How much energy do we expend thinking and using our brain?

After a long day of work or study, your brain may feel like it’s run out of energy. But do our brains burn more energy in mental athletics than in other activities, such as watching TV?

To answer this question we have to look at the engine room of our brain: the nerve cells. The main energy currency of our brain cells is a molecule called adenosine triphosphate (or ATP), which our body makes from sugar and oxygen.

Brain energy use can be tracked using both sugar and oxygen, but oxygen is the more accessible option.

Tracking oxygen consumption, the brain accounts for about 20% of the body’s energy consumption, despite representing only 2% of its weight.

That’s about 0.3 kilowatt-hours (kWh) per day for an average adult, more than 100 times the typical smartphone required daily. And it equates to 260 calories or 1,088 kilojoules (kJ) per day (the total energy intake of an average adult is about 8,700 kJ per day).

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How do we know?

In 2012, British neuroscientist David Attwell and colleagues measured oxygen consumption rat brain slices.

They found that while 25% of energy needs are used for household activities, such as cell wall maintenance, the bulk of 75% is used for information processing, such as calculating and transmitting neural signals.

We can’t measure brain energy use in humans this way, but we can track oxygen, because increased brain activity requires more oxygen.

One way to measure the changes in our body’s oxygen consumption is to measure the CO₂ level through a capnography device (where air passes into a tube). This requires participants to wear a mask, but is different non-invasive.

Indeed, research shows that increased mental workload (such as mental arithmetic, reasoning or multitasking) is associated with increased oxygen consumption (as measured by release CO₂).

However, the increased oxygen consumption may also be due to the whole body reacting to an emotional, stressful situation and not reflecting the actual changes in brain activity.

Can we measure oxygen consumption only in the brain?

It’s complicated. Increased brain activity causes an increased supply of oxygenated blood. That extra supply of oxygenated blood is region-specific and can be channeled (literally) to active neurons with micrometer precision.

Because blood and its oxygen are weakly attracted to it magnetic fieldswe can use MRI (magnetic resonance imaging), a radiation-free tool, to obtain an, albeit indirect, measurement of brain activity.

But unfortunately we can’t use MRI to tell us how much energy our brain uses for different mental activities. MRI studies can only identify relative differences in brain activity and energy expenditure rather than absolute values.

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However, this makes sense since our brains are always on and thus always need energy. Even at times when we casually think of a state of idle mind, we are still processing massive amounts of information.

First, there’s the ever-present sensory input: we don’t usually spend our day in one dark floatation tank.

Second, even in a seemingly taskless state, our mental activity will bounce from reminiscing about past events and planning our future.

Finally, there are our emotions, which, even if subtle (such as feelings of serenity or uncertainty), are the product of brain activity and therefore come with a constant energy cost.

So, how much does brain activity increase?

Let’s take something simple, like paying attention. MRI studies have shown that following moving objects intently compared to passively viewing them increases brain activity in our visual cortex about 1%.

This doesn’t seem like much, especially considering that the occipital lobe, which houses the visual cortex (which gives meaning to what we see), has only the about 18% of our brain mass.

But interestingly enough, processing visual information leads to a reduction in activity in auditory regions, meaning we spend less energy processing the sounds in our environment. This also works the other way around: when we pay attention to auditory information, we reduce our visual processing activity.

At the whole-brain level, the cost of paying attention to a visual stimulus is likely already offset by savings in auditory processing.

In short, research shows that mental activity is indeed linked to increased energy expenditure. Yet the increase is minimal, region-specific and often offset by energy declines in other areas.

Then why do we feel exhausted after too much mental activity?

It is probably a result of mental stress. Complex mental tasks are also typically emotionally challenging and lead to increased activation of our sympathetic nervous system, ultimately leading to mental and physical fatigue.

The good news is that we don’t have to worry that too much mental activity will deplete our brain energy. But it’s still a good idea to keep up the pace to avoid mental overload, stress, and fatigue.

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