Fatigue and performance


Prolonged periods of activity or emergency calls during the night are frequent in many disciplines such as the military, search and rescue and medicine. The resultant sleep deprivation, both acutely and chronically, compromises our ability to perform. 

Sleep deprivation and poor sleep quality significantly impair our cognitive and physical performance. Psychologists describe two types of sleep deprivation. Acute total sleep deprivation occurs when we experience a prolonged period of being awake. Chronic partial sleep deprivation occurs when we have a number of nights with poor quality sleep or sleep periods which are shorter than we need. 

A review of all US naval aviation mishaps from 1990 to 2004 (1) found that fatigue was the most common contributory factor. Similar studies of medical errors in hospital have shown significant increases in error rates when staff were sleep deprived. 

The need for sleep is driven by two processes: homeostatic and circadian. Homeostatic mechanisms create the need for sleep as periods of wakefulness increase. Circadian rhythms, the body’s in-built clock, alter hormone levels at different times of the day which create feelings of tiredness or wakefulness. 

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Effects of fatigue on performance

When fatigued, an overall reduction in alertness and attention is related to frequent, brief moments of inattentiveness or micro sleeps. These are more likely during prolonged, simple and monotonous tasks such as driving or monitoring a set of machine controls. Fatigue also has the effect of impairing the function of selected parts of the brain. The decrease in function in these areas has been demonstrated on functional magnetic resonance imaging. The frontal lobes are particularly vulnerable to sleep deprivation. Functions of the frontal lobes include attention and short term, working memory which are important for performance of complex tasks. 

Fatigue has the following effects on our ability to gather information, make decisions and carry out physical tasks. 

·     Auditory and visual attention. Our ability to detect, listen to, see and process information passed to us verbally and visually is impaired.

·     Selective attention. The ability to concentrate and focus on individual tasks becomes poorer.

·     Executive functions. The prefrontal cortex of the brain is responsible for controlling the brain’s function when shifting between different tasks. This involves moving attention from one task to another and inhibiting information which isn’t required for the task in hand. The brain’s ability to shift from one task to another is compromised with sleep deprivation. 

·     Decision making. Sleep deprivation reduces our ability to reason and make decisions. Fatigue has been shown to reduce our ability to recognise and respond to increasing complexity in decision making. Innovation, the ability to create new behaviours and responses based on experience, is also impaired. Decision making becomes more rigid when we’re tired.

·     Emotional processing. Changes in facial expression are less well assessed and understood in subjects who are tired. When given a range of solutions to interpersonal problems and conflict, fatigued individuals will tend to take the path of least resistance.

·     Risk taking. Fatigued individuals have a reduced ability to assess risk and are more likely to undertake activities and make decisions which involve risk of harm.

·     Planning. Studies in military subjects have shown that when given complex tasks which require planning, their outcomes deteriorated when they were tired and the time to complete the task lengthened.

·     Memory. Tests have shown that our capacity to develop short term, working memories which are required when undertaking complex tasks is poorer in those who are not adequately rested.  This resulted in a higher frequency of errors being made. We are also poorer at retrieving information we already have stored in our long-term memories. 

·     Insight. Our ability to assess how well we are performing is impaired when we are sleep deprived. Our ability to pick up errors also deteriorates. Some studies have shown increased confidence in producing answers to problems. One experiment showed higher confidence when the answer was wrong than when it was correct!


It’s not simply the duration of being awake which leads to fatigue induced performance impairment. Activity levels during the period of deprivation also play a significant part. Physical and mental exertion for prolonged periods adds to levels of tiredness. For those of us involved in aviation, aircraft noise, vibration and flying in unpressurised cabins also increase fatigue.

The timing of the activity is also an important factor. Following a period of sleep deprivation, completion of a task in the middle of the night, is more challenging than following the same period without sleep during the day. This is due to the effect of our circadian rhythms. 

Studies have shown that the amount of sleep required by different people to maintain a state of alert wakefulness differs. They also show that people have differing susceptibility to sleep deprivation There is some evidence to suggest that sleep deprivation is better tolerated by older people and by females - cognitive function deteriorated further with fatigue in younger subjects and in males. People who are physically fit are also less susceptible to impaired performance with sleep deprivation.

Managing fatigue

There may be merit in training ourselves for fatigue. This training is akin to stress inoculation and is commonly used in the military. During training exercises military personnel are intentionally sleep deprived to induce fatigue. They develop awareness of the signs of fatigue in themselves and their colleagues. They also develop insight into how their decision making, risk assessment, judgement and physical capabilities are affected. 

If we anticipate a period of acute sleep deprivation how can we prepare for it to minimise the effect on our performance? Taking a 90-minute nap in the afternoon preceding a night without sleep allows us to experience one full sleep cycle. This goes some way to replicating the benefits of a full night’s sleep. Sleeping for only 30 to 90 minutes means we are likely to wake when in a deep phase of sleep and for us to gain significantly less benefit. 

A period of exercise in the day preceding an all-night period of work is also beneficial in maintaining alertness, as is adequate hydration and ingestion of foods rich in protein.  It’s tempting to ingest carbohydrates to boost blood sugar levels. This is beneficial for short periods but results in insulin release which can have a sedative effect. Caffeine, used in moderation, is beneficial as a stimulant to boost wakefulness and alertness. Restricting caffeine use in the period before a potential sleep deprivation renders us more responsive to the beneficial stimulant effect.

If we do find ourselves sleep deprived but still having to perform to a high standard, there are a number of contingencies we can employ. If it’s possible, taking a 20 to 30-minute nap is helpful in reducing fatigue and improving our cognitive abilities. If we sleep for less than 30 minutes we are less likely to enter a deep sleep phase. If we do, we will wake up feeling more tired and cognitively impaired than when we went to sleep due to sleep inertia. 

Fresh, cool air, bright lights and short periods of light exercise are also beneficial. If we are working as a team it’s important to let our colleagues know that we are fatigued. This allows cross checking of decision making and task completion. 

Experiments have shown that in emergency circumstances, or when given warning of an impending task, the brain can make an active effort to succeed in the task when its function is impaired by sleep deprivation. This is known as attentional focusing. This can be seen on functional magnetic resonance imaging scans as increased thalamic activity to maintain alertness. 

We must ensure we take the opportunity to fully recover from periods of acute or chronic sleep deprivation. Sleep following a period of deprivation is called recovery sleep. The quality of recovery sleep is better than normal sleep. Periods of light sleep are shorter, and the duration of deep REM sleep is greater. Recovery from chronic sleep deprivation takes longer than from acute deprivation. Research evidence on how our cognitive abilities recover from sleep deprivation is not strong.

Fatigue calculators

Having insight into the fact that we are fatigued is essential when deciding to undertake complex or hazardous tasks. Being able to objectively assess the degree of fatigue would be ideal. This is especially important as, paradoxically, our ability to judge our level of tiredness and cognitive impairment is compromised when we’re tired. 

Fatigue calculation tools aim to objectively assess whether we are in a fit state to undertake tasks. A number of fatigue calculators are used in military and aviation settings. These tools attempt to objectively quantify the level of our fatigue and suggest how our cognitive function may be impaired according to how tired we are. 


1.   Davenport N, Lee J. Assessing how fatigue causes mishaps. Approach: The Naval Safety Center's Aviation Magazine; Sep/Oct2007, Vol. 52 Issue 5, p6

PerformanceStephen Hearns