Do you ever feel spacey, distracted and worn down toward the end of a long work-related task – especially if that task is entirely a mental one? For over a century, psychologists have been trying to determine whether mental fatigue is fundamentally similar to physical fatigue or whether it is governed by different processes.
Some researchers have argued that exerting mental effort depletes a limited supply of energy – the same way physical exertion fatigues muscles. The brain consumes energy in the form of glucose, which can run low.
Other researchers see mental fatigue as more of a psychological phenomenon. Mind-wandering means the current mental effort is not being sufficiently rewarded – or opportunities to do other, more enjoyable activities are being lost.
My colleagues and I have been trying to resolve this question. Our research suggests mental fatigue is in large part a psychological phenomenon – but one that can be modified by setting goals.
Tedious tasks can be especially hard to stick with diligently.
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Vigilance is hard to sustain
We began by reviewing the science related to mental fatigue.
Psychologists in the World War II era studied why soldiers monitoring radar were losing focus during their shifts. Psychologist Norman Mackworth designed the “clock test,” in which military participants were asked to watch a large “clock” on a wall for up to two hours. The second hand ticked at regular intervals. But rarely and unpredictably, it would jump twice the usual distance. The task was to detect those tiny variations.
Within the first 30 minutes, the subjects’ performance dropped dramatically – and then continued to decline more gradually. Psychologists named the necessary mental focus “vigilance” – and concluded it was fundamentally limited in humans.
Decades of research since has confirmed that vigilance is difficult to maintain, even over brief intervals. In studies, people report feeling stressed and fatigued following even a brief vigilance task. In 2021, one study even showed a reduction of blood flow through the brain during vigilance.
My colleagues and I wondered: Are all forms of mental work like vigilance? Surely, there are instances where people can engage with mental work without feeling fatigued.
Setting goals
We decided to study whether goal-setting could improve mental focus and ran three experiments to test this idea.
Where’s the X?
Lauren D. Garner, CC BY-ND
In the first experiment, we showed 108 undergraduate students at the University of Oregon a screen with four empty white boxes against a gray background. Every one to three seconds, an X appeared in one of the four boxes. Their task was to indicate where that symbol appeared as quickly as possible. After each response, the participant was given feedback about both their accuracy and their speed, such…
