The brain is 2% of your weight but burns 20% of your energy

By mass, the brain is almost an afterthought — about 2% of an adult’s body weight. By appetite, it is the body’s single most expensive organ, consuming roughly 20% of your resting energy, drawn mostly from glucose.

That gap is striking because, by standard biological scaling laws, an organ this size should demand far less. The brain instead runs at a furiously high, near-constant metabolic rate to keep its billions of neurons charged and ready to fire.

Most of that energy isn’t spent on conscious thought. The bulk goes to background “housekeeping” — maintaining electrical gradients and recycling neurotransmitters — so hard concentration adds only a small amount on top.

Much of the bill is paid by the electrical work of signalling: pumping ions back across membranes after each nerve impulse, and clearing the neurotransmitter glutamate from synapses. Thinking, it turns out, is metabolically pricey even when you feel idle.