How Fatigue Affects our Decisions and Desires

What is fatigue?

There is no general agreement yet on the definition of fatigue. The feeling of exhaustion is at its core, but there is much more to it. It may be characterized by a decline in vigor or performance, which show up as lapses in attention, slower reaction times, and decreased accuracy. These changes are likely associated with alterations in motivation. Some research suggests that impulsivity in decision-making might increase along with fatigue, with fatigued individuals opting for lower, immediate rewards over higher, delayed rewards.

We know that fatigue increases with effort and accumulates over time, but interestingly, research suggests that fatigue is not static. Rather, the brain may shift its valuation of a task moment-by-moment, creating sensations of fatigue that are partially recoverable through brief rest periods.

How does fatigue show up in the brain?

We don’t yet know the exact neural correlates of fatigue, but it likely involves a cost-benefit evaluation that takes place in the ventral striatum and parts of the prefrontal cortex — two brain areas that have also been associated with motivation and reward.

During my PhD at the University of Oxford, we used functional magnetic resonance imaging (fMRI) to study brain activity in participants who were offered grip force tasks of varying effort in exchange for varying monetary rewards. The lateral prefrontal cortex, which is also typically involved in decision-making and cognitive control, works in concert with the anterior cingulate cortex, which has been linked to the ability to persevere through a task. We saw that brain activity in these areas decreased with fatigue but rebounded in a specific sub-region of the brain with periods of rest.

As activity in these areas fluctuates, a person might experience a desire to stop the task and then, moments later, a willingness to continue. A participant who eagerly exerted, say, 39% of her grip for a certain amount of money at the beginning of the study might decline an offer to exert the same amount of effort for an even higher amount after repeatedly exerting effort. This suggests that this fronto-striatal system of the brain shifted its subjective valuation of that task. Rest became more important than money at that moment.

How do we determine what’s motivating or worth the effort?

The degree to which a reward motivates you depends on the context, your personal life history, and many other factors. On the neural level, evidence suggests that differences in individual motivation levels may be related to the concentration and diffusion of dopamine, the neurotransmitter often associated with reward, in different brain regions. More specifically, dopamine levels affect your valuation of a reward as worth the effort, which subsequently affects aspects of fatigue.

We see that patients with neurological disorders associated with dopamine deficiencies, such as Parkinson’s disease, typically experience pronounced fatigue and decreased motivation. Some evidence also suggests that fatigue is linked to viral infections such as long COVID.

However, it is probably more complicated than just a linear relationship between levels of dopamine and levels of motivation and fatigue. Low dopamine in the ventral striatum, for example, might signal decreased motivation or fatigue. But low dopamine in a different brain region could have a different effect, and we still don’t know exactly how these regions interact.

Other processes that potentially influence fatigue and decisions to exert effort are interoception — the awareness of one’s internal bodily signals — and metacognition — thinking about one’s own thoughts. A person on a run might feel compelled to stop by internal feelings of hunger. Still, he might continue if he recognizes through metacognitive self-assessment of his running history that he runs best on an empty stomach.

So far, I have mainly used physical tasks to study fatigue, but we assume that the neural mechanisms underlying physical and cognitive fatigue are related. The differences probably lie in which task-specific areas of the brain are recruited, along with the previously mentioned areas and regions connected to them. A physical task like exerting grip force would recruit motor regions to a greater extent, whereas a cognitive task like memorization would recruit areas associated with memory formation to a greater extent.

Why do people experience fatigue differently?

There are numerous factors influencing differences in fatigue and its effects. On the neural level, different aspects of fatigue might be more pronounced depending on which brain areas and neural systems are affected. Someone with a particularly active interoception pathway might readily pause a task to adjust the temperature or get a snack. The overvaluation of metacognitive processes telling you you haven’t studied enough to get an A on this exam might influence your performance. And yet evidence suggests that enough motivation could override many of these factors.

Fatigue affects motivation, and in turn, motivation affects fatigue, but whether motivation affects feelings of exhaustion directly or indirectly — through our thoughts and behavior — is still debated. An aspiring painter might visualize her work hanging in an art museum while staring at a blank canvas, but as fatigue sets in, her exhaustion might obscure her view of this once-tangible dream. Reconnecting with that dream could propel her to finish. Some researchers assume that factors such as the degree of autonomy one has over a task, motivational self-talk, or imagination of a reward might facilitate perseverance despite fatigue. 

Interestingly, the order in which people receive or seek out information about a task may influence their willingness to do it. In our research into effort-based choices, we gave participants the option of knowing about the monetary reward first or the effort level first — in this case, grip force strength. People hugely differed in their choices. Some almost always chose to know how much strength they would have to exert before agreeing to the task. These participants were more likely to reject the high-effort offers, even if the reward was relatively high.

They also tended to report lower levels of physical activity in everyday life and higher susceptibility to fatigue, which might suggest that this group had generally lower motivation and perhaps even lower confidence in their ability to persist on a physical task.

This study showed that the ways in which people search for information, their motivation, and levels of fatigue are all related. Future research might illuminate in even greater detail how precisely they affect each other and how this knowledge could best be used to help people prevent and recover from excessive or persistent fatigue and its consequences.