Between October and June they shuffle out of auditoriums, gymnasiums and classrooms, their eyes adjusting to the sunlight as their fingers fumble to awaken cell phones that have been silent for four consecutive hours. Some raise a hand to their foreheads, as though trying to rub away a headache. Others linger in front of the parking lot, unsure of what to do next. They are absolutely exhausted, but not because of any strenuous physical activity. Rather, these high school students have just taken the SAT. "I was fast asleep as soon as I got home," Ikra Ahmad told The Local, a New York Times blog, when she was interviewed for a story on "SAT hangover."

Temporary mental exhaustion is a genuine and common phenomenon, which, it is important to note, differs from chronic mental fatigue associated with regular sleep deprivation and some medical disorders. Everyday mental weariness makes sense, intuitively. Surely complex thought and intense concentration require more energy than routine mental processes. Just as vigorous exercise tires our bodies, intellectual exertion should drain the brain. 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.

Brainpower
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:

—Resting metabolic rate: 1300 kilocalories, or kcal, the kind used in nutrition
—1,300 kcal over 24 hours = 54.16 kcal per hour = 15.04 gram calories per second
—15.04 gram calories/sec = 62.93 joules/sec = about 63 watts
—20 percent of 63 watts = 12.6 watts

So a typical adult human brain runs on around 12 watts—a fifth of the power required by a standard 60 watt lightbulb. Compared with most other organs, the brain is greedy; pitted against man-made electronics, it is astoundingly efficient. IBM's Watson, the supercomputer that defeated Jeopardy! champions, depends on ninety IBM Power 750 servers, each of which requires around one thousand watts.

Energy travels to the brain via blood vessels in the form of glucose, which is transported across the blood-brain barrier and used to produce adenosine triphosphate (ATP), the main currency of chemical energy within cells. Experiments with both animals and people have confirmed that when neurons in a particular brain region fire, local capillaries dilate to deliver more blood than usual, along with extra glucose and oxygen. This consistent response makes neuroimaging studies possible: functional magnetic resonance imaging (fMRI) depends on the unique magnetic properties of blood flowing to and from firing neurons. Research has also confirmed that once dilated blood vessels deliver extra glucose, brain cells lap it up.

Extending the logic of such findings, some scientists have proposed the following: if firing neurons require extra glucose, then especially challenging mental tasks should decrease glucose levels in the blood and, likewise, eating foods rich in sugars should improve performance on such tasks. Although quite a few studies have confirmed these predictions, the evidence as a whole is mixed and most of the changes in glucose levels range from the miniscule to the small. In a study at Northumbria University, for example, volunteers that completed a series of verbal and numerical tasks showed a larger drop in blood glucose than people who just pressed a key repeatedly. In the same study, a sugary drink improved performance on one of the tasks, but not the others. At Liverpool John Moores University volunteers performed two versions of the Stroop task, in which they had to identify the color of ink in which a word was printed, rather than reading the word itself: In one version, the words and colors matched—BLUE appeared in blue ink; in the tricky version, the word BLUE appeared in green or red ink. Volunteers who performed the more challenging task showed bigger dips in blood glucose, which the researchers interpreted as a direct cause of greater mental effort. Some studies have found that when people are not very good at a particular task, they exert more mental effort and use more glucose and that, likewise, the more skilled you are, the more efficient your brain is and the less glucose you need. Complicating matters, at least one study suggests the opposite—that more skillful brains recruit more energy.*

Not so simple sugars
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. Claude Messier of the University of Ottawa has reviewed many such studies. He remains unconvinced that any one cognitive task measurably changes glucose levels in the brain or blood. "In theory, yes, a more difficult mental task requires more energy because there is more neural activity," he says, "but when people do one mental task you won't see a large increase of glucose consumption as a significant percentage of the overall rate. The base level is quite a lot of energy—even in slow-wave sleep with very little activity there is still a high baseline consumption of glucose." Most organs do not require so much energy for basic housekeeping. But the brain must actively maintain appropriate concentrations of charged particles across the membranes of billions of neurons, even when those cells are not firing. Because of this expensive and continuous maintenance, the brain usually has the energy it needs for a little extra work.

Authors of other review papers have reached similar conclusions. Robert Kurzban of the University of Pennsylvania points to studies showing that moderate exercise improves people's ability to focus. In one study, for example, children who walked for 20 minutes on a treadmill performed better on an academic achievement test than children who read quietly before the exam. If mental effort and ability were a simple matter of available glucose, then the children who exercised—and burnt up more energy—should have performed worse than their quiescent peers.

The influence of a mental task's difficulty on energy consumption "appears to be subtle and probably depends on individual variation in effort required, engagement and resources available, which might be related to variables such as age, personality and gluco-regulation," wrote Leigh Gibson of Roehampton University in a review on carbohydrates and mental function.

Both Gibson and Messier conclude that when someone has trouble regulating glucose properly—or has fasted for a long time—a sugary drink or food can improve their subsequent performance on certain kinds of memory tasks. But for most people, the body easily supplies what little extra glucose the brain needs for additional mental effort.

Body and mind
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. Studies have shown that something similar happens when people exercise and play sports: a large component of physical exhaustion is in our heads. In related research, volunteers that cycled on an exercise bike following a 90-minute computerized test of sustained attention quit pedaling from exhaustion sooner than participants that watched emotionally neutral documentaries before exercising. Even if the attention test did not consume significantly more energy than watching movies, the volunteers reported feeling less energetic. That feeling was powerful enough to limit their physical performance.

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.

18 Comments

1. 1. sapbucket 11:15 AM 7/18/12

   Fantastic article! I really appreciate the extra effort taken by the author, especially the math converting resting metabolic rate to a measure of power. Editor please promote this guy.
   
   Those of you who frequent articles on this website should know what I mean: how many times have you clicked on an interesting article title only to find 2 or 3 paragraphs of dribble and fluff? (Complete with typos, misquotes, inaccuracies, lack of basic science, faulty hypothesis, etc. - many other writers for this website lack basic math and science knowledge, and in fact, can just barely string together a small story putting to question their training as a English and/or technical writer).

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2. 2. Tony_Who 11:59 AM 7/18/12

   Thanks for the article. I have often wondered about how much energy it takes to think. Sometimes, when working on a complex machine design for months on end, I have to eat twice as much.
   
   I also wonder, do heavy thinkers typically have a higher body temperature? What is the typical range of body temperatures for people?
   
   Thanks,
   -Tony

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3. 3. krohleder 01:57 PM 7/18/12

   One assumption made by most when first learning about neuroscience is that a neuron fires, kind of like a circuit getting a charge, and that firing is what processes the information. More firing neurons equals more thinking, which equals more energy consumption. Well this is entirely wrong. Only some of the time does it work this way. However quite often a neuron or network of neurons are firing and when they are inhibited, that is turned off, that is when the information is processed. So in that case energy consumption is reduced to process the information (at least locally and in the short term). To my knowledge nobody knows yet what the ratio of inhibitory versus excitatory neural networks is although some have estimated that more than half could be inhibitory.

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4. 4. jhewitt123 04:15 PM 7/18/12

   Yes, we don't really need to prove this idea because both it and the opposite can be true. As contributor Krohleder might agree, neuron groups can be like a class of noisy students, processing disjointed trivial thoughts at perhaps relatively high energy expenditure, and when tuned down by an instructor may be directed to a more widespread and coherent attention

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5. 5. Dr. Strangelove 01:52 AM 7/19/12

   Thinking hard does not burn more calories because the brain already burns a lot of calories even when you're not thinking. Some put it at 25 watts. That's 24 hours a day. Put a dynamo to power a 25-watt bulb in your stationary bike. Pedal to keep the lights on for 24 hours. That's how much energy the brain consumes even while you sleep.

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6. 6. Neosteo 02:44 AM 7/19/12

   Interesting article however not a single ounce of energy comes from food.
   Energy comes from the Mind and glucose is the fuel, with oxygen being its burning medium. This accounts for the apparent increase of energy after exercise and the variations in sugar consumption. Not all sugars are made equal, Sugars from fresh fruit and vegetables have the highest nutrient, oxygen and water content hence they are the easiest to digest. Cooked starches on the other hand have far less nutrients and are deprived of the oxygen needed to burn the fuel the body needs in an efficient manner hence the sluggishness and high fluctuations in blood sugar levels.

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7. 7. Dr. Strangelove in reply to Neosteo 03:09 AM 7/19/12

   Energy comes from the fuel which is glucose which comes from food. Energy in the form of electrical signals and heat is expended in the brain.

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8. 8. Dr. Strangelove in reply to Neosteo 03:17 AM 7/19/12

   Exercise increases the rate of metabolism, the conversion of fuel into energy. Sugar is fuel so blood sugar level and energy level in the form of heat are affected by exercise. Notice you feel warm and sweat when exercising.

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9. 9. krohleder 09:56 AM 7/19/12

   Neosteo, I am glad you are interested but almost everything you wrote is somewhat inaccurate. All energy consumption in the body comes from food; specifically sugar. Glucose is type of converted sugar. It is burned to produce heat and adenosine triphosyphate, a molecule that stores and releases energy as needed by the cell. In terms of energy most sugars are sort of equal and I am not sure about what nutrients you mean; but that would have nothing to do with food energy conversion. You do not burn fuel in your body like a combustion engine or fire where oxygen is needed. Also the word nutrient is a vague term which refers to chemicals an organism needs to grow and live. So this can be protein, iron, vitamins, carbohydrates (sugars), etc.

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10. 10. bucketofsquid in reply to Neosteo 12:22 PM 7/25/12

    "Energy comes from the Mind and glucose is the fuel, with oxygen being its burning medium."
    
    You really need to learn some basic physics. Fuel is the energy source. Try driving your car without gas or batteries. The engine of a vehicle, like the brain in an animal, releases and directs that energy. It does not create that energy. (you can skip the whole steering and transmission bit because I am simplifying)
    
    Had you specified the conversion of potential energy to kinetic energy under the direction of the mind with the potential energy coming from glucose you would have been fine but the way you constructed your sentence creates an incorrect or incomplete statement.

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11. 11. cautionflame in reply to sapbucket 11:43 PM 7/25/12

    I find it ironic that the first comment I read on this article is such effusive praise, when I found the conclusions to the studies presented rather illogical.
    
    For example the article states:
    "Complicating matters, some studies have found that when people are not very good at a particular task, they exert more mental effort and use more glucose, whereas at least one study suggests the opposite—that the more skilled you are, the more efficient your brain is and the less glucose you need."
    
    That seems perfectly reasonable to me. When you are not good at something you need to exert more energy and burn more glucose. When you are good at something, you exert less energy and use less glucose. How does that complicate things? The use of the word opposites implies that it's contradictory, but really, it's complementary.
    
    The paragraph about children exercising states;
    "children who walked for 20 minutes on a treadmill performed better on an academic achievement test than children who read quietly before the exam. If mental effort and ability were a simple matter of available glucose, then the children who exercised—and burnt up more energy—should have performed worse than their quiescent peers."
    
    Energy in the body isn't in a ready to burn state, like a gas tank. The body has to convert stored energy into usable energy. The kids who were exercising were in energy conversion mode. Their body was freeing stores of glucose to "power" the exercise, so the energy/glucose was more readily available for thinking activities as well.
    
    I think the key points of the topic, which were only lightly touched on at the end, are the sustained concentration required in a situation like the SATs and the duration of time. The studies stated are like comparing a short distance sprinter to a marathon runner.

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12. 12. mindclone 05:12 PM 8/1/12

    From all of the research I've read over the years, it appears that the brain does burn significantly more calories while thinking extra hard.
    I think that individuals are too complex in their biology to explain why some feel an increase in appetite after an intense period of studying or whatnot.
    Perhaps its from stress or merely just increased hunger.
    http://evolutionarypsychiatry.blogspot.com/2010/12/your-brain-on-creatine.html
    and http://aging.info

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13. 13. dpitkow 04:19 PM 8/15/12

    I think there are many things going on in the brain that have not been studied, so the focus on glucose consumption is shortsighted. How about all the neurotransmitters? How about distinguishing between different type tasks? Watching a movie and reading a book are different tasks mentally, very different from performing a math calculation.

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14. 14. dmoffittsmith 01:23 AM 8/17/12

    I find it counterintuitive that concentrated thinking would increase energy consumption in the brain. The logic, to me, being that focus on a problem is probably accompanied by inhibition of competing threads of activity. And focus is perceived to be difficult.

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15. 15. agpsciam 10:17 AM 10/23/12

    On the topic of whether mental exhaustion is mostly about energy consumption/availability: The energy that you consume while sleeping for 8 hours is very similar to what you would use if you instead lie awake in bed for the same amount of time. However, the subsequent mental states are of course very different. Wouldn't this suggest that mental exhaustion is not about energy depletion, but is instead connected to somehow not getting the brain "re-set" (or whatever it is that sleep does...)?

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16. 16. upload70 10:29 AM 12/13/12

    thinking hard is stressful that increases heart rate so prob does increase calorie requirements. http://buysteroidsuk.co/

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17. 17. Brazenhawk 11:09 PM 3/13/13

    I can appreciate certain "theories", however, most are simply that, unless you have good evidence. It's true that brain activity will never compare to moderate types of physical exercise, however, I have been a type 1 diabetic for almost 34 years. I can notice a sizable difference in my blood sugars when consumed in deep thought and having a "mental workout" in a day. I notice the patterns of simular activity in front of the computer, from a day of simple regular work and comparing that with heavy "thinking" days which require constant focus and deep thought (such as drawing up legal and patent papers). The same structured enviroment with the same physical activity (sitting in front of the computer) and thesame diet and insulin usage. However, in a day of deep thought, my sugar can go into the lower ranges several times during the heavy mental days, when n typical days it never does. I can actually feel my pulse if I put my hand on top of my head due to the extra blood flow. So in my own personal experience, I would have to say that heavy thought does burn significantly more calories. Being an insulin dependent diabetic for almost 3 1/2 decades makes me much more sensitive to such changes and I do notice a signifcant difference.

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18. 18. sandalf 11:55 AM 6/5/13

    It doesn't seem to me that ANY of these tests used anything but the most fundamental mental abilities. High brain activity means being able to determine the color of a word? Sudoko? Hitting one key or another? Let's talk to mathematicians involved in regularly creating work in 700+ dimensional-space, who require hours to get into the heart of the problem and cannot be disturbed less those hours of work crash, and they have to re-enter the problem. Let's talk to people actually using more of their brains, rather than students taking SATs. I do have BG issues myself. When I keep my sugar around 90-100 as the docs want, I cannot think in more than 5 or 6 dimensions without making costly mistakes. My best, most productive mental productivity/functioning level seems to be between 120 and 140. Since ideal BG levels are set for that mythical AVERAGE person {often ideal AVERAGE levels are determined by using institutionalized, hospitalized, or paid off-the-street-I-need-money patients}, why would any of us believe it had to apply to us?
    It could even be the same issue as the iron level of spinach. The researcher was off in his decimal points, we now know, and spinach has no more significant iron than iceberg lettuce. People keep republishing his flawed numbers. We lost a Mars landing craft a few years back when it skipped off Mars' atmosphere because one set of researchers were using metric measures and the other was using English (ft, etc.) measures, and none had bothered doing what their high school math teacher required, write the units. How do we know BG research applying to all of us is any more valid than these faulty results? Are you AVERAGE?
    

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