Your brain needs a lot of energy to function. Despite the fact that the average adult brain is only about 2% of your total body mass, it uses a full 20% of the energy you spend while at rest. To keep all your brain functions going, neurons have to constantly be at the ready, and that's no easy task.
Your brain has about 86 billion neurons that are interconnected at a hundred trillion to a quadrillion synapses. When you're thinking, your neurons are shooting messages back and forth across your brain; the cells are "firing." And because someone could jump out and scare you at any time, they have to be ready to fire at a moment's notice.
One study found that the firing of neurons uses about two thirds of your brain's energy. And now scientists have found out how brain cells recharge and fire! Turns out they operate much the same way your muscle cells do when you're exercising.
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Scientific American: Does Thinking Really Hard Burn More Calories?
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Medical Xpress: Brain cells mobilize sugar in response to increased activity