What exactly does it mean to be mentally tired? What’s actually happening in your brain? Photo from Geber86/iStock.

Back in 2009, a research team at Bangor University in Wales published a study showing that mental fatigue impairs physical performance. Subjects spent 90 minutes sitting in front of a computer screen either watching a bland documentary or playing a simple but focus-demanding computer game. Afterwards, when they hopped on an exercise bike for a time-to-exhaustion test, those who’d played the computer game immediately reported higher levels of perceived effort, and gave up 15 percent earlier than the documentary watchers.



The results might seem predictable, but the magnitude of the effect was a surprise. In performance terms, sitting at a computer for 90 minutes was roughly equivalent to the negative effects on your leg of jumping off a 14-inch box 100 times. Mental fatigue, a topic that had been mostly neglected by exercise physiologists since the late 1800s, suddenly became a hot topic. This year alone, there have been studies of its effects on soccer, swimming, table tennis, and cycling.



The big practical question, of course, is how to avoid or counteract the effects of mental fatigue. There are several ideas out there, but it’s hard to draw any firm conclusions when the feeling itself remains a black box. What exactly does it mean to be mentally tired? What’s actually happening in your brain? Proposing an answer to these riddles is the challenge that a paper in Sports Medicine, from a team at the University of Canberra led by Kristy Martin, takes on.



The basic hypothesis that Martin and her colleagues present (drawing on a suggestion from 2014) is that mental fatigue results from the accumulation of a brain chemical called adenosine. In this picture, sustained cognitive activity burns up glucose, particularly in certain regions of the brain associated with “effortful mental processes,” such as the anterior cingulate cortex. This temporary and localized fuel shortage triggers a rise in adenosine levels, which in turn blocks the release of neurotransmitters like dopamine. The result is a rise in perception of effort and a decrease in motivation—in other words, a feeling of mental fatigue.



It’s almost impossible to monitor fluctuating levels of brain chemicals in humans in real time, but there are various lines of evidence that bolster this view. For instance, it’s well-established that adenosine levels progressively increase when you’re deprived of sleep, then decrease when you finally conk out. In rats, injections that increase adenosine levels in the brain lead them to make “lazier” decisions, choosing easily available but unappetizing food instead of going to the trouble of pressing a lever to get better food. In contrast, caffeine blocks the action of adenosine: it has a similar molecular structure, so it can attach itself to the same receptors in the brain, preventing adenosine from doing its dirty work—and we all know the mental-fatigue-fighting powers of caffeine.



The net result (if this picture turns out to be correct) is that having a bunch of adenosine in your brain makes everything feel harder. This seems to matter most in sustained endurance activities, when you have to fight an ongoing battle against your desire to quit. In shorter, more intense activities like sprinting, on the other hand, you can suck it up and overcome mental fatigue. That means that in an intermittent running test that alternates sprinting and jogging—think soccer game—mental fatigue doesn’t affect how much ground you cover while sprinting, but it does slow you down during the jogging portions. Late in a soccer game, in other words, you can still sprint for a loose ball, but you’re less likely to be in the right position to start the sprint.



The practical takeaway, according to the researchers, is that anything that lowers the levels of adenosine accumulating in your brain has the potential to improve your endurance performance. Caffeine is a prime example, though hardly a new one.



Simply avoiding cognitively challenging activities before a big competition—a “mental taper”—is another strategy worth considering. In a crazy study a few years ago, cyclists had to watch a video of a woman eating her own vomit before riding a 10K time trial. When they were asked to maintain a poker face rather than show their disgust—a mentally fatiguing act of emotional regulation—they cycled an average of 25 seconds slower. I’ve heard Samuele Marcora, the lead researcher in the original 2009 mental fatigue study, compare the demands of watching this video with the demands placed on athletes to be polite during interviews with “nosy journalists” right before big competitions (a comparison I try not to take personally).



Finally, there’s the seductive lure of training your brain to resist mental fatigue, so that you accumulate less adenosine. Martin and her colleagues suggest that repeated exposure to mental fatigue might eventually make your brain more efficient, so that the neurons themselves in areas like the anterior cingulate cortex consume less glucose. Such brain training might also increase the amount of fuel your brain is able to store, just as exercise leads to an increase in how much glycogen your muscles can store. Both mechanisms would help keep your brain adequately fueled, and in turn reduce the build-up of adenosine.



Marcora has conducted some pilot studies using the mental fatigue-inducing computer games as a form of “brain endurance training,” with promising results. (I tried it myself for a few months, and can’t say it was a lot of fun.)



But it’s important to realize that endurance training itself is a pretty potent source of mental fatigue, and thus may be its own form of brain endurance training. After all, what is a marathon other than several hours of resisting the powerful (and eminently logical) urge to slow down or stop? In one of Martin’s earlier studies, she compared professional and amateur cyclists, and found that the pros were better at the mentally fatiguing computer tasks. The pros also didn’t get slower on their bikes after a mental fatigue session, unlike the amateurs. That suggests that all their training had make the pros better at handling mental fatigue, perhaps by reducing how quickly adenosine builds up in their brains during sustained cognitive effort.



So the open question, to me, is how much headroom is left for improving mental fatigue. If you’re training twice a week and have some free time, you’re obviously going to be better off doing more physical training (with mental benefits built in automatically) rather than thinking about mental training. But if you’re already training pretty hard, is there any benefit to adding some mental exercises to potentially boost your performance without added load on your body? My suspicion is that the more practical insights will come from thinking of ways to avoid mental fatigue in the first place, like the mental taper concept. But having a well-articulated physiological model of what mental fatigue actually is should pave the way for some experiments to test these ideas out.

Alex Hutchinson's new book, Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance, with a foreword by Malcolm Gladwell, is now available. For more, join him on Twitter and Facebook, and sign up for the Sweat Science email newsletter.