The strange truth about why thinking hard makes you feel exhausted

THE myth that we use only 10 per cent of our brains has been comprehensively debunked. Perhaps it persists because it is so appealing to believe that you could become a genius simply by learning to engage the dormant 90 per cent. In reality, no part of your brain is surplus to requirements, and it is always switched on, even when you are asleep or not thinking about much at all.

But that doesn’t necessarily mean that your brain burns the same amount of energy while daydreaming as it does when you are concentrating. We have all experienced that feeling of mental exhaustion after focusing on a tricky problem. Detailed thinking certainly feels like hard work, but is it? The answer is a touch more subtle than you might suspect.

The brain is certainly a hungry organ. “It is the most energy-consuming part of the body,” says Nilli Lavie at University College London. Although it accounts for around 2 per cent of our body weight, it uses some 20 per cent of the energy we burn at rest.

Most of this energy is used to maintain different levels of electrical charge across the membranes of neurons – an imbalanced state that needs to be restored after a neuron has fired off a signal. “That requires a lot of fuel,” says Ewan McNay at the University at Albany in New York.

Intriguingly, when it comes to energy use, the brain doesn’t distinguish between tasks that we traditionally regard as “hard” and those that come more naturally. This was first…