Fatigue is one of the most common symptom experiences, both among people with chronic illnesses and among healthy adults. Unfortunately, it's one with no easy medical solution: Caffeine helps in the short term, but its effects aren't as restorative as sleep. And the same factors that cause fatigue can also interfere with getting restful sleep or true recovery. Unfortunately, this post isn't going to be about how to reduce fatigue; instead, it's about what happens in the brain when we are tired. We're all likely to be fatigued at some point, so it might be useful to understand how that affects us.
A 2022 study using fMRI brain scan technology found that the amount of inter-connection between different parts of the brain tended to decrease after people completed a cognitively demanding task. That this means is that the conscious, deliberate, Narrative Mind becomes isolated from the Intuitive Mind that actually controls behavior. Similarly, a 2021 study showed that cognitive effort (in this case, tiring people out with the Stroop color-naming task) led to reduced blood flow in the frontal lobes of the brain, which is the area most clearly associated with the Narrative Mind or what we usually call "executive functioning." This situation is similar to what happens to the brain over time in people with substance use disorders, and also in people with psychological trauma -- they become more likely to react based on immediate rewards and punishments, rather than thoughtfully pursuing their long-term goals. Prioritizing short-term goals can lead to some obvious problems: For instance, a tendency to choose sugary and fatty foods, or an increased tolerance for risk. A study of Chinese miners found that creating a state of mental fatigue did indeed make the workers more likely to take risks in everyday situations, which in their line of work might create life-threatening dangers. Another way in which fatigue impairs task performance is by creating an "aversion to effort" -- tired people are less motivated to gather needed information or to think through their decisions carefully. Hospital-based physicians order fewer tests as they become more tired over the course of their shifts, and soccer referees make worse decisions as the game wears on, in part because they exert less effort to investigate potential rules violations. Finally, fatigue may deny us access to creative processing that requires input from both the Narrative and the Intuitive Minds. Tired soccer players are less creative in their thinking, and these changes are associated with less alpha-wave EEG activity (active mental processing) in the frontal cortex areas associated with the Narrative Mind. Basketball players similarly show less effective use of their skills as they become more fatigued, due to impaired practical decision-making on the court.
So far, these findings are very much in line with what we might expect from Kahneman's Thinking, Fast and Slow. The "lazy" Narrative Mind is easy to derail in various ways, and tired people default to an Intuitive mode of thinking. But not so fast: There's plenty of evidence that fatigue impairs the Intuitive Mind as well. The easiest way to see this type of impairment is using a test of lower-level skills like reaction time or response accuracy. In Cao et al.'s study of basketball players, it wasn't just higher-level strategic thinking that was impaired. The players also had deficits in technical aspects of play like free throw accuracy, three-point shot completion, and turnover of the ball. Soccer players similarly have worse response time, worse shot accuracy, and worse passing performance as they become more fatigued. A review of studies from 3 different sports -- soccer, basketball, and table tennis -- found that players' offensive game showed impairment based on declines in accuracy and speed (Intuitive Mind), and that these deficits were similar in magnitude to those seen for players' decision-making in the game (Narrative Mind); interestingly, players' defensive game showed much less impairment. Outside the realm of sports, the managers of Chinese mining operations had a slower reaction time to decisions under conditions of fatigue, as well as the impaired critical thinking that was found in studies of the miners themselves, and the signs of fatigue could be detected using brain scans. All of these studies suggest that the Intuitive Mind is also significantly impaired under conditions of fatigue.
If both mental systems show similar deficits when we are fatigued, what can we do to cope? A study of fighter pilots showed that fatigue did produce the expected decline in reaction time after 7 hours in the cockpit of a simulator. But that study did not show the expected decrease in strategic decision-making performance. The authors' interpretation is that the pilots were able to exert more effort in order to overcome the effects of fatigue, essentially using the Narrative system to compensate for Intuitive-level impairment. Interestingly, although the pilots' accuracy was no worse, their confidence in their performance dropped significantly -- and more so, the more complicated the task that they were asked to perform. The study of Chinese mine managers also suggested more questioning of one's own judgment as fatigue set in. In both of these cases, a tendency to second-guess one's own performance probably helped the study participants to keep working at a high level of accuracy, and perhaps they were motivated to do so by the high-stakes nature of their tasks. People who are less motivated (remember that fatigue inherently decreases motivation) might be less likely to exert the herculean levels of effort required to maintain accuracy.
Although most strategies to combat fatigue do tend to rely on increased use of the Narrative Mind (double-checking, structured checklists, prompts and reminders, pop-up windows, etc.) there may be another approach. Another study under real-world conditions found that physicians' accuracy in detecting breast cancer did not decrease after an hour of reading mammograms. In fact, they had faster performance speed and fewer false alarms as time went on, without any corresponding increase in missed diagnoses. One possibility is that the repetitive task allowed these experts to get "in the zone" where their established skills could be deployed more automatically and with less conscious thought. The study duration might not have been long enough to produce significant fatigue -- presumably, after a few more hours of work, the physicians would have shown the usual declines in performance speed and accuracy. But over the short-term, they actually seemed to do better by shifting the work to their Intuitive Mind. That makes a kind of sense: For tasks where the person has a high level of expertise, the best strategy under conditions of fatigue may be simply to "trust their gut." Of course, if confidence decreases with fatigue, that might become increasingly hard to do.
Overall, the Narrative Mind is probably the more limited of the two systems: It has a small funnel for information in the form of our ability to pay attention to only one thing at a time, and it seems to be harder to restore, with some persistent deficits in Narrative-Mind performance even after a rest. Narrative-Mind processing also requires conscious effort, which is impaired with fatigue. The Intuitive Mind of course has its own risks, and the review above shows that it is similarly less efficient under conditions of fatigue. But as the physicians' study suggests, it's possible to make practiced Intuitive-level judgments with a high degree of accuracy without using the Narrative Mind much at all. Intuitive-Mind skills, therefore, are our first line of defense when fatigued. Beyond that, we can follow the fighter-pilots' example and exert extraordinary effort to offset the effects of fatigue, but we will probably pay a price in terms of second-guessing ourselves. As people get more fatigued, the gap between their actual performance and their confidence seems to get larger -- and not necessarily in the direction of overconfidence! It may therefore help to have an external standard against which to judge our performance, to see whether we are actually doing as well or as badly as we think. And ultimately, we have to accept that we just don't work as well when fatigued: The only real solution is rest.
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