… the brain requires a disproportionately high amount of the body’s energy, accounting for about 20 percent of the oxygen consumed by the body. “Your muscles normally aren’t sucking a lot of oxygen out of you,” says Feinsilver. “With exercise, they will. But the brain always takes a lot of your energy.” In other words, if you’re conscious, your brain demands your energy, and lots of it. Using your brain takes real, honest, physical work — it’s just not visible to us the way using our muscles to exercise is.