Why Do We Experience Fatigue?

In a previous blog post, I covered what fatigue is, what happens when we feel fatigued, and why fatigue intuitively seems to contradict how we think of other subjective experiences. In this one, I will review how psychologists tended to think about fatigue for the past century and two interesting studies and concurrent theories of fatigue. Last, based on all of this, I will speculate a bit on how we might delay the onset of fatigue while working towards our goals.

A Short History of Fatigue

Psychologists have studied fatigue pretty much since psychology was “founded” as a science in the late 19th century. For the majority of this time, they thought of fatigue as reflecting the draining of a mental battery. The idea was that when we work hard, some mental resource gets depleted. Fatigue was then thought of as a byproduct of mental activity that indicates that this resource is running low and we need to take a break from work.

Sounds like it makes sense. Except that it doesn’t. Allow me to explain.

First, let’s look at this idea from a theoretical perspective. The first thing you should ask yourself is “What is this resource”? Most of the work done on this topic does not deem it necessary to specify the resource—rather, it seems that the resource is thought to be some mental, non-material construct. In my opinion, every scientist should immediately reject such an idea on the terms of what we call unfalsifiability—if you do not specify what the resource is, how I can measure it and where I can observe it, I cannot prove you wrong. Making predictions that are falsifiable should be the lowest bar for a proper scientific theory.

Then, it was proposed that glucose is the resource that is being depleted, since glucose is the primary source of energy in the brain. The brain does consume a large amount of glucose, but given how vital it is that our brain does not run out of energy, nature made sure to give the brain priority over other body parts when it comes to glucose usage. Unsurprisingly, it was shown convincingly that the brain does not consume more glucose when it is working harder (though interestingly, changes in the rest of the body, such as the heart and the lungs, can be observed with increasing mental activity).

While I am criticizing these ideas a bit now, I am actually very fond of them, because they vividly capture how science is meant to work—an idea is put forward, rigorously tested, abandoned, and eventually replaced by a new working model. I just wish it didn’t take us 100 years to get there. Anyways, before we turn to the modern theories on fatigue, let’s quickly take a look at two cool studies that have contributed to refuting the resource metaphor.

Empirical Studies on Fatigue

Using a super cool and simple design, Veronika Job and colleagues showed that the typical effect of a short fatigue manipulation was modulated by people’s beliefs about willpower. First, participants reported to which extent they believed that willpower is based on a limited resource. Then, participants were divided into two groups. The two groups got a similar task that differed in difficulty, so one group should have become more fatigued than the other group (though the strength of this manipulation itself could be the subject of a blog post).

Unsurprisingly, participants in the group that had done the more difficult task felt more fatigued and performed worse at a subsequent task. However, this was only true for participants that believed in the resource idea! I hope it is obvious why this contradicts the resource idea itself: If a resource is depleted, it should not matter what one believes.

In another study with a simple but effective design, Jasper Hopstaken and colleagues asked participants to do a mentally demanding task for 90 minutes (yep, no questions about the strength of this fatigue manipulation). As expected, over time participants reported feeling more fatigued and performed worse on the task.

After these 90 minutes, the experimenter told the participants that how long the experiment was going to last from that moment on would depend on their performance. The better they performed, the quicker they would be able to get out of there. After this manipulation, participants reported less fatigue and performed even better than at the start of the experiment! Again, this challenges the resource idea: If a resource is truly used up, there is no way that it suddenly refills just because you regain motivation for the task at hand.

A Motivational Perspective on Fatigue

Okay, so the resource idea is dead (at least to me and hopefully to you; it continues to be popular in multiple fields of research). This raises some questions: Why do we feel fatigued? What’s the point? Why, if working hard is good for us, does fatigue not feel pleasant? Multiple psychologists have come up with potential answers to this question, but I personally like the proposed explanation by Robert Kurzban and colleagues best. They explain that while all of us have multiple goals and hence would like to do multiple things at all times, we usually can only do one thing at a time. Thus, everything we do comes at a cost, namely not being able to do something else we also would also like to do (this is called an opportunity cost). Fatigue, then, might reflect these costs.

More specifically, we might start to feel fatigue when our mind comes to the conclusion that we should stop doing what we are doing right now and switch to doing something else. From this perspective, fatigue is not the result of a depleted resource but of a loss of motivation for the current activity. The two studies reviewed above are more in line with this idea, but a lot of empirical work still needs to be done to test this idea as rigorously as the resource idea has been tested.

The reason why I like this model a lot is because Kurzban et al. aim to include predictions in the theory that are testable and falsifiable. They propose that the mind compares what is currently being done to what could be done instead. If the current activity is judged to have the highest value, the theory predicts no or little experience of fatigue even when it is performed for a prolonged amount of time and requires substantial (mental) activity. On the other hand, when the alternative activity is judged to have a higher value, we should quickly start to feel more and more fatigued.

A Potential Way to Prevent Fatigue?

As noted, the idea that we feel fatigued because we are losing motivation needs to be tested a lot more. However, I believe that this idea best explains all the data from psychological studies on fatigue. Additionally, in my own work, we have found some preliminary support for a motivational role in fatigue.

What then, could we do to prevent or delay feeling fatigued? In situations in which we try to be productive (in the library, at work, doing chores) we could try to identify the most rewarding alternative activities in our environment and temporarily disable them (e.g., turning off our smartphone). Maybe, by removing rewarding alternatives from the environment, we can trick our minds into being motivated to pursue our obligatory goals a little longer.

Would I bet a lot of money that this works? At this point, certainly not. But it probably doesn’t hurt to try it for a while. If you do, let me know how it worked out.