Brain Energy Demands

Your brain is about 2% of body weight but consumes roughly 20% of your energy — and it runs near full capacity even at rest. It stores almost no fuel of its own, so it depends on a constant supply of glucose delivered by a dense network of blood vessels.

When glucose drops, the brain switches to ketones made from fat — a backup for times of food scarcity. Some researchers think ketones may even be a preferred fuel.

The brain never truly rests. Even during sleep it keeps consuming significant energy to clear waste, consolidate memories, and maintain its cells.