Why Do You Get Tired After Taking a Test?

By Veronique Greenwood
Jul 18, 2012 9:17 PMNov 19, 2019 11:51 PM


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So. Tired. From reading email.

A day of hard mental labor---writing emails, taking the SAT, competing in the national crossword competition---can leave you beat. But how, exactly, is that possible? You haven't done any heavy lifting, at least not with your muscles. Ferris Jabr at Scientific American MIND takes a crack

at investigating this phenomenon, exploring the science on whether thinking really hard burns calories, or whether the exhaustion is coming from something else. He writes:

Although the average adult human brain weighs about 1.4 kilograms, only 2 percent of total body weight, it demands 20 percent of our resting metabolic rate (RMR)—the total amount of energy our bodies expend in one very lazy day of no activity.RMR varies from person to person depending on age, gender, size and health. If we assume an average resting metabolic rate of 1,300 calories, then the brain consumes 260 of those calories just to keep things in order. That's 10.8 calories every hour or 0.18 calories each minute. (For comparison's sake, see Harvard's table of calories burned during different activities). With a little math, we can convert that number into a measure of power.

A typical brain, he finds, runs on 12 watts, a fraction of the power need to run a lightbulb. And does that go up when you're doing the infamously tough Sunday crossword puzzle? Well, it's not exactly clear, but scientists have devised various experiments involving sugarwater, all-you-can-eat buffets, and tasks of varying difficulty. It seems like that pile of cells you call your brain might need slightly more juice to answer a tough question, but is it enough to matter? To read more about what they've found, head over to Scientific American


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