The myth that we only use 10% of our brains has been comprehensively debunked. Perhaps it persists because it’s so tempting to believe that you can become a genius just by learning to be 90 percent passive. In fact, no part of your brain is redundant, and it’s always on, even when you’re sleeping or not thinking much.

But that doesn’t necessarily mean your brain burns as much energy while daydreaming as it does when you’re concentrating. We’ve all experienced that mental exhaustion after focusing on a difficult problem. Detailed thinking sure feels like hard work, but is it? The answer is more subtle than you might suspect.

The brain is indeed a hungry organ. “It’s the most energy-consuming part of the body,” he says. Blue lava at University College London. Although it makes up about 2 percent of our body weight, it uses up about 20 percent of the energy we burn at rest.

Much of this energy is used to maintain different levels of electrical charge across the neuron’s membranes—an imbalance that needs to be restored after the neuron stops signaling. “It requires a lot of fuel,” says. I don’t know McNay at the University at Albany in New York.

Surprisingly, when it comes to energy use, the brain does not distinguish between tasks that we traditionally consider “difficult” and those that come more naturally. It was the first…