What is Mental Energy?
Metabolic energy supply, neurotransmitters and motivation.
When the brain is active, it sends messages from one nerve cell (neuron) to another. These messages travel through a combination of electrical and chemical signals. When a message is passed from one neuron to the next, an electrical impulse triggers the release of chemicals into the gap between two neurons. This space is called the synaptic cleft.
One of the main chemicals released to help the message continue over the synaptic cleft is called glutamate.
Glutamate is an excitatory neurotransmitter, meaning it increases the likelihood that the next neuron will also fire an electrical signal, allowing the message to carry on.
During mentally demanding tasks, many of these messages are sent rapidly and repeatedly.
This leads to large amounts of glutamate being released into the synaptic cleft. When this happens continuously over time, glutamate can start to build up in the synaptic cleft.
The situation is exacerbated because astrocytes, which are specialized support cells that are responsible for taking up glutamate from the synaptic cleft and recycling it, can get overwhelmed.
During prolonged mentally demanding work, the astrocytes can’t clear the synaptic cleft fast enough to counteract a buildup of glutamate. And as a result, glutamate accumulates in the synaptic cleft. (Pellerin, L., & Magistretti, P. J. (1994), (Falkowska, A, et al 2015)
In order for astrocytes to clear glutamate from the synaptic cleft, they must take up glutamate molecules from the extracellular space (outside the cells). They do this by co-transporting glutamate together with sodium ions (Na⁺) across their cell membrane.
This process takes advantage of the fact that Na⁺ naturally flows into cells due to its electrochemical gradient - a combination of its higher concentration outside the cell and the negative charge inside the cell.
By opening specific transporters in the membrane, the cell uses this inward flow of Na⁺ to drive the uptake of substances like glutamate.
However, as astrocytes take in glutamate along with Na⁺, Na⁺ begins to accumulate inside the cell. To maintain ionic balance, the astrocyte activates the Na⁺/K⁺ pump, which uses ATP to actively transport Na⁺ out of the cell in exchange for K⁺.
This pumping process is energy demanding. To meet the increased energy need, astrocytes break down their internal glycogen stores into glucose.
The glucose is then metabolized through glycolysis, generating both ATP and lactate. The ATP powers the Na⁺/K⁺ pump, while the lactate is exported to nearby neurons, where it can be used as a valuable energy source.
(Falkowska, A, et al 2015)
The usage of glucose or lactate to produce energy, essentially produces ATP, which is the energetic coin of the body. ATP, which is short for adenosine triphosphate, is made up of three phosphate-bound adenosine units.
When one of these bonds is broken, energy is released, and it’s this energy that powers many of the cell’s functions.
Everything the body does, requires energy. And when the brain is active and sending messages from neuron to neuron, this too consumes energy.
Both the astrocytes and the neurons themselves use ATP by breaking it down, releasing adenosine in the process.
When we engage in sustained mental work, this breakdown happens faster than the body can recreate ATP.
As a result, adenosine begins to build up in the brain.
The accumulated adenosine binds to something called A1 receptors on neurons. When that happens, it becomes harder for messages to pass between nerve cells, because the adenosine blocks other signal-carrying molecules from binding and continuing the message across the synaptic cleft.
As adenosine builds up, it becomes harder to focus and process information.
Over the course of the day, a broader biological process known as sleep pressure develops. This is essentially the body’s natural drive to sleep.
Adenosine accumulation is one important contributor to this process. A long day of mentally demanding work, especially into the evening hours, can intensify this buildup and make the need for sleep even stronger. (Reichert, C. F., et al. 2022), (Kok, A 2022).
These processes act as a natural brake, slowing the brain's processes down and protecting against overactivation.
The Anterior Cingulate Cortex (ACC) really is a key region involved in mechanisms related to mental energy.
It serves as a central hub for monitoring cognitive effort and task difficulty, regulating performance and attention, and tracking internal energy availability.
The ACC evaluates how energy-demanding a task is, by receiving input from astrocytes and monitoring levels of neuromodulators like adenosine, glutamate and dopamine.
In general, if the ACC detects low energy availability – for example via high levels of adenosine - it may prompt restorative behaviors like taking a break or initiating sleep.
It constantly balances the energy required for a task against current capacity and the perceived outcome.
As adenosine builds up and binds to A1 receptors, it becomes harder for signals to pass across the synaptic cleft. This increases the energy cost of communication between neurons, making tasks feel more cognitively demanding.
Maybe because mentally draining tasks such as focusing and concentrating mainly involves an activation of the frontal part of the brain, when the energy cost outweighs the expected reward, the ACC may reduce top-down control from the frontal regions of the brain.
This reduction in executive activation can lead to lower motivation and reduced focus.
Therefore, activity in the Default Mode Network (DMN) may increase, leading to mind-wandering, which will make it harder to stay concentrated. As a result, you’re more likely to shift to easier tasks, as for example checking your phone or taking a break.
Another key player in this system is dopamine.
The ACC receives dopaminergic input and uses it to help assess the cost-benefit of mental effort.
During demanding tasks, dopamine supports the stability of working memory, especially in the prefrontal cortex, by enhancing the likelihood of these messages continuing over the synaptic cleft from one nerve cell to another.
But when energy is low or the predicted reward is not sufficient, dopamine release decreases. This weakens the signaling strength and makes it harder to stay focused.
The involvement of dopamine therefore links motivation to the equation of mental energy.
One must assume, that if one is strongly intrinsically motivated by a given task, more dopamine would be available, strengthening the brains ability to send these messages.
Frontal Midline Theta (FMθ) activity, which is a brain rhythm associated with cognitive control, typically increases during focused work as it amongst else, plays a role in suppressing DMN activity.
But as the task continues and the brain has less and less energy, FMθ plateaus or drops, allowing DMN activity to rise and focus to decline. (Kok, A 2022), (Westbrook, A.. & Beaver, T. S, 2016), (Wascher, et al, 2013), (Scheeringa, R. 2008)
Mental energy is shaped by both biology and motivation.
So, from this, we can see that when you are feeling mentally tired, in most cases, it’s actually your brain hitting the brakes because energy levels are running low.
Your brain shifts into Default Mode Network activity, which is linked to self-focused, repetitive thoughts. When executive control is low, due to mental fatigue, these thoughts can become intrusive or harder to regulate.
When you focus for long periods, your brain uses real fuel. Signals between brain cells slow down, and a chemical called adenosine builds up, making it harder to think clearly.
Sleep pressure naturally builds throughout the day, and demanding mental work, makes this biological drive for rest more powerful.
At the same time, your brain is always ”weighing” the reward vs the price: Is this worth the effort? If energy is low or the reward isn’t considered big enough, it’ll start pulling back. Focus fades, and distractions get more tempting.
I believe we should use that insight.
Although motivation plays a part here, its rarely meaningful to power through extreme periods of mentally demanding work. Because by then its not as much a question of your willpower as you properly think. It’s a question of having enough energy like when fatigue it building up by the end of a marathon.
We should start managing our mental energy, just as one would manage physical energy. After a tough run, one would feel tired and relax on the couch for an extended period of time before doing squats in the gym.
This does not mean pulling up the phone.
Use small breaks, naps and meditations strategically to optimize your mental energy.
References:
Pellerin, L., & Magistretti, P. J. (1994). Glutamate uptake into astrocytes stimulates aerobic glycolysis: A mechanism coupling neuronal activity to glucose utilization. Proceedings of the National Academy of Sciences, 91(22), 10625–10629. https://doi.org/10.1073/pnas.91.22.10625
Falkowska, A., Gutowska, I., Goschorska, M., Nowacki, P., Chlubek, D., & Baranowska-Bosiacka, I. (2015). Energy Metabolism of the Brain, Including the Cooperation between Astrocytes and Neurons, Especially in the Context of Glycogen Metabolism. International Journal of Molecular Sciences, 16(11), 25959–25981. https://doi.org/10.3390/ijms161125939
Reichert, C. F., et al. (2022). Sleep pressure and mental fatigue: A neurocognitive perspective. Journal of Sleep Research, 31(3), e13597. https://doi.org/10.1111/jsr.13597
Kok, A. (2022). Cognitive control, motivation and fatigue: A cognitive neuroscience perspective. Brain and Cognition, 160, 105880. https://doi.org/10.1016/j.bandc.2022.105880
Westbrook, A., & Braver, T. S. (2016). Dopamine does double duty in motivating cognitive effort. Neuron, 89(4), 695–710. https://doi.org/10.1016/j.neuron.2015.12.029
Wascher, E., Getzmann, S., & Falkenstein, M. (2013). Frontal theta activity reflects distinct aspects of mental fatigue. Biological Psychology, 96(1), 57–65. https://doi.org/10.1016/j.biopsycho.2013.01.001
Scheeringa, R., Petersson, K. M., Oostenveld, R., Norris, D. G., Hagoort, P., & Bastiaansen, M. C. M. (2008). Frontal theta EEG activity correlates negatively with the default mode network in resting state. International Journal of Psychophysiology, 67(3), 242–251. https://doi.org/10.1016/j.ijpsycho.2007.05.017
Disclaimer:
This summary is based on the scientific references listed directly in the text and is intended to provide a simplified overview of complex brain processes. While care has been taken to reflect the core ideas from the original research, some explanations have been adapted or rephrased to improve clarity and accessibility.
OptiMindInsights and any contributors cannot take responsibility for how this information is interpreted or applied. The content is not medical advice and should not replace professional consultation. If you're curious or want to dive deeper, we encourage you to explore the original sources.