Your brain is small. It is only about 2% of your body weight. But it uses about 20% of your body's energy when you are resting. It needs about 20 watts, like an old light bulb.

The brain uses energy even when you sleep. Young children's brains use even more. Hard thinking adds only a little extra energy, so studying will not help you lose weight.

The adult brain weighs only about 1.2 to 1.4 kilograms, about 2 percent of body weight. But it uses around 20 percent of the body's energy at rest. That is roughly 20 watts, similar to a dim light bulb.

Where does all this energy go? Much of it keeps nerve cells ready to send electrical signals. After each signal, tiny pumps in the cell membranes move ions back into place, and this constant work needs a lot of energy.

Because the brain is always this busy, concentrating hard on a problem adds only a little extra. Children's brains are even hungrier. One study estimated that at around age five, the brain uses about two-thirds of the body's resting energy.

Weighing only about 2% of an adult's body, the brain nevertheless consumes roughly 20% of the energy the body uses at rest, about 20 watts. Gram for gram, it burns several times more energy than resting muscle, and it never switches off, even during sleep. Neuroscientists have argued that most of this budget is spent maintaining baseline activity rather than responding to tasks, so concentrating hard adds only a small fraction.

A major cost is keeping neurons ready to fire: ion pumps continually restore the balance of sodium and potassium after each signal, using large amounts of ATP. Children's brains are even more demanding. By combining existing PET and MRI data, a 2014 study estimated that brain glucose use peaks at around age five, when the brain may account for about two-thirds of resting energy, and that body growth slows when brain demand is highest.

However, this was a cross-sectional estimate, not a study following the same children. Scientists cannot yet say why mental work feels tiring. A 2022 study found glutamate building up in the prefrontal cortex after a long day of difficult tasks, but this single study has not been confirmed, and rival explanations exist.