How much energy does the brain consume?

Simon Laughlin, a professor in the Department of Zoology at the University of Cambridge, explains why the brain needs so much energy and how it uses that energy.

For the average adult at rest, the brain consumes about 20% of the body’s energy. The brain’s primary function—processing and transmitting information via electrical signals—is extremely energy-intensive. Of the two main types of brain tissue—gray matter and white matter—gray matter requires much more energy than white matter.

Some functions require more energy than others. The areas of the brain responsible for auditory processing require more energy than the olfactory system or the areas of the brain responsible for memory. Listening requires very rapid and precise signaling—it would be disadvantageous if a danger sound were delayed in any way.

Two important factors must be taken into account. The brain requires a lot of energy to function. And your brain never shuts down. Even when you’re sleeping at night, your brain consumes about as much energy as it does during the day. If a specific task requires a particular area of the brain, that area’s energy demands do indeed increase.

Unlike the muscles in your leg, where energy consumption can increase three- or fourfold when walking compared to when you’re sitting, the brain requires a relatively constant amount—whether you’re doing something complex, such as solving a difficult math problem, or watching cartoons.

The brain has no energy reserve to draw on when it needs it. Unlike muscles, which can store excess carbohydrates, the brain must be constantly supplied with oxygen and energy to function properly. If blood flow to the brain is interrupted or disrupted—as during a stroke or head injury—neurons begin to shut down rapidly.