The brain uses a ton of energy all the time and thinking hard probably doesn’t burn all that much more fuel.Mental exhaustion often results when we have to work hard on something we don’t like (the SAT) but rarely on something we do enjoy (a fun but challenging puzzle.)
So feeling brain-tired may literally be “all in our heads” or it might just be the result of stress.
What the latest science reveals, however, is that the popular notion of mental exhaustion is too simplistic. The brain continuously slurps up huge amounts of energy for an organ of its size, regardless of whether we are tackling integral calculus or clicking through the week’s top 10 LOLcats. Although firing neurons summon extra blood, oxygen and glucose, any local increases in energy consumption are tiny compared with the brain’s gluttonous baseline intake. So, in most cases, short periods of additional mental effort require a little more brainpower than usual, but not much more. Most laboratory experiments, however, have not subjected volunteers to several hours’ worth of challenging mental acrobatics. And something must explain the feeling of mental exhaustion, even if its physiology differs from physical fatigue. Simply believing that our brains have expended a lot of effort might be enough to make us lethargic.
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. Studies have shown that something similar happens when people exercise and play sports: a large component of physical exhaustion is in our heads… In the specific case of the SAT, something beyond pure mental effort likely contributes to post-exam stupor: stress.
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