Cognitive Fatigue is a common issue in our everyday life, defined as a psychobiological state caused by prolonged and/or intense periods of demanding cognitive activity and characterised by subjective feelings of “tiredness” and “lack of energy” (1). In the same way that our muscles feel tired after physical activity, so too does our brain get exhausted after large bouts of mental activity. Put simply, mental exhaustion happens if the brain receives too much stimulation or is required to maintain an intense activity level without rest.
Cognitive Fatigue can occur in many environments and circumstances, such as work or studying for long hours with few or no breaks, a sense of overwhelming workplace responsibilities, or conferred from the overseeing of multiple other people. Existing mental health conditions such as anxiety or depression can predispose an individual to stress, worry and negative thinking and therefore compound Cognitive Fatigue.
Cognitive fatigue includes a spectrum of deficits affecting motivation, emotion, cognition and behaviour.
The sequela is often associated with difficulties in maintaining attention and information processing, as well as memory and executive functions (2). Further, sustained performance, in this definition, does not necessarily involve the same task but can extend over different tasks that require mental effort, such as fatigue because of a day in the office (which often also involves several different tasks). While there isn’t a universally accepted definition of fatigue, researchers have attempted to distinguish between peripheral and central fatigue. Peripheral fatigue is attributed to known mechanisms such as the failure of neuromuscular transmission, metabolic disturbances, defects of muscle membranes, or peripheral circulatory failure. In contrast, central or Cognitive Fatigue represents a failure to sustain attention that requires self-motivation to optimize performance (3).
A decrease in alertness and difficulty focusing are the most common effects of Cognitive Fatigue, and other signs such as a depressed or anxious mood, anger or irritability, difficulty managing emotions or decreased productivity can result from Cognitive Fatigue. Further, physical signs may be noticed, such as head and body aches, stomach upset, sleep issues, and appetite or weight changes. The relationship is cyclical; physical fatigue can induce and prolong mental fatigue, and vice versa.
In industry, many incidents and accidents have been related to mental fatigue as the result of sustained performance (4). This is particularly true of industries that value long work hours and around-the-clock availability, such as the healthcare, mining and industrial sectors. Aspects of these work environments and intrinsic job factors can also predispose to fatigue. Mentally exhausting tasks that demand constant attention, quick reaction, or vigilance are physically draining, while less mentally challenging, monotonous or unstimulating tasks or occupations have their own set of problems. Driving performance can decrease after 90 minutes due to this very reason (5). In order to prevent or deal with fatigue-related errors, it is important to understand the nature of mental fatigue and its specific effects on behaviour and physiology. However, despite many studies on fatigue, it is remarkably difficult to get a grip on mental fatigue and the cognitive processes underlying its different manifestations (6).
Research into Cognitive Fatigue has investigated the ways in which our behaviour is affected by fatigue. Many studies have identified that performance, as measured by accuracy or reaction time on a cognitive task, declines when Cognitive Fatigue is induced (7, 8). Some have proposed that Cognitive Fatigue affects the overarching organisation of our actions, rather than the discrete actions themselves. In the famous Cambridge Cockpit studies, after more than 2 h of skilled work, pilots in a flight simulator were still able to perform individual actions well, but it was the overall organization of these actions that seemed to suffer.
Further evidence supporting this ‘control view' of fatigue is that performance on simple or well-learned tasks, which can be executed in a more or less automatic way, can be upheld over long periods of time, after sleep deprivation, or after (mentally) demanding activities. On the other hand, complex tasks that require deliberate control of behaviour are generally difficult to perform under such circumstances (9). It is easier to drive a familiar route home after a busy day, which becomes an automatic process than to drive to an unfamiliar location in the same conditions. Research into the time course of recovery from mental fatigue has also been undertaken. Rivers et al. was one of the first groups to evaluate the lasting effects of Cognitive Fatigue by exploring the effect of a 30- or 60-min rest period following 30 min of mental work (i.e., addition calculation). Their results indicated that 30 minutes of total rest was insufficient to neutralise the effects of the induced mental fatigue and that 60 min of total rest only partially eliminated the effects (10).
Ultimately, there remains several gaps in neuropsychiatric research into Cognitive Fatigue, but there are many challenges that may account for this. Standardised neuropsychological tests measure different outcome variables that vary in terms of their sensitivity to Cognitive Fatigue. Additionally, it is important to distinguish between and separately measure Cognitive Fatigue and cognitive performance to ensure that the two are not confounded. Worse cognitive performance should not be confused with cognitive fatigue, and subjective and objective measures of cognitive fatigue should always be attained (11). Chronic Cognitive Fatigue requires long-term management and support. Ideally, removing the stressor or source of fatigue is the best solution, but practically, this is not always possible. Other strategies include taking regular breaks, using relaxation techniques such as deep breathing or yoga, restoring and maintaining sleep hygiene, and exercising (5).
Acute Cognitive Fatigue needs immediate intervention, however. If acutely in a dangerous situation that may threaten the safety of yourself or those around you, however, it’s important to speak up and remove yourself if possible to avoid accidents, injury or death. The strategies listed above are useful in preventing Cognitive Fatigue also. Pre-emptively taking steps to avoid fatigue can lessen the chance of developing it and reinforce healthy habits that reduce the duration of Cognitive Fatigue if it does strike.
Rozand, V., & Lepers, R. (2017). Influence de la fatigue mentale sur les performances physiques. Movement & Sport Sciences-Science & Motricité, (95), 3-12.
Chaudhuri, A., & Behan, P. O. (2004). Fatigue in neurological disorders. The lancet, 363(9413), 978-988.
Wascher E, Rasch B, Sanger J, et al. Frontal theta activity reflects distinct aspects of mental fatigue. Biol Psychol. 2014;96:57–65.
Mockel T, Beste C, Wascher E. The effects of time on task in response selection—an ERP study of mental fatigue. Sci Rep. 2015;5:10113.
Sanders, A. F., & Sanders, A. (2013). Elements of human performance: Reaction processes and attention in human skill. Psychology Press.
Rivers, W. H. R. (1896). On mental fatigue and recovery. J. Ment. Sci. 42, 525–530. doi: 10.1192/bjp.42.178.525
Holtzer, R., Shuman, M., Mahoney, J. R., Lipton, R., & Verghese, J. (2011). Cognitive fatigue defined in the context of attention networks. Neuropsychology, development, and cognition. Section B, Aging, neuropsychology and cognition, 18(1), 108–128. https://doi.org/10.1080/13825585.2010.517826