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.

A small but revealing study suggests that even mildly stressful intellectual challenges change our emotional states and behaviors, even if they do not profoundly alter brain metabolism. Fourteen female Canadian college students either sat around, summarized a passage of text or completed a series of computerized attention and memory tests for 45 minutes before feasting on a buffet lunch. Students who exercised their brains helped themselves to around 200 more calories than students who relaxed. Their blood glucose levels also fluctuated more than those of students who just sat there, but not in any consistent way. Levels of the stress hormone cortisol, however, were significantly higher in students whose brains were busy, as were their heart rates, blood pressure and self-reported anxiety. In all likelihood, these students did not eat more because their haggard brains desperately needed more fuel; rather, they were stress eating.



Messier has related explanation for everyday mental weariness: "My general hypothesis is that the brain is a lazy bum," he says. "The brain has a hard time staying focused on just one thing for too long. It's possible that sustained concentration creates some changes in the brain that promote avoidance of that state. It could be like a timer that says, 'Okay you're done now.' Maybe the brain just doesn't like to work so hard for so long."

*Editor's note: The last two sentences of the seventh paragraph were edited after publication for clarity and accuracy