The brain spends a fifth of the budget

The human brain is about two percent of body mass and, at rest, uses about twenty percent of the body's oxygen and metabolic energy.

Measured by oxygen consumption or by glucose, a resting adult brain draws about a fifth of the body's energy. The fraction is higher in children, whose brains are a larger share of a smaller body and whose cost can approach half. Most of the energy goes to signalling — pumping ions back across membranes after spikes and synapses — not to conscious effort.

Hard thinking barely moves the total. The brain is expensive at idle. What changes during exercise is the denominator: muscle adds a large new demand, and the brain's share of the whole-body budget falls even though the brain's own consumption stays about the same.