Sleep will eventually force itself upon you if you go long enough without it. When scientists carry out sleep-deprivation experiments with human volunteers they often have to work quite hard to keep their subjects awake. Someone who has been deprived of sleep for two or three days requires stimulation, activity and variety to stop them falling asleep, no matter how well motivated they are to stay awake. Ian Oswald, a British scientist who conducted sleep-deprivation experiments in the 1960s, recalled walking the streets of Edinburgh flanked by exhausted volunteers whom he was working hard to keep awake. Even then, he would often see their eyelids closing as they walked along in a twilight state somewhere between waking and sleep.
Sleepiness and its flip side wakefulness determine how alive you feel and how well you perform every day of your life. But until the late 1970s there was no standard tool for measuring sleepiness. Then Mary Carskadon of Brown University in the USA invented a simple technique called the multiple sleep latency test (MSLT). It measures sleepiness like this. The subject is invited to go into a dark, quiet room in a sleep laboratory, to relax and fall asleep. Sensors detect the physiological signs of sleep (of which more later). As soon as the subject falls asleep, he or she is woken up. The key measurement is the time taken to fall asleep. The purpose of the MSLT is to measure daytime sleepiness, so the tests are generally carried out between ten in the morning and eight in the evening. The procedure is usually repeated four or five times during the course of a day.
The length of time between lying down and falling asleep is called the sleep latency. A daytime sleep latency of 15–25 minutes is generally interpreted as a reassuring sign of normality. Someone who is sleep-deprived will fall asleep faster. A sleep latency of less than ten minutes raises questions; there is a good chance that the person concerned will be experiencing intrusive daytime sleepiness, especially during the normal afternoon dip in wakefulness or when bored. A daytime sleep latency of less than five minutes usually signifies excessive tiredness, implying that the individual is substantially sleep-deprived or suffering from a sleep disorder.
Not everyone who falls asleep quickly in the MSLT is sleep-deprived, however. There are a few healthy people who can fall asleep in a trice, given the opportunity and the will. These individuals have the ability to go out like a light even after they have had unlimited amounts of sleep and without displaying any other signs of sleep deprivation. They just seem to be unusually good at relaxing and going to sleep during the day, regardless of their need for sleep. An unusually short sleep latency should therefore be viewed as strong evidence of sleep deprivation, but not conclusive proof.
The MSLT is conducted in a sleep laboratory, using electronic sensors to determine the onset of sleep objectively. There is, however, a crude, do-it-yourself version of the MSLT. This entails lying on a bed under sleep-friendly conditions – lights off, shoes off, TV off, no noises off, and so on – with one hand dangling over the edge of the bed and holding a metal object such as a spoon. Under the dangling hand rests a plate or some other hard surface. When you fall asleep your muscles relax and the spoon drops onto the plate with a loud clatter. All being well, you should then wake up, check your stopwatch and write down your sleep latency. If, on the other hand, you wake up several hours later to find your spoon lying on the plate, it may be that you are sleep-deprived. An even simpler way of assessing your own subjective sense of sleepiness is with a self-rating method called the Stanford Sleepiness Scale. This is a seven-point scale, ranging from 1 (defined as feeling active, vital, alert, wide awake), through 2 (functioning at a high level, not at peak), 3 (relaxed, not full alertness, responsive), 4 (a little foggy, not at peak, let down), 5 (tired, losing interest, slowed down), and 6 (drowsy, prefer to be lying down), to 7 (almost in a reverie, hard to stay awake).
Sleepiness is affected by many other factors besides the length of time since you last slept. Foremost among these is the time of day. Other things being equal, it is easier to fall asleep at certain times of the day and harder at other times. Wakefulness fluctuates naturally over the 24-hour cycle, with low points occurring in the early hours of the morning and again in the afternoon. This daily rhythm in wakefulness is mirrored by fluctuations in performance, which manifest themselves in many ways. For instance, you are more likely to crash your car or make a mistake when performing a difficult task in the early hours of the morning or in the afternoon. Sleep deprivation amplifies the effects of time of day, so the more sleep-deprived someone becomes, the deeper their daily troughs in wakefulness.
Stimulation and activity can mask sleepiness, at least for a while. Busy people often fail to notice how tired they really are until they relax at the weekend or on holiday. Their true state then becomes apparent. Sleepiness is even influenced to some extent by the nature of your previous activity. Experiments have confirmed, for example, that people feel sleepier and fall asleep more quickly after watching television than after taking a walk.
Fighting the beast (#ulink_5606a271-0a1e-5188-9fbc-1b32ee472e7b)
Thou art inclined to sleep; ’tis a good dullness,
And give it way: I know thou canst not choose.
William Shakespeare, The Tempest (1611)
What does it feel like to be seriously tired? For most of us real exhaustion is thankfully a rare experience. But for some, it is all too real. One of the most graphic accounts ever written about crushing fatigue can be found in the autobiography of the American aviator Charles Lindbergh. In 1927 Lindbergh made the first solo non-stop flight across the Atlantic in his single-engine plane The Spirit of St Louis. It lasted 33.5 hours. In his 1953 Pulitzer Prize-winning autobiography, Lindbergh described how his flight from New York to Paris came close to disaster – not because of mechanical failure or bad weather, but because of simple lack of sleep.
Before his marathon flight Lindbergh was told that it would be impossible for one man to fly an aeroplane alone for 30 or 40 hours without sleep. Lindbergh disagreed: he had worked for more than 40 hours at a time without sleep, so he did not see why he should be incapable of flying for that long sitting down. He was almost proved wrong.
The timing of Lindbergh’s departure remained uncertain until the last moment. It all depended on the weather. When an opportunity suddenly arose to take off the following morning at daybreak, Lindbergh went back to his hotel to cram in a few hours’ sleep. As a professional pilot he had learned from experience that every little bit of sleep helps. Even so, it was close to midnight before he reached his room, leaving barely three hours if he was to be ready to fly at dawn. As he eventually lay down to sleep, Lindbergh regretted the flawed planning that would force him to set off in a state of sleep deprivation. He knew he would have been better off starting his marathon fully rested – not just because that would make him a better pilot, but also because tiredness would prevent him from fully appreciating the experience. Lindbergh wanted to enjoy his flight. Even then, in those precious few hours, his sleep was further eroded when someone woke him to ask a stupid question. By three o’clock that morning Lindbergh was at the airfield preparing to take off. And so began his epic, sleepless flight across the Atlantic.
After only a few hours in the air, Lindbergh felt tiredness creeping up on him. How pleasant it would be, he mused, to doze off for a few seconds. He shook himself. He could not afford to feel like that so early in the trip. Later that day, and still less than nine hours into the flight, fatigue hit him again:
My eyes feel dry and hard as stones. The lids pull down with pounds of weight against their muscles. Keeping them open is like holding arms outstretched without support. After a minute or two of effort, I have to let them close … My mind clicks on and off, as though attached to an electric switch with which some outside force is tampering. I try letting one eyelid close at a time while I prop the other open with my will. But the effort’s too much. Sleep is winning. My whole body argues dully that nothing, nothing life can attain, is quite so desirable as sleep.
If sleepiness weighed so heavily upon him now, how could he get through the night, to say nothing of the dawn and another day and its night and possibly even the dawn after that? Lindbergh was ashamed. How could he let something as trifling as sleep ruin the record-breaking flight he had spent so many months planning? How could he face his sponsors and admit he had failed to reach Paris because he was sleepy? This must be how an exhausted sentry feels, he thought: unable to stay awake, yet knowing he will be shot if he is caught napping. He had no choice but to battle against his fatigue, minute by minute. In the end, it would all come down to sheer will power.
As the first traces of dawn began to appear on the second morning, Lindbergh felt the overwhelming desire to sleep falling over him like a quilt. Dawn was the time he had dreaded most:
Like salt in wounds, the light of day brings back my pains. Every cell of my being is on strike, sulking in protest, claiming that nothing, nothing in the world, could be worth such an effort; that man’s tissue was never made for such abuse. My back is stiff; my shoulders ache; my face burns; my eyes smart. It seems impossible to go on longer. All I want in life is to throw myself down flat, stretch out – and sleep.
Lindbergh searched for some way to stay alert. Shaking his body and stamping his feet no longer did any good. He had no coffee with him, but consoled himself with the thought that he had long since passed the stage when coffee could have helped. He pushed the stick forward and dived down into a ridge of cloud, pulling up sharply again after clipping through its summit. That woke him up a little, but not for long. He was thankful that The Spirit of St Louis had not been designed to be a stable aeroplane. The very instability that made it difficult to fly now guarded him against catastrophic errors. The slightest relaxation of pressure on stick or rudder would start a climbing or a diving turn, hauling him back from the borderland of sleep.
In the twentieth hour sleepiness temporarily gained the upper hand. Lindbergh suddenly awoke to find the plane diving and turning: he had been asleep with his eyes open. The realisation that he had lost control of himself and the plane was like an electric shock, and within seconds he was back in command. But as time passed, and no new emergencies occurred, he lapsed back into a dreamlike state, unsure whether he was dreaming through life or living through a dream. Over and over again he fell asleep with his eyes open, knowing he was falling asleep and unable to prevent it. Extreme measures were needed. He struck his face sharply with his hand, but felt hardly any sensation. He hit his face again, this time with all his strength. All he felt was numbness. Not even pain would come to his rescue. He broke open a capsule of ammonia and inhaled, but smelt nothing. Lindbergh realised how deadened his senses had become.
After 24 hours in the air and with more than a thousand miles still to go, Lindbergh seriously doubted whether he could stay awake long enough to avoid crashing into the Atlantic. But just as he felt death and failure staring him in the face, he began to turn the corner. The seriousness of his crisis had at last broken the spell of sleep and summoned up his last reserves of mental strength. He felt as though he was recuperating from a severe illness. And, as the history books relate, Charles A. Lindbergh made it to Paris and became an international hero. His remarkable last-minute rally was almost certainly a reflection of his circadian rhythm. His next peak of alertness came just in time. If his flight had continued much longer, he would inevitably have plunged back down into another circadian trough of fatigue from which he might never have ascended.
Lack of sleep has always been one of the least glamorous aspects of life at sea. In 1938 the 18-year-old Eric Newby signed on as an apprentice aboard one of the last square-rigger sailing ships. Newby’s autobiographical account of his voyage, The Last Grain Race, charts his experiences as he made the round trip from Belfast to Australia and back again via Cape Horn. The ship had a minimal crew, so each man worked hard and slept little. ‘I had never been so tired in my whole life,’ wrote Newby, ‘far too exhausted to appreciate the beautiful pyramids of sail towering above me.’ After coming off watch he would fall into a dreamless sleep, so deep that when he awoke and went up on deck again he felt like a sleepwalker. On the return voyage his ship was hit by an awe-inspiring storm that soon had the crew’s compartment awash in six inches of water. But the sailors who were not on duty snored through it all, so great was their appetite for sleep. Like soldiers preparing for the next battle, they lay ‘absorbing sleep greedily like medicine’.
The writer C. S. Forester charted the sleepiness of the long-distance mariner in his Hornblower novels, which relate the fictional Royal Navy career and Napoleonic War adventures of Horatio Hornblower, a character partly modelled on Admiral Horatio Nelson. In one story, the gallant Hornblower is exhausted from lack of sleep after a daring escape from France and a prolonged battle with pursuing enemy ships. Like Charles Lindbergh, Hornblower experiences the disturbing sensation that his mind has become disconnected from his body:
His voice sounded strange and distant in his own ears, like that of a stranger speaking from another room, as he issued his orders; the very hands with which he held the ropes seemed not to belong to him. It was as if there was a cleavage between the brain with which he was trying to think and the body which condescended to obey him.
Sleep deprivation was a fact of life for Hornblower’s real-life counterpart as well. When Horatio Nelson was commanding a Royal Navy warship he seldom had more than two hours of uninterrupted sleep and sometimes stayed on deck all night. However, like many leaders famed for coping with little sleep, Nelson had a well-developed ability to take catnaps during the day. He would nap in his cabin in a black leather armchair, his feet up on a chair. As we shall see later, a faculty for napping has enabled many high achievers to cope with meagre rations of night-time sleep.
Even in the twenty-first century, napping is a crucial skill for sailors – especially when sailing single-handed. In February 2001 the British yachtswoman Ellen MacArthur crossed the finishing line of the Vendée Globe boat race after 94 days alone at sea. She had travelled 24,000 miles across three oceans to become the fastest woman to sail single-handedly around the globe. For 13 weeks she had managed her 18-metre yacht in some of the world’s roughest seas by herself. The only way Ellen MacArthur could survive was to become a past master of napping, and divide her sleep into multiple brief naps. During the 94-day voyage she took 891 naps, each lasting on average 36 minutes, giving her a total of about five and a half hours of sleep a day. Even sleeping in such short bursts, MacArthur frequently had to rely on what she called her ‘sixth sense’ to wake her when something urgently required her attention.
A soil for peevishness (#ulink_31da901f-b36f-5d30-93c2-10b97a1ef0e7)
He that sleeps feels not the toothache.
William Shakespeare, Cymbeline (1609–10)
Lack of sleep does far more than just make us feel sleepy, however: its tentacles reach out and twist our emotional, cognitive and physical states. One of the first casualties is mood. Tired people are emotionally less resilient and more prone to irritation or sadness. Tiredness also impairs our social and emotional skills, with potentially damaging consequences for personal relationships. A tired person can be physically present but psychologically and emotionally absent. In The Screw-tape Letters, C. S. Lewis imagines an experienced devil instructing his neophyte nephew on how to corrupt a young human. The best way, he advises, is through fatigue:
The paradoxical thing is that moderate fatigue is a better soil for peevishness than absolute exhaustion … It is not fatigue simply as such that produces the anger, but unexpected demands on a man already tired.
After a night without sleep, healthy people exhibit clear disturbances in mood, which are characterised by irritability, tension and reduced vigour. These symptoms normally evaporate after a good night’s slumber. Sleep-deprived people, like drunks, lose their social inhibitions and behave in inappropriate ways; they are prone to outbursts of childish humour, which others around them do not always find hilarious. (And strangely, for reasons that remain unclear, acute sleep deprivation can also have the counterintuitive effect of stimulating the libido.)
Severe sleep deprivation can induce feelings of persecution and mild paranoia. It is well known among sleep scientists that the volunteers who take part in their sleep-deprivation experiments often become irritable and impatient. Some subjects become slightly paranoid, convinced that the researchers and fellow volunteers are plotting against them. In rare instances, exhaustion can provoke more dramatic changes. In one documented case, a previously healthy man became psychotic after four nights of badly disrupted sleep and believed he was the Messiah.
Chronic sleep deprivation – that state of never getting quite enough sleep, day in day out, over a prolonged period – is far more common in everyday life than the acute deprivation that comes from having no sleep at all for one or two nights. And chronic sleep deprivation can have just as much cumulative impact, leaving even the saintliest person with a shorter fuse. The writer John Seabrook described the debilitating fatigue that goes with having a small baby like this:
The burning eyes; the band of fatigue that tightens around the skull, a sensation some liken to the feeling that you’re always wearing a hat; the irritation – at each other, at friends, at the cat’s water bowl, which I kept kicking by accident …
Lack of sleep and tiredness are obviously not wholly responsible for the tetchiness, aggression and petty violence of everyday life, but it is a racing certainty that they contribute towards making the world a nastier place. Conversely, there is little doubt that good sleep makes us feel better. In one study, researchers issued volunteers with pocket computers on which they logged their sleep patterns, moods and social interactions over a two-week period. The results showed that going to sleep earlier in the evening was consistently associated with better mood and better social interactions the following day.
Tired people are stupid and reckless (#ulink_bbd9505f-01f8-5e16-b9e2-966c10e0ea01)
Fatigue makes women talk more and men talk less.
C. S. Lewis, The Screwtape Letters (1942)
Besides making us grumpy and poor company, sleep deprivation impairs our mental abilities in many subtle and not-so-subtle ways. In brief, tired people are stupid and reckless. Sleep deprivation damages our ability to perform tasks that require attention, thought, judgment, memory, social skills or communication skills – which covers the ground fairly comprehensively. Tired people also seem to lose sight of the consequences of their actions, liberating them to do silly and sometimes catastrophic things: the Chernobyl and Exxon Valdez disasters were just two examples.
Even modest sleep loss will measurably reduce your mental performance. After one night without sleep, you will have slower reactions, make more mistakes and find it harder to maintain attention. Unsurprisingly, two or three days of sleep deprivation produce even bigger impairments. Young adults are no more resistant than older people. If anything, they are more vulnerable. When researchers compared the consequences of one night of total sleep deprivation on healthy 80-year-olds and 20-year-olds, they recorded larger disturbances in the mood and cognitive performance of the younger subjects.
Reducing someone’s sleep for several nights in a row can undermine their performance just as much as completely depriving them of sleep for one or two nights. When scientists limited healthy young adults to an average of only five hours’ sleep a night for a week, the subjects became progressively sleepier and their performance deteriorated significantly. Two full nights of catch-up sleep (equivalent to a restful weekend) were needed to reverse the decline.
The armed forces have understandably maintained a long-standing interest in how well people hold up when they are deprived of sleep, as often happens in conflict. Experience shows that soldiers can continue to perform reasonably well after days of sleep deprivation under combat conditions, buoyed up by adrenaline, physical exertion and strong motivation. With sufficient stimulation and will power, military personnel can usually keep going without sleep for three or four days before they keel over. In one study, for example, men were assessed throughout a strenuous combat training course lasting several days. Some of the trainees were allowed no sleep at all, while others were permitted a few hours in the middle of the course. All the men displayed a substantial deterioration on measures of mood, vigilance and reaction time, with those who got no sleep performing even worse than those who got some. By the end, the trainees who had not slept at all were suffering from clinical symptoms including sensory disturbances.
Sleep-deprived people can often perform certain tasks reasonably well in short bursts, despite their fatigue. The real problems arise if they have to sustain their effort for any length of time. This lack of staying power was highlighted by experiments conducted in the 1950s at the Walter Reed Army Institute of Research in the USA. Sleep-deprived volunteers scored well in tests of reaction times when they were only required to react a few times over a period of one minute. But when they had to make lots of rapid responses spread irregularly over a period of 15 minutes, their performance fell apart. Sometimes they reacted quickly and sometimes they reacted very slowly or not at all. The more sleep-deprived they were, the more their responses varied. That experiment illustrates a general pattern that has emerged from studies of sleep deprivation – namely, that the performance of tired people becomes much more variable and inconsistent. Their accuracy and speed can fluctuate wildly over periods of a few minutes. In one study, for example, scientists monitored the performance of volunteers throughout an 88-hour period of total sleep deprivation. The subjects’ performance on a task requiring vigilance and coordination was much more variable (as well as just plain worse) than a comparison group, and the longer they went without sleep the more variable their performance became. These wild fluctuations in performance probably arose because their attention was repeatedly blotted out by microsleeps. As we saw earlier, someone who is very tired can lurch from wakefulness to sleep and back again in a matter of seconds without even noticing.
Much of the research on sleep deprivation has focused on how it impairs people’s ability to perform tasks requiring rapid but relatively simple decisions, involving very basic skills such as the ability to maintain attention under monotonous conditions. These skills are sensitive to sleep deprivation (which is partly why they have been so frequently measured) but they also have only a tenuous bearing on what happens in reality. The sorts of judgments and decisions we all have to make in real life usually demand far more than just the ability to stay awake and press buttons. Real decisions require us to assimilate and process complex, incomplete and often contradictory information, to keep track of our actions, assign priorities, ignore distractions and communicate with other people. The truly frightening aspect of sleep deprivation is the way it erodes all of these capacities.
Sleep-deprived people are bad at making complex decisions that require them to revise their plans in the light of unexpected news, to ignore irrelevant information and to communicate effectively. These are precisely the capabilities that we all need most when dealing with the vagaries of normal life – as do politicians, managers, doctors, military commanders and other key decision makers.
Even a single night without sleep will impair your ability to think flexibly and creatively. Sleep-deprived people perform badly on all aspects of creative thinking, including originality, flexibility, generating unusual ideas, being able to change strategy, word fluency and nonverbal planning. Tired people tend to persist with their current activity regardless of whether it is appropriate – a characteristic that psychologists call perseveration. In one cleverly revealing experiment, sleep-deprived volunteers played a realistic marketing game that required them to make complex decisions and then continually update those decisions in the light of new information. After 36 hours without sleep, the well-motivated subjects were still able to read and absorb written information. But their ability to make sound decisions based on that information deteriorated markedly. As they became more sleep-deprived they found it increasingly difficult to recognise when to change tactics in light of changed circumstances. Their thinking became rigid and they tended to persist with incorrect responses. Eventually, after 36 hours without sleep (the equivalent of losing just one night’s sleep) their ability to play the game collapsed.
Sleep-deprived people reveal their blunted creativity and mental inflexibility through alterations in their spoken language. After 36 hours of sustained wakefulness, people display a marked deterioration in verbal fluency and inventiveness. They are more reliant on routine responses and tend to become fixated with a particular category of words. They also start using inappropriate, monotonous or flattened intonation when reading out loud. Think about how people talk when they are drunk and you will get the picture. Tired people are bad at finding the right words. Their language becomes less spontaneous and expressive, and they are less willing to volunteer information that others might need to know. All in all, they are worse at communicating their thoughts, feelings, decisions and actions. Being a poor communicator is unhelpful, whether you are a head of state trying to deal with a crisis or simply someone who values their social life.
The supposedly robust adolescent is, if anything, even more vulnerable than older people. One study found that when youths aged 10–14 years were restricted to only five hours in bed for one night, their verbal creativity, verbal fluency and ability to learn new abstract concepts were all impaired. Less complex mental functions, such as rote learning, were unaffected by this modest degree of sleep restriction. So the sleep-deprived adolescent would still be able to go through the basic motions at school the next day, but would be unable to perform to anything like their true potential. And it is not entirely unknown for adolescents to go to school after getting less than a full night’s sleep.