacéïne. She deprived puppies of sleep by keeping them constantly active. They all died within four or five days, despite every effort to keep them alive. The younger the puppy, the more rapidly it succumbed. Marie de Manacéïne also noticed a progressive decline in the body temperature of the sleep-deprived animals, a phenomenon that is now known to be a standard symptom of prolonged sleep deprivation in humans and other species. She concluded that sleep was even more crucial for survival than food:

As a rule, the puppy deprived of sleep for three or four days presents a more pitiful appearance than one which has passed ten or fifteen days without food. I can speak from observation, as I was obliged to make experiments on the results of want of food as well as of sleep, and I became firmly convinced that sleep is more necessary to animals endowed with consciousness than even food.

Italian scientists working at the end of the nineteenth century kept adult dogs awake by making them walk. The sleep-deprived dogs all died after 9–17 days, regardless of how much food they ate.

One objection to experiments such as these (apart from the obvious ethical one) is that the scientists had to use increasingly stressful methods to keep the animals awake, so perhaps it was the stress that killed them rather than the sleep deprivation itself. Prolonged stress can impair the immune system and make an animal more vulnerable to infection. However, more recent research has managed to sidestep this methodological problem.

In a long series of experiments, Alan Rechtschaffen and colleagues at the University of Chicago systematically investigated how prolonged sleep deprivation affects rats. They used an experimental procedure known as the disc-over-water method, which works like this. Two rats – the experimental subject and the ‘yoked control’ – are placed on a turntable mounted over a shallow bath of water. The brain-wave patterns of both animals are continuously monitored to detect the onset of sleep. When the experimental rat’s brain waves indicate that it is falling asleep, the turntable automatically revolves slowly, waking the unfortunate rat and forcing it to walk in the opposite direction to avoid being pitched into the water. The control animal, which is on the other side of the turntable and separated by a partition, receives precisely the same treatment at precisely the same times. The crucial difference is that the turntable movements are unaffected by its sleep. The control animal is therefore able to get some sleep when the experimental animal is awake. This cunning technique has the advantage – from the human experimenter’s point of view – of preventing the experimental subject from sleeping without having to subject it to other noxious stimuli.

Rats that are prevented in this way from sleeping invariably die after two or three weeks. The control animals, which experience the same stimuli but not the complete loss of sleep, survive and display relatively few symptoms. Before they die, the sleep-deprived rats all exhibit the same horrible syndrome. This is characterised by a debilitated appearance, skin lesions, increased food intake, weight loss, increased metabolic rate, increased levels of the hormone noradrenaline, and declining body temperature. Some of these changes are symptomatic of excessive heat loss from the body, which has led some scientists to suggest that sleep is crucial, among other things, for the regulation of body temperature.

A progressive rise in metabolic rate (the rate at which the body consumes energy) is an early symptom of sleep deprivation. Sleep-deprived rats eat more to compensate for their rising energy expenditure, but their weight and body temperature nonetheless continue to fall. Feeding them an easily digestible diet helps to slow this process somewhat, but it does not prevent them from dying. An increase in appetite is one of the less obvious effects of sleep deprivation in humans as well. Might it be that chronic sleep deprivation is one of the factors helping to fuel the epidemic of obesity that is currently sweeping the USA, UK and other industrialised nations?

Body and soul (#ulink_056f000a-6d8d-504c-a021-e7d92fc8b9f5)

Health may be as much injured by interrupted and insufficient sleep as by luxurious indulgence.

William Kitchiner, The Art of Invigorating and Prolonging Life (1822)

What of people? Research on humans has stopped short of the lethal sleep deprivation imposed on rats and puppies, but it has delved systematically into the consequences of a few days’ sleep loss. The results consistently show that moderate sleep deprivation has pervasive effects on the human body as well as the human mind. Sleep loss impairs vision, for example, causing blurring and errors in judging distances. It also triggers the familiar decline in body temperature that Marie de Manacéïine observed in her puppies, together with a reduction in blood glucose levels and changes in various hormones.

Set against this, sleep loss has surprisingly little impact on our ability to keep moving around and doing physical work. Moderate sleep deprivation does not greatly diminish our capacity for labour. Physically fit young adults can withstand several days of sleep deprivation without a substantial deterioration in their muscle strength, muscle endurance or cardiovascular responses to exercise. In one experiment, for example, the exercise capacity of young women was assessed following 60 hours without sleep. The sleep deprivation had no significant effect on their aerobic capacity or their endurance for exhausting exercise. In another study, researchers monitored two men while they played a marathon tennis match lasting a week, during which time the players got very little sleep. Although their mental performance deteriorated during the match, the players were able to sustain a high level of physical work. Our muscles can mostly keep going even when our brains are flagging.

Sleep deprivation does disturb many aspects of physiological functioning, however. Breathing is one example. A single night of sleep loss impairs breathing in healthy people, provoking a small but significant reduction in the maximum amount of air that can be exhaled after maximum inhalation. Sleep loss also leads to a substantial blunting of the normal respiratory responses to reduced blood-oxygen levels. After 30 hours without sleep there are marked deteriorations in the strength and endurance of the muscles used for breathing – as revealed, for example, by a reduction in the time for which people can breathe in against a sustained pressure. Such changes could be important in patients with respiratory diseases, who often suffer from chronic sleep loss. Sleep deprivation also slows the rate of cardiovascular recovery from intense exercise. When someone has been deprived of sleep for 24 hours, their breathing rate and oxygen uptake after a burst of intense exercise remain higher for longer.

Sleep loss is accompanied by many changes in body chemistry. People who have been kept awake for more than three days have altered liver functions, marked by large increases in the levels of key liver enzymes, changes in various types of fat and a rise in the amount of phosphorus circulating in the blood. Thyroid hormone levels are affected and biochemical changes can be detected at the level of gene activity.

Glucose metabolism is particularly perturbed by sleep loss. Healthy young men whose sleep was experimentally restricted to four hours a night for six nights became less tolerant to glucose. They took 40 per cent longer than normal to regulate their blood-sugar levels after eating high-carbohydrate food, and their ability to produce insulin fell by nearly a third – a condition resembling the early signs of diabetes. These abnormalities vanished after the men had slept for 12 hours. Fatigue-induced physiological changes like these could contribute to the development of chronic conditions such as diabetes, obesity and high blood pressure, all of which are associated with a shortened lifespan.

Sleep, immunity and health (#ulink_1f2df845-d1fc-5f97-86ab-ea2ec9fcf366)

Our foster-nurse of nature is repose.

William Shakespeare, King Lear (1605–6)

Some of the most interesting, least well understood, and potentially important consequences of sleep deprivation are found within the immune system. In short, lack of sleep can impair the body’s immune defences and thereby make us more susceptible to infection by bacteria, viruses and parasites.

The evidence comes mostly from research with other species. In one experiment, for example, mice that were immunised against the influenza virus were resistant to infection if they were exposed again to the virus a week later. But if the immunised mice were deprived of sleep for seven hours immediately after being exposed to the virus, they were no more resistant to infection than mice that had not been immunised at all. A mere seven hours of sleep deprivation disturbed their immune response enough to erase the benefits of immunisation.

Some scientists have suggested that one reason why prolonged sleep deprivation is ultimately fatal is that it breaks down the animal’s immune defences, making it vulnerable to infection by any opportunistic bacteria and viruses that happen to be in the vicinity. Experiments with rats have shown that following severe sleep deprivation, the lymph nodes and other organs are invaded by potentially dangerous bacteria, which appear to have migrated there from the intestines. However, the role of infection in killing sleep-deprived animals remains a controversial issue.

Sleep loss impairs the human immune system as well. Even modest sleep deprivation evokes measurable changes. One night of sleep loss lowers the activity of natural killer cells and reduces the numbers of several different types of white blood cells circulating in the bloodstream. (Natural killer cells are a special type of lymphocyte, or white blood cell, that attack virus-infected cells and certain types of cancer cells.) Depriving healthy adults of sleep for seven hours on one night suppressed their natural killer-cell activity by 28 per cent. It bounced back to normal after a night of uninterrupted sleep. Moderate sleep loss will also reduce the body’s production of interleukin-2, a chemical messenger substance that plays an important role in regulating immune responses. After two or three days of sleep deprivation there is a marked decline in the responsiveness of lymphocytes and an even bigger fall in the activity of natural killer cells.

Sleep loss might play a role in the well-established connection between severe depression and impaired immune function. Depressed people generally sleep badly and have poorer immune responses. The more disrupted their sleep, the bigger the decline in their immune function. One study, for example, found that people who were suffering from depression following bereavement had fewer natural killer cells. The bereaved subjects were troubled by intrusive thoughts that often woke them or kept them awake during the night. The extent of the reduction in their natural killer-cell numbers was correlated with the amount of time they spent awake during the night: the more troubled someone was by their loss, the more disrupted their sleep and the fewer natural killer cells circulating in their blood. Sleep deprivation could be one of the mechanisms by which depression makes people more vulnerable to illness.

The relationship between sleep and immunity works in both directions. Not only does sleep affect the immune system, but the immune system also affects sleep. The immune reactions triggered by infection and illness can elicit alterations in sleep patterns. That is why infections are often accompanied by lethargy, loss of appetite, depressed mood and general malaise. Animals infected with influenza virus display a large increase in sleep about 24 hours after exposure to the virus. These changes in wakefulness are part of the body’s defence mechanisms and assist the recovery process. Human experiments, in which noble volunteers were injected with bacterial toxins, found that sleep is highly sensitive to the activation of the immune defences. Low-level infection tends to promote deep sleep. However, a full-blown infection accompanied by fever induces lethargy but typically disrupts sleep. You might have noticed that you sleep more deeply for a night or two when your body is fending off a potential infection, whereas when you are in the throes of a galloping illness you feel exhausted but lie for hours without sleeping.

The immune response to infection stimulates the release of chemical messenger substances that act on the brain to induce malaise, drowsiness, loss of appetite and sleep. During infection, a substance known as interleukin-1 stimulates the brain to induce deep sleep, while other interleukins trigger the fever that often accompanies infections. They do this by adjusting the brain’s temperature control centres – in effect, putting the body’s thermostat on a higher setting. That is why we feel hot and sleepy when we have a bad infection. The fever response is a defence mechanism found in all animals: the rise in body temperature makes life harder for the offending bacteria or viruses, and the lethargy forces the infected organism to curl up in a dark corner and sleep until it has recovered. It all makes good biological sense.

The brain and the immune system are interconnected through an elaborate network of chemical and neural communication channels. One important link between sleep, immune function and psychological stress is the steroid hormone cortisol. Sleep deprivation and prolonged stress both provoke an increase in the level of cortisol. After one night of sleep loss, your cortisol levels would typically be raised by about 45 per cent the next evening. It is not good to have elevated cortisol levels for too long, since cortisol has a powerful suppressive effect on the immune system. The functioning of the immune system is also intimately bound up with the 24-hour sleep – wake cycle and the circadian rhythms in hormone levels. Various aspects of immune function fluctuate in tune with the circadian cycle. Anything that disrupts the normal cycle of sleep and wakefulness therefore tends to disturb the immune system, with potential consequences for the body’s ability to defend itself against infection and disease.

The intimate relationship between sleep and immune function takes on a potentially huge practical significance when you consider how widespread sleep deprivation has become in society. Tired people are more likely to become sick people.

The Battle of Stalingrad (#ulink_9b00653d-4c92-584b-bd9a-f2e3f6199cb4)

O, I have passed a miserable night,

So full of fearful dreams, of ugly sights

William Shakespeare, Richard III (1591)

Prolonged sleep deprivation, uncontrollable stress and starvation make a lethal cocktail, as Hitler’s troops found to their cost during the Battle of Stalingrad in World War Two. In June 1941 German forces invaded the Soviet Union and were soon threatening Moscow. The capture of Stalingrad on the River Volga became a key strategic objective. Stalin decreed that the city must be defended to the bitter end. The titanic struggle that ensued cost the lives of at least 800,000 Axis soldiers and 1.1 million Soviet soldiers.

The fight for Stalingrad (now renamed Volgograd) began in earnest in the summer of 1942, as the Germans advanced rapidly towards its suburbs. There was fierce Soviet resistance and the fighting dragged on into the harsh Russian winter. By September 1942 the battle was being waged at close quarters among the buildings, cellars, sewers and bunkers of ‘the Stalingrad Academy of street-fighting’.

To increase the pressure on their opponents, the Soviet commanders ordered continual raids to be carried out by night. They did this partly because the Germans lacked protection from their air force at night, but mainly to induce exhaustion among the enemy. To augment the night raids, the Soviets fired flares indicating that an attack was imminent even when it was not. Their air force also attacked German positions every night. The Soviets kept up the psychological pressure throughout the night, with loudspeakers blaring out propaganda broadcasts, surreal tango music, or the sound of a ticking clock. The strategy was highly effective. ‘We lie exhausted in our holes waiting for them,’ wrote one German soldier. The German commanders begged for air support, citing their men’s exhaustion.

The German troops’ health started to deteriorate badly even before the dreadful Russian winter had begun to bite. There was a sharp rise in deaths from infectious diseases including dysentery, typhus and paratyphus. The actual prevalence of these diseases was not much worse than it had been a year earlier, but the numbers of infected men who were dying from them increased fivefold. It was as though the German soldiers had lost their capacity to resist infection. The Russians noticed this phenomenon, which they referred to as ‘the German sickness’.

In November 1942 the Russians launched a huge and ultimately successful counteroffensive that soon had the Germans encircled within the ruined city. But the Germans were under orders from Hitler not to surrender, and so they fought on through December while the Russians gradually tightened the noose. Conditions for the German troops became appalling as their supply lines were cut off and the Russian winter froze them. There was hardly any food and little or no medical care.

In mid-December 1942 the German military doctors in Stalingrad noticed a new phenomenon: more and more apparently healthy troops were suddenly dying for no obvious reason. The Germans were unsure whether the deaths were the result of starvation, exposure, exhaustion or an unidentified disease. A German army pathologist named Girgensohn, who was sent to Stalingrad to investigate the problem, became convinced that a combination of exhaustion, stress, cold and lack of food was responsible for the much higher death rate. The Russian night attacks and round-the-clock activity had caused severe sleep deprivation, and Girgensohn concluded that this had amplified the effects of the food shortage by ‘upsetting the metabolism’ of the exhausted Germans. We know now that one symptom of prolonged sleep deprivation is a marked increase in metabolic rate and hence the requirement for food. Whatever the precise explanation, the pressure was too much for the Germans. In February 1943 the Battle of Stalingrad finally ground to a halt, as the crushed and starving remnants of the German army surrendered.

Sleepless in hospital (#ulink_970a2ca6-868e-5135-aa5d-1fce1f4248e3)

I have the feeling that once I am at home again I shall need to sleep three weeks on end to get rested from the rest I have had.

Thomas Mann, The Magic Mountain (1924)

Sleep is good for you and lack of sleep is bad. It therefore seems odd that hospitals, which are supposed to promote recovery, are usually dreadful environments for sleeping. ‘The hospital bed,’ wrote one historian, ‘is one in which normal sleep is forbidden.’ A Punch cartoon of 1906 shows a patient being told to ‘wake up and take your sleeping-draught’. Things have improved since 1906, but not much.

Sick people really do benefit from sleep. We saw earlier how the brain and the immune system respond naturally to infection by inducing sleep. This helps the body cope with disease in several ways. The production of growth hormone occurs mainly during sleep, and growth hormone aids physical recovery by promoting the healing of mucous membranes and in other ways. The hormone melatonin, which is also produced at night, boosts immune responses, inhibits the growth of tumours and enhances resistance to viral infections. Conversely, sleep deprivation impairs immunity and slows the healing process. Given the importance of sleep for recovery, it is ironic that hospital patients are routinely subjected to conditions that make normal sleep almost impossible.

Sick people start with big disadvantages, of course. Pain is a powerful disrupter of sleep. Patients suffering from chronic, severe pain often become exhausted. Disrupted sleep is a common complication of burn injuries, for example. Studies have found that between half and three quarters of burns patients experience significant sleep disturbances. Sleep problems are common among cancer patients too. Fatigue can become one of the most distressing aspects of having cancer, severely reducing the quality of life. While medical treatments for cancer have advanced apace, efforts to improve patients’ quality of life by alleviating their fatigue have lagged behind.

To make matters worse, tired people are more sensitive to pain. Sleep deprivation lowers pain thresholds, generating a vicious cycle in which pain disrupts sleep, the resulting sleep loss makes the pain feel even worse, and so on. An investigation of patients with burns injuries uncovered a systematic link between the quality of their sleep and subsequent pain. Patients who slept poorly during the night experienced more intense pain the following day, because the fatigue intensified their perception of pain.

One of the worst places imaginable if you need a good night’s sleep is an intensive care unit (ICU). The combination of serious illness, serious drugs, constant monitoring, bright lighting and the aftereffects of surgery ensures that ICU patients are often subjected to severe sleep deprivation. And yet the ICU houses the sickest people in the hospital, with the greatest need for sleep. American researchers conducted an experiment to see if implementing regular ‘quiet times’ during the day would help ICU patients to get more sleep. Each day for two two-hour periods, lighting levels in the unit were reduced and the staff made a concerted effort to minimise noise. The ‘quiet-time’ regime worked: the patients were 60 per cent more likely to sleep during these quiet periods than at other times of the day. More flexibility over when patients are given their medication can also help them to get more sleep.

Sadly, the apparent neglect of sleep in hospitals is another reflection of the general disregard for sleep in medicine and society as a whole. Lack of sleep really does have very little to recommend it.

PART III Mechanisms (#ulink_be9ab661-a886-5eee-a451-329f6d4fd927)

5 The Shapes of Sleep (#ulink_70b8113c-3786-5379-a11c-7351d23b0377)

Sleep rock thy brain.

William Shakespeare, Hamlet (1601)

Now it is time to peer beneath the surface at the strange state of existence known as sleep – or, to be more precise, the two strange states of existence known as sleep.

All of human life is spent in one of three states. You are very familiar with one of them: it is called the waking state, or consciousness, and it forms the subject matter for almost everything that has ever been said, written, acted, painted or composed about humanity. When scientists analyse the mind, when novelists dissect the human condition and when biographers portray the lives of eminent individuals, it is the waking state they almost invariably describe. However, there are two other distinct states of existence that together account for at least a third of each life. They labour under the workaday names of Rapid Eye Movement (REM) sleep and Non-Rapid Eye Movement (NREM) sleep, and we are about to take a closer look at them.

A night’s sleep is a complex and cyclic process, comprising several distinct patterns of brain activity and behaviour, with alternating episodes of NREM sleep and REM sleep. We will follow the sleep cycle from the beginning, starting with the transition from the waking state. But before we do that, a quick word about how scientists know what is going on when we are asleep.

Measuring sleep (#ulink_1726e0c8-aadf-5969-b848-c38cd4d3615b)

Brains wave.

Owen Flanagan, Dreaming Souls (2000)

The sleeping brain reveals what is going on inside itself in various ways, both electrically and chemically. Since the middle of the twentieth century, the main tool for monitoring sleep has been the electroencephalograph. This machine exploits the fortunate fact that varying patterns of electrical activity within the brain manifest themselves as varying patterns of voltage changes on the surface of the scalp.

The brain comprises billions of nerve cells, or neurons, and although the electrical activity of an individual neuron is too faint to be detected outside the skull, it is possible to monitor the gross patterns generated collectively by large numbers of neurons. These show up as minute voltage changes, which can be detected by electrodes stuck onto the scalp. (The very first electrodes were small pins that were stuck into the scalps of stoical volunteers.) Thus, the brain emits electrical signals revealing information about its inner state. These tiny voltage patterns are amplified and displayed as the familiar ‘brain waves’ of the electroencephalogram, or EEG. (Confusingly, the machine is called an electroencephalograph, while the graph it produces is called an electroencephalogram, or EEG. To avoid nausea, I will use EEG to denote both the machine and its output.)

The EEG was invented in the 1920s by a psychiatrist named Hans Berger. It really came into its own in the 1950s when, as we shall see, it enabled the discovery of REM sleep. Before the invention of the EEG, scientists could only assess sleep by observing overt body movements, or the lack of them. Scientists still find it useful to record sleepers’ body movements, especially in studies of sleep patterns under natural conditions where the use of EEG would be too intrusive or too expensive. Nowadays, body movements are usually logged automatically, using a miniature recorder worn on the wrist.

Sleep laboratories use an extension of the EEG called the polysomnograph – a sort of somnolent variation on the polygraph. A polysomnograph records the EEG brain waves, together with other informative measures of the sleeper’s physiological state and behaviour. Electrodes placed near the corners of the eyes detect movements of the eyeballs, producing a trace known as the electro-oculogram, or EOG. Other electrodes placed on the chin and neck monitor the muscle tone (producing an electromyogram, or EMG) while electrodes on the chest record the heart rhythms (electrocardiogram, or ECG). Additional devices may record whole body movements, breathing, the flow of air through the nose and mouth, and the concentration of oxygen in the blood. In the early days of sleep science, these measurements were recorded as continuous pen traces on miles of rapidly unfurling paper, but nowadays the outputs are usually stored digitally.

In recent decades, brain scanning has become an increasingly important tool in sleep research. One of the main brain-scanning techniques is called positron emission tomography (PET). PET scans reveal the local patterns of blood flow and oxygen uptake within small areas of the brain by measuring how rapidly the tissue is using energy. Unlike some brain-scanning techniques, PET does not require the subject to sleep inside a large, claustrophobia-inducing scanning device. It therefore allows scientists to monitor sleep under conditions that are slightly closer to normality. Even so, the sleeping subject’s head needs to be kept absolutely still, which is usually achieved by pinning the head down with a special mask (the stuff of some people’s nightmares).

Most measurements of sleep are made in specialised sleep laboratories rather than people’s own homes. The underlying assumption is that the sleep patterns observed in the laboratory closely resemble the real thing. Fortunately, this turns out to be a broadly valid assumption. Comparisons have confirmed that for most people there is a reasonably good concordance between their sleep patterns at home and in the sleep laboratory. But there are some systematic differences. In particular, people tend to sleep for a slightly shorter period under laboratory conditions and to wake up slightly earlier than they would normally. They also have less bizarre dreams and fewer wet dreams. (Wouldn’t you?)

Falling asleep again, what am I to do? (#ulink_b330ab61-a9f6-5a33-a7d6-2ff7e618681b)

Warm beds: warm full blooded life.

James Joyce, Ulysses (1922)

Falling asleep is not an abrupt process, like turning off a light, although it can seem like that because you usually forget about it. Recordings of brain-wave activity and other physiological variables show that falling asleep is in fact a continuous process, which starts from a state of relaxed drowsiness and ends in the first or second stages of unequivocal sleep.

During that process of falling asleep you may find yourself temporarily suspended for several minutes between the worlds of waking consciousness and sleep. This transition phase is often accompanied by strange thoughts, dreamlike images and occasional hallucinations. In one of his short stories, Washington Irving described how the mind can roam far and wide while it is in this pre-sleep state:

My uncle lay with his eyes half closed, and his nightcap drawn almost down to his nose. His fancy was already wandering, and began to mingle up the present scene with the crater of Vesuvius, the French Opera, the Coliseum at Rome, Dolly’s Chop house in London, and all the farrago of noted places with which the brain of a traveller is crammed – in a word, he was just falling asleep.

These dreamlike experiences occur when we are in what is known as the hypnagogic state – a twilight zone partway between wakefulness and sleep. They are referred to as hypnagogic (or sleep-onset) dreams and they are distinct from ordinary dreams, which do not occur until much later in the sleep cycle. Hypnagogic dreams can contain all the basic elements of ordinary dreams, including bizarre plots, visual images and sounds, but there are fewer of these features in any one dream, suggesting that hypnagogic dreaming is a reduced version of normal dreaming. Similar dreamlike experiences can also occur at the other end of a night’s sleep, during the transition from sleep to wakefulness, when they are known as hypnopompic dreams.

The hypnagogic and hypnopompic states are strange and fascinating. In comparison with true sleep and ordinary dreams, they are also poorly researched and poorly understood. Indeed, the English language does not even have a decent name for them – unlike Italian, which has a single word for both (dormiveglia, or ‘sleep-waking’). In English, hypnagogic dreams are colloquially referred to by a variety of vague terms such as ‘faces in the dark’ or ‘visions of half-asleep’. As we shall see in a later chapter, many famous creative flashes and inspired thoughts have come to people while in the hypnagogic state.

During hypnagogic dreams we may see strange sights, hear strange sounds and think strange thoughts. As our wakefulness fluctuates, we may wake up again and consciously remember the strange things we have briefly been dreaming. This hypnagogic nonsense sometimes includes bizarre, invented words. The sleep researcher Ian Oswald recalled waking from one hypnagogic dream with the phrase ‘or squawns of medication allow me to ungather’ running through his mind. On another occasion he found himself musing on the hypnagogic thought that ‘it’s rather indoctrinecal’. A British magazine once printed a collection of hypnagogic ramblings sent in by readers. These included the immortal verse ‘Only God and Henry Ford have no umbilical cord.’

Hypnagogic thoughts and images can be more coherent, however. Charles Dickens often fell into a half-sleeping state while on one of his long nocturnal walks, and he could compose poetry while in this reverie. Dickens wrote of how, one night, he got out of bed at two in the morning and walked thirty miles into the countryside:

I fell asleep to the monotonous sound of my own feet, doing their regular four miles an hour. Mile after mile I walked, without the slightest sense of exertion, dozing heavily and dreaming constantly … It is a curiosity of broken sleep that I made immense quantities of verses on that pedestrian occasion (of course I never make any when I am in my right senses), and that I spoke a certain language once pretty familiar to me, but which I have nearly forgotten from disuse, with fluency. Of both these phenomena I have such frequent experience in the state between sleeping and waking, that I sometimes argue with myself that I know I cannot be awake, for, if I were, I should not be half so ready.

People who play lots of computer games sometimes experience ‘screen dreams’ as they fall asleep, in which they see vivid images of the game they have been playing. These screen dreams are also products of the hypnagogic state. The computer game Tetris, which requires the player to fit together coloured shapes as they cascade down the screen, is well known for provoking hypnagogic dreams. Scientists at Harvard Medical School investigated screen dreams by getting volunteers to play Tetris for several hours. Many of them experienced vivid dreams about Tetris as they fell asleep. Among the subjects in this experiment were five amnesiac patients who had extensive brain damage in their temporal medial lobes – brain regions crucial for conscious memory. Three of the five amnesiacs experienced hypnagogic dreams of Tetris even though they had no conscious memory of playing the game. This implies that the brain can generate hypnagogic dreams without input from conscious memory.

The length of time it takes you to fall asleep, once you have lain down and shut your eyes, is known as your sleep latency. It varies according to lots of factors. As we saw earlier, very short sleep latencies usually indicate sleep deprivation, whereas very long sleep latencies may signify other problems. A study of people living in rural Oxfordshire found that those with the longest sleep latencies typically described themselves as bored or mildly ill. You can make yourself fall asleep faster if you are minded to do so. Researchers proved this by giving volunteers a financial incentive to fall asleep quickly at various times during the day. The paid volunteers fell asleep faster than subjects who had no financial incentive.

Your body temperature has a big influence on how fast you fall asleep. A night’s sleep is normally preceded by a drop in core body temperature, and scientists have established that this drop in temperature actively facilitates the onset of sleep. Under normal conditions, the maximum rate of decrease in body temperature occurs about one hour before the onset of sleep. If the onset of sleep is artificially delayed, the drop in body temperature is attenuated – further evidence that the two are closely linked. The polymath Benjamin Franklin realised the importance of a falling body temperature in triggering sleep. He set out this practical advice in a 1786 essay called ‘The Art of Procuring Pleasant Dreams’:

Get out of bed, beat up and turn your pillow, shake the bedclothes well with at least twenty shakes, then throw the bed open and leave it to cool; in the meanwhile, continuing undressed, walk about your chamber. When you begin to feel the cold air unpleasant, then return to your bed, and you will soon fall asleep, and your sleep will be sweet and pleasant.

Benjamin Disraeli found that he was more comfortable when sleeping in hot weather if he used two beds, moving periodically from the hot, sweaty bed into the cooler one. Benjamin Franklin lit upon the same trick years earlier, but Franklin reckoned he needed four beds to be really cool. William Harvey, the seventeenth-century English physician who discovered the circulation of blood, similarly appreciated that cooling the body helps to induce sleep. According to his contemporary, the biographer John Aubrey, Harvey would tackle his insomnia by cooling himself down until he began to shiver:

He was hot-headed, and his thoughts working would many times keep him from sleeping. He told me that then his way was to rise out of his Bed, and walk about his Chamber in his Shirt, till he was pretty cool, i.e. till he began to have a horror [began to shiver], and then return to bed, and sleep very comfortably.

Another scholar who stumbled across the sleep-inducing properties of cool air was Lord Monboddo, an eccentric eighteenth-century Scottish nobleman and pioneering anthropologist. When Samuel Johnson and James Boswell visited Monboddo, the great sage and his biographer were surprised by their host’s behaviour. As Boswell recorded:

Lord Monboddo told me he awaked every morning at four, and then for his health got up and walked in his room naked, with the window open, which he called taking an air bath; after which he went to bed again, and slept two hours more. Johnson, who was always ready to beat down any thing that seemed to be exhibited with disproportionate importance, thus observed: ‘I suppose, Sir, there is no more in it than this, he awakes at four, and cannot sleep till he chills himself, and makes the warmth of the bed a grateful sensation.’

A less irksome way of achieving a similar effect is to take a hot bath an hour or two before bedtime. The bath will temporarily raise your body temperature. Over the following hours, your temperature will drop again and, all being well, this will help to trigger sleep. Experiments have confirmed that people do feel sleepier at bedtime after taking a hot bath. But the bath must not be too hot, too long or too close to bedtime, or it may have the reverse effect.

The fall in core body temperature that precedes sleep is accompanied by a small rise in the temperature of the hands, feet and other appendages. Blood vessels in your appendages dilate when you lie down to sleep at night, causing them to warm up. As they warm up so your body cools down, helping to send you off to sleep. Experiments have shown that warm feet assist the onset of sleep, bearing out another piece of folk wisdom. One of the best ways of predicting how quickly someone will fall asleep is to measure the temperature gradient across their body. The hands and feet are normally a degree or two cooler than core body temperature, but the temperature difference dwindles to nothing as sleep approaches.

A further demonstration of the linkage between warm appendages and the onset of sleep came from a study of people suffering from a disorder known as vasospastic syndrome. This condition is caused by faults in the physiological mechanisms controlling the peripheral blood vessels, which become less able to dilate. One of the main symptoms is cold hands and feet. As predicted, the cold-toed victims of vasospastic syndrome took longer than normal to fall asleep at night.

The importance of a declining body temperature means that artificial heat sources like electric blankets can disturb sleep. An electric blanket operating between the early hours of the morning and waking will typically increase your core body temperature by about 0.2 degrees Celsius. Even this small increase in body temperature is enough to disrupt sleep.

The sleep-inducing effect of a falling body temperature helps to explain why vigorous physical exercise, which raises body temperature, is not a good idea just before going to bed. It also reminds us why it is inadvisable to eat a large meal shortly before bedtime. The digestive processes that follow a large meal evoke a rise in metabolic rate, which in turn raises body temperature. In an ideal world, a large evening meal would be eaten at least three hours before bedtime. However, this helpful advice is of little use to the many people who work long hours and face long journeys to get home afterwards. They may have barely enough time to prepare and eat an evening meal before going to bed – another example of how lifestyles can conflict with good sleep.

One popular notion that reportedly fails to stand up to scientific scrutiny is that we fall asleep faster after orgasm. A group of enterprising researchers conducted an experiment in which they monitored the sleep of men and women under three different conditions: after the subjects had masturbated to orgasm, after they had masturbated without orgasm, and after they had simply read some nonsexual material. Recordings of their subsequent sleep yielded no evidence that masturbation, with or without orgasm, affected any aspects of sleep, implying that post-coital sleepiness has nothing to do with the attainment of orgasm. (You may find this hard to believe.) This is clearly an area crying out for more research.

The sleep cycle (#ulink_8aefcd07-4655-52e8-bff0-4d696acbd200)

Sleep flooded over him like a dark water.

Jorge Luis Borges, Labyrinths (1964)

Two broadly different states are conventionally bracketed together under the general heading of sleep: rapid eye movement (REM) sleep and non-rapid eye movement (NREM) sleep. NREM sleep is further subdivided into four different stages, based on their characteristic EEG patterns. Each sleep stage has its own distinctive pattern of brain activity. The various stages and types of sleep alternate cyclically throughout a night’s sleep.

As you become sleepier, your EEG pattern changes. If you are tired enough, this can happen even when you are walking around and supposedly wide awake. The pre-sleep state of quiet restfulness is heralded by the appearance of brain waves of lower frequency and higher voltage, called alpha waves. If you are very sleepy, but still awake, your alpha waves will be accompanied by slow, rolling movements of your eyes. Nearly there. Then you are asleep.

The initial phase of sleep, which has the prosaic name of stage 1, typically lasts only a few minutes. Your muscles start to relax. If you are trying to sleep in a sitting position, the relaxation of your neck muscles will allow your head to slump forward, briefly waking you; your head straightens, you nod off again, and so on. hat is why you ‘nod off’. You can easily be roused into wakefulness from stage 1 sleep. If someone does wake you during stage 1 sleep you may be aware that you have been asleep, or you may be equally convinced that you have been awake the whole time. Stage 1 sleep is accompanied by a further slowing of the brain-wave patterns.

The next phase is known, predictably, as stage 2 sleep. This is signalled by the appearance on the EEG of two specific brain-wave patterns called K complexes and sleep spindles. The K complex is a single, strong wave that lasts less than a second. The sleep spindle is a brief burst of waves lasting less than a second. A sleep spindle on an EEG trace looks like a spindle moving along a loom, hence its name. During stage 2 sleep your eyes are still and your muscles are relaxed. You are less easily awoken by stimuli and you appear to an observer to be sound asleep. Altogether, stage 2 occupies about 45–50 per cent of a night’s sleep.