The adult human brain weighs about three pounds, about two percent of total body weight. However, the brain uses about 20 percent of resting metabolic energy, or about 20 Watts. As such, it is about nine times more hungry for energy than the rest of the body on average (other organs like the heart are also overachievers).
Interestingly, most of our muscle tissue, as well as the organs of digestion or immunity, may use a little or a lot of energy depending upon how circumstances exercise them. Our brain tissue can never 'relax' very much...there is no such thing, really, as a 'resting' state. So what happens when the brain has to exercise? And what does it mean for the brain to 'exercise?'
The brain is an adaptive system, adjusting its function based on experience. When new experiences are very similar to expectations, perceptual and behavioral functions tend to show 'consolidation', which is like a form of data compression. If everything is going according to plan, meaning if my brain understands what is going on and how I can act to get what I want, my brain is perhaps as close to being 'at rest' as it can be. When my expectations are violated, or when I have to track a lot of surprising events and make sense of them, my brain is being exercised.
Under these conditions, neuroscientists have observed that local parts of the brain use up more oxygen and glucose, and local blood flow increases in response to the need. Another observation is that local metabolic activity in the brain goes up about five percent during heavy cognitive loading. This can be sustained for several seconds or even minutes, but if it continues, cognitive failures are likely to result. Neurons will get tired from overuse.
Informational stress can cause cognitive failures, just as exhaustion can result from overworked muscles. Is there an equivalent notion of being cognitively in shape, or fit, from regular exercise?
The author's affiliation with The MITRE Corporation is provided for identification purposes only, and is not intended to convey or imply MITRE's concurrence with, or support for, the positions, opinions, or viewpoints expressed by the author.