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
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
