ReviewMental fatigue: Costs and benefits
Introduction
Mental fatigue refers to the feeling that people may experience after or during prolonged periods of cognitive activity. These feelings are very common in everyday modern life and generally involve tiredness or even exhaustion, an aversion to continue with the present activity, and a decrease in the level of commitment to the task at hand (Holding, 1983, Hockey, 1997, Meijman, 2000). In addition, mental fatigue has been associated with impaired cognitive and behavioural performance (e.g. Boksem et al., 2005, Lorist et al., 2005, Van der Linden and Eling, 2006).
Over the past decades, work has changed to a large extent from demanding physical effort, to demanding mental effort. This has resulted in a substantial increase in complaints related to mental fatigue: in the Netherlands, half of the women in the working population complain about being fatigued, while a third of the men report such complaints. Fifteen years ago, only 38% of the women and 24% of the men reported such complaints. A recent survey of the U.S. workforce (Ricci et al., 2007) showed that 38% of workers reported being fatigued. Of these fatigued workers, 66% reported lost productivity time, compared to 26% of workers without fatigue. Ricci and colleagues estimated that lost productivity time from fatigued workers costs employers an excess of a hundred billion dollars annually, compared to non-fatigued workers. On a more personal level, complaints of fatigue can have a profound and negative impact on people's social and occupational life. Moreover, mental fatigue is a common symptom in a large number of chronic medical conditions such as cancer, Human Immunodeficiency Virus, Multiple Sclerosis, Chronic Fatigue Syndrome, Alzheimer's disease, Huntington's disease and Parkinson's disease (see Chaudhuri and Behan, 2000). It is important to realize however, that fatigue as a symptom of disease may involve different biological substrates compared to acute mental fatigue in otherwise healthy people.
Mental fatigue has been shown to result in various deteri,orations in cognitive functioning. Van der Linden et al., 2003a, Van der Linden et al., 2003b, Van der Linden and Eling, 2006 showed that fatigued subjects had difficulties in focusing their attention, planning, and adaptively changing strategies in the face of negative outcomes. Our own work showed that fatigued subjects had difficulties in adequately preparing their responses (Boksem et al., 2006) and had difficulties in sustaining attention and ignoring irrelevant information (i.e. they suffered from increased distractibility; Boksem et al., 2005). In addition, fatigued subjects corrected their mistakes less often and post error performance adjustment was impaired (Lorist et al., 2005, Boksem et al., 2006).
Common sense would dictate that mental fatigue is the direct result of working for a prolonged period of time: the longer one works on a demanding task, the more fatigue one will experience. This, however, has been shown not to be the case. Fatigue may be experienced after working for a relatively short period of time, while working long hours does not always lead to fatigue (Sparks et al., 1997, Park et al., 2001). Indeed, working long hours has been shown not to lead to fatigue at all when the rewards of working (in terms of payment, but also appreciation by peers and co-workers) are perceived as high (Siegrist, 1996, Van der Hulst and Geurts, 2001).
In other words, high workload only results in fatigue when rewards associated with work are low. Here, we will propose a framework for mental fatigue that involves an integrated evaluation of both expected rewards and energetical costs associated with continued performance, resulting in a reduction in motivation and the effort invested in continued performance.
Two complimentary and interacting motivational systems are proposed to result in producing goal directed behaviour: the motivation to obtain rewards and the motivation to avoid harm and punishment. While both rewards and punishments can motivate to engage in activities, both types of motivated behaviour are associated with energetical costs. In the present review, we will propose that the feeling of fatigue may result from the subconscious analyses of cost and benefits to expend energy, or to conserve energy. People will only expend energy on a certain task when (energetical) costs are comparably low and benefits (high reward, low punishment) are comparably high. However, when tasks have to be performed for prolonged periods of time, the amount of energy invested in performance will build up until it eventually outweighs potential rewards, resulting in a decrease in the motivation to work for rewards that fail to be procured. Thus, we propose that the feeling of fatigue corresponds to a drive to abandon behaviour when energetical costs exceed perceived benefits of continued performance.
The purpose of this review is to provide a coherent framework for mental fatigue. Because of this, we inevitably focus more on some aspects of this complex phenomenon that on others. This is especially true for the motivational systems and neural substrates we propose are primarily involved. While we focus on the role the ‘reward system’ of the brain and on the role of the dopamine (DA) neurotransmitter in motivated behaviour, it is certain that other neural structures, neurotransmitters and hormones are also involved. However, by narrowing our focus on these particular aspects of mental fatigue, we are able to present our views on what is a central aspect of this important and complex psychophysiological phenomenon.
Section snippets
Goals
So, what is mental fatigue? This question has proved to be very difficult to answer. Despite its mundane nature, mental fatigue appears to be a highly complex phenomenon that involves changes in mood, information processing and behaviour (Desmond and Hancock, 2001), making it a difficult subject to study. Perhaps asking ourselves the opposite question could clarify matters: what makes us tick? What makes us get out of bed every morning and go to work, to school, or do whatever else is planned
Rewards and punishments
There are at least three psychological components of reward (Berridge and Robinson, 2003, Schultz, 2004). First, the consumption of rewards involves feelings of pleasure (‘liking’). Second, these pleasurable feelings are highly motivating to obtain this reward more often (‘wanting’). Third, in order to do so, one has to learn about the relationships among stimuli and the consequences of actions that led to the procurement of the reward (‘learning’). The obvious benefit of such a reward system
To act or not to act
Despite the importance of the evaluation of potential reward for successful goal directed behaviour, reward seeking activities would only seem to be a part of what constitutes adaptive goal directed behaviour. Rewards always come at a price. To keep with our example: to catch an antelope, the cheetah will have to leave the safety and comfort of its shelter and venture into the open savannah, spending valuable energy. So, to select the best course of action, an organism has to take into account
Mental fatigue: The costs of acting
The fact that our brain can process information on both the potential benefits and the potential costs involved in working in order to reach goals enables it not only to determine whether an expected outcome of behaviour is desirable, it can also determine whether a particular action is actually worth performing (Rushworth et al., 2004). For any action to be worth performing, the desirability of the expected outcome should outweigh the potential energetical costs.
We propose that the feeling of
Mental fatigue and control
As already mentioned in the introduction, there will be a reduced probability that the selection of actions will be controlled by high-level regulatory control processes (Lorist et al., 2000, Meijman, 2000), when people become fatigued. Van der Linden et al., 2003a, Van der Linden et al., 2003b, Van der Linden and Eling, 2006 showed that fatigued subjects had difficulties in focusing their attention, planning, and adaptively changing strategies in the face of negative outcomes. In addition, our
Why fatigue?
Although this seems to suggest that fatigue is only detrimental to performance and goal-directed behaviour, we argue that this is not the case. Instead, we propose that fatigue can best be considered as an adaptive signal that the present behavioural strategy may no longer be the most appropriate, because it continues to demand effort while substantial effort has already been invested and the goal evidently has not yet been achieved. Fatigue may provide the cognitive system with a signal that
Beyond control
Choosing courses of action based on an analysis of costs and benefits is highly adaptive and efficient, but only when one is free to choose between various action alternatives (i.e. the situation is controllable). In everyday life, especially in the work environment, this control over the situation is often lacking. High workload, high demands on cognitive functions, and fixed production quotas limit the possibilities of employees to choose lower effort alternatives when they perceive effort to
Individual differences
Are some more at risk for becoming fatigued than others? Several studies have noted that people with personality characteristics that increase chances of overriding the signal of fatigue are at risk. These personality characteristics include high perfectionism (Magnusson et al., 1996, White and Schweitzer, 2000), high neuroticism (Johnson et al., 1996, Prins et al., 2006) and low extraversion (Prins et al., 2006, Watson et al., 1999). All these personality characteristics relate to the way
Summary
Our environment typically supports multiple goals, associated with different magnitudes of reward. The OFC, BLA and insula have been shown to be primarily involved in coding the expected appetitive and aversive value that each option may provide. This reward-related information may be relayed to the ACC, where these sources of reward information may be integrated to bias behaviour in a particular direction that is predicted to result in the largest reward value.
In addition to information on
Acknowledgments
This work was supported by the Netherlands Organisation for Scientific Research (NWO), as part of the Netherlands concerted research action “Fatigue at Work”.
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