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About what share of the body's energy does the brain use, though it is only about 2% of body weight?
Even at rest the brain is very energy hungry, running on glucose and oxygen.
An adult brain weighs roughly 1.3 to 1.4 kilograms, about 2 percent of body mass, yet it accounts for around 20 percent of the body's resting oxygen and energy use. That works out to something on the order of 20 watts, comparable to a dim light bulb. The figure is an approximation that varies with age: in young children the brain's share of energy use is considerably higher than in adults.
Most of this energy is not spent on conscious problem solving. Estimates attribute a large fraction to maintaining electrical gradients across neuron membranes, so that cells are ready to fire, and to restoring ion balance after signals are sent, including the work of synapses passing chemical messages. The brain also stores very little fuel of its own, so it depends on a steady supply of glucose and oxygen delivered by blood flow.
A common misconception is that hard thinking sharply raises the brain's energy bill. Measurements of metabolism during demanding mental tasks show only small increases over resting levels, since the brain's activity is already high in the background. Likewise, the idea that people use only 10 percent of their brains is not supported by imaging; most regions are active over the course of a day.