Oliver Roeder writes that the fourth game between Magnus Carlsen and Sergey Karjakin was another marathon that, like the third, ended similarly with a thrilling draw after Karjakin managed to recover from a poor situation.

Today is a rest day and the players will need it after two grueling days. People who have not played high level chess may be surprised, given that there is so little physical exertion involved, at the intense feeling of exhaustion one feels at the end of a hard-fought game. Others may have had similar feelings after studying for a test or otherwise engaging in intense intellectual activity. Could it really be that the brain is using up that much more energy during periods of intense concentration?

It is true that although the brain makes up only 2% of our body weight, it consumes 20% of our resting metabolic rate. But there is no clear evidence that concentrating alone drives up energy consumption by that much. So why do we feel so tired?

Unsatisfying and contradictory findings from glucose studies underscore that energy consumption in the brain is not a simple matter of greater mental effort sapping more of the body’s available energy.
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If challenging cognitive tasks consume only a little more fuel than usual, what explains the feeling of mental exhaustion following the SAT or a similarly grueling mental marathon? One answer is that maintaining unbroken focus or navigating demanding intellectual territory for several hours really does burn enough energy to leave one feeling drained, but that researchers have not confirmed this because they have simply not been tough enough on their volunteers. In most experiments, participants perform a single task of moderate difficulty, rarely for more than an hour or two. “Maybe if we push them harder, and get people to do things they are not good at, we would see clearer results,” Messier suggests.

Equally important to the duration of mental exertion is one’s attitude toward it. Watching a thrilling biopic with a complex narrative excites many different brain regions for a good two hours, yet people typically do not shamble out of the theater complaining of mental fatigue. Some people regularly curl up with densely written novels that others might throw across the room in frustration. Completing a complex crossword or sudoku puzzle on a Sunday morning does not usually ruin one’s ability to focus for the rest of the day—in fact, some claim it sharpens their mental state. In short, people routinely enjoy intellectually invigorating activities without suffering mental exhaustion.

Such fatigue seems much more likely to follow sustained mental effort that we do not seek for pleasure—such as the obligatory SAT—especially when we expect that the ordeal will drain our brains. If we think an exam or puzzle will be difficult, it often will be.
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In the specific case of the SAT, something beyond pure mental effort likely contributes to post-exam stupor: stress. After all, the brain does not function in a vacuum. Other organs burn up energy, too. Taking an exam that partially determines where one will spend the next four years is nerve-racking enough to send stress hormones swimming through the blood stream, induce sweating, quicken heart rates and encourage fidgeting and contorted body postures. The SAT and similar trials are not just mentally taxing—they are physically exhausting, too.

It may be that in playing championship chess, people are constantly making minute movements that, combined with the stress, leads the feelings of tiredness.