When we think and use our brains, how much energy do we expend?
Your brain might feel drained after a long day at work or in the classroom. When we engage in mental athletics, does our brain burn more energy than when we watch television?
Our brain’s engine room: the nerve cells, is the key to answering this question. We make adenosine triphosphate (or ATP) from sugar and oxygen, which is the primary energy currency of our brain cells. Both sugar and oxygen can be used to track energy consumption, but oxygen is more accessible.
According to oxygen consumption data, the brain consumes about 20% of the body’s energy, despite only representing 2% of its weight [1]. A typical adult consumes around 0.3 kilowatt hours (kWh) per day, more than 100 times what a typical smartphone uses [2]. It is equivalent to 260 calories or 1,088 kilojoules (kJ) a day (an average adult consumes 8,700 kJ per day).
Why do we know this?
British neuroscientist David Attwell measured oxygen consumption in slices of rat brain in 2012 [3]. Approximately 25% of energy is used for housekeeping, such as maintaining cell walls, but 75% goes towards information processing, such as computing and transmitting neural signals.
Human brain energy consumption cannot be measured this way, but we can monitor oxygen levels, since increased brain activity requires more oxygen. Through a capnography device (where air is blown into a tube), we can measure our bodies’ oxygen consumption changes. The method requires participants to wear a mask, but it is otherwise non-invasive [4].
Research has shown a correlation between mental load (such as mental arithmetic, reasoning, or multitasking) and oxygen consumption (measured by CO2 release) [5]. Oxygen consumption may also increase when the entire body reacts to a stressful situation, rather than indicating actual changes in brain activity.
Is it possible to measure oxygen use only in the brain?
There’s no easy way to do it. As brain activity increases, oxygen-rich blood is supplied to the brain. The extra oxygen-rich blood can be channeled with micrometer precision to active neurons in specific regions.
Due to the weak attraction of blood and oxygen to magnetic fields, MRI (magnetic resonance imaging) is a radiation-free method of measuring brain activity [6].
Unfortunately, MRI can’t tell us how much energy our brain uses for different mental functions. It is impossible for MRI studies to determine absolute differences in brain activity or energy consumption, but only relative differences can be identified.
In light of the fact that the brain is always on and requires energy, this makes sense. In moments, we might casually consider idle states of mind. A vast amount of information is still processed by us.
Firstly, sensory input is ever-present. We don’t spend our days in dark floatation tanks. Second, even in a seemingly task-less state, our mental activity will bounce from reminiscing about the past to planning the future. Finally, we have our emotions, even when subtle (like feelings of serenity or uncertainty), which are products of brain activity.
How much does brain activity rise?
Consider something simple, such as paying attention. According to MRI studies, paying attention to moving objects increases activity in our visual cortex by around 1% [7].
The occipital lobe, which contains the visual cortex (which interprets what we see), only makes up 18% of our brain’s mass [8]. However, processing visual information reduces activity in auditory areas, so we spend less energy processing sounds [9].
We also lessen our visual processing activity when we attend to auditory information. An auditory stimulus offsets the cost of attention to a visual stimulus on a whole-brain level.
There is a link between mental activity and energy consumption, according to research. However, the increase is minimal, region-specific, and often offset by other energy decreases.
Why do we feel exhausted after too much mental activity?
Stress is likely to be the cause. In addition to being cognitively challenging, complex tasks are also emotionally demanding, leading to increased sympathetic nervous system activity, resulting in fatigue on the physical and mental levels [10].
We shouldn’t worry that too much mental activity will drain our brain energy. It’s still a good idea to pace yourself in order to avoid mental overload, stress and fatigue [11].
[1] https://www.pnas.org/doi/10.1073/pnas.172399499
[2] https://www.forbes.com/sites/christopherhelman/2013/09/07/how-much-energy-does-your-iphone-and-other-devices-use-and-what-to-do-about-it/
[3] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3390818/
[4] https://en.wikipedia.org/wiki/Capnography
[5] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4923594/
[6] https://en.wikipedia.org/wiki/Paramagnetism
[7] https://doi.org/10.1093/cercor/13.4.340
[8] https://faculty.washington.edu/chudler/facts.html
[9] https://direct.mit.edu/jocn/article-abstract/14/3/420/3621/Deactivation-of-Sensory-Specific-Cortex-by-Cross
[10] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3113724/
[11] https://theconversation.com/how-much-energy-do-we-expend-thinking-and-using-our-brain-197990
