Your body is constantly burning calories. Calories are a unit of measurement for energy, and it takes a lot of energy to breathe, keep blood flowing, digest food, and regulate your temperature. The brain orchestrates almost all bodily functions, and it requires a lot of energy to do so. 

This vital organ uses about 20% of the energy 1the body burns at rest. So, say you burn 1,600 at rest, about 320 calories would be diverted to power the brain alone. 

Like other organs in the body, blood vessels deliver energy to the brain mainly in the form of glucose (blood sugar). When a region of the brain is activated, blood vessels dilate to increase blood flow and nutrients like glucose to meet the higher energy demands of that region.

And blood flow seems to increase more to regions of the brain involved in more complex mental tasks. Researchers of a small study published in 2021 found that completing a higher-demand cognitive test was associated with increases in blood flow in the middle cerebral artery2, whereas lower-demand tasks or resting was not.