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.
Advertisement
The brain is certainly a hungry organ. “It is the most energy-consuming part of the body,” says 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 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 demonstrated in the 1950s in a study showing that the , regardless of whether we are concentrating on mental arithmetic or letting our mind wander. “Daydreaming takes up neural energy too,” says Lavie.
Your brain allocates resources to its different parts depending on the mental activity being carried out. But there is a trade-off. “When the demand of a mentally challenging task increases, you see increased metabolism in the neurons that are responsible for the task,” says Lavie. At the same time, you see corresponding decreases in other brain areas. For instance, in a study published in November, Lavie and her colleagues measured energy use in the brain region responsible for daydreaming and found that it .
So thinking hard does burn more energy in the brain region involved, but this is offset by energy savings in other parts of the brain. However, the amounts of energy involved are minuscule. For example, a self-control task, such as keeping your hand in icy water for as long as you can, “burns up 1 calorie of glucose”, says McNay. However, although this is a tiny amount of fuel, your brain doesn’t see it that way. “It worries about an imbalance of supply over demand,” he says. If the brain detects local drainage of glucose – the sugar that fuels the brain – it perceives it as something bad, says McNay. This is what gives rise to the feeling of being exhausted after prolonged focus.
One , which is a breakdown product of adenosine triphosphate (ATP), the key compound involved in metabolism. “When your brain is burning through ATP, it’s producing adenosine and literally signalling to itself: ‘Hey, I’m tired now,'” says McNay. Research from 2022 also revealed that thinking hard for several hours leads to the build-up of a compound called glutamate – the main molecule used by neurons for signalling – in the front of the brain. An overload of glutamate is potentially harmful and the brain seems to protect itself by avoiding further mental workout, hence the feeling of mental fatigue.
So, after a tough exam, the sensation of having burned through a supercomputer’s worth of energy is real, even though your brain has used few or no extra calories overall. “There is a constant rate of neural energy in the brain,” says Lavie. “It is irrelevant whether is it spent on a task requiring concentration or on a distraction.”
