So a typical adult human brain runs on around 12 watts-a fifth of the power required by a standard 60 watt lightbulb. 15.04 gram calories/sec = 62.93 joules/sec = about 63 watts 1,300 kcal over 24 hours = 54.16 kcal per hour = 15.04 gram calories per second Resting metabolic rate: 1300 kilocalories, or kcal, the kind used in nutrition With a little math, we can convert that number into a measure of power: (For comparison's sake, see Harvard's table of calories burned during different activities). That's 10.8 calories every hour or 0.18 calories each minute. 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. RMR varies from person to person depending on age, gender, size and health. Simply believing that our brains have expended a lot of effort might be enough to make us lethargic.Īlthough 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. And something must explain the feeling of mental exhaustion, even if its physiology differs from physical fatigue. Most laboratory experiments, however, have not subjected volunteers to several hours' worth of challenging mental acrobatics. So, in most cases, short periods of additional mental effort require a little more brainpower than usual, but not much more. 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.
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. What the latest science reveals, however, is that the popular notion of mental exhaustion is too simplistic. Just as vigorous exercise tires our bodies, intellectual exertion should drain the brain. Surely complex thought and intense concentration require more energy than routine mental processes. Everyday mental weariness makes sense, intuitively. 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. "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." Rather, these high school students have just taken the SAT. They are absolutely exhausted, but not because of any strenuous physical activity. Others linger in front of the parking lot, unsure of what to do next. Some raise a hand to their foreheads, as though trying to rub away a headache.
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.
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