If you feel beat after a day in the office, then you're not alone. Concentrating for long periods of time on mental tasks, like critical thinking and decision-making, can cause cognitive fatigue, which reduces mental alertness. By the time you stop working, you might feel "brain fogged" or "fuzzy."

And all that thinking can make you feel physically tired, too, even if you've been sitting at a desk all day. Recent neuroscience research has revealed fascinating insights into how prolonged cognitive efforts – from writing code to counting change – reshape not only the brain, but also the body's ability to manage its energy.

The mind-body connection

Feeling bone-tired after an afternoon combing through spreadsheets isn't just in your imagination. Studies involving brain scans show that prolonged periods of performing demanding mental tasks can change the function and structure of the brain in several ways:

  • A key neurotransmitter called glutamate builds up in the prefrontal cortex, causing an imbalance in brain function.1
  • Activity decreases in the right middle frontal gyrus, right insula, and right anterior cingulate gyrus – areas of the brain that manage various aspects of executive function, like decision-making and memory, as well as emotional processing.2
  • Functional connectivity within the brain decreases – meaning, the different areas of the brain don't communicate as well with one another – when transitioning to a more difficult mental task.3,4
  • The caudate and putamen brain structures tend to appear smaller in individuals who are experiencing clinical mental exhaustion when compared to the brain scans of those who feel more mentally energized.5 In addition, the brains of people experiencing burnout show more electrical activity, indicating their brains are working harder.4

Periods of deep mental work can affect the body in other ways too. For example, challenging cognitive tasks can trigger the body’s stress response, which suggests a link between mental work and increased cortisol levels.6 Other physical responses to thinking include faster breathing and dilated pupils.7-9

It's clear that mental work triggers a variety of physical responses throughout your body. Yet unraveling how these responses add up to a feeling of exhaustion is a deep dive into the intricate relationship between mind and body.

How mental fatigue affects your ability to make decisions 

Consider this scenario: You've just spent an 8-hour workday drafting emails, speaking with clients on the phone, and managing invoices. In the corner of your office lies your gym bag, a reminder of your new goal to hit the elliptical every day after work.

But even though you hardly left your desk chair today, the thought of working out sounds exhausting. So, with a promise to exercise tomorrow, you head straight home.

Scientists assure us this isn't laziness. Decreasing your intentions for, and overall output of, physical exercise is a common symptom of mental fatigue. One study found that individuals who otherwise have a strong intention to exercise change their minds after 30 minutes of a challenging mental task.10

More recent research suggests that metabolic changes in the brain during periods of intense mental work might be at play. A task that requires concentration can lead to excess glutamate in the prefrontal cortex, which can cause potentially toxic effects. To regulate the excess, the brain must divert energy from other executive functions, like decision-making or impulse control.1

Other research shows that during cognitive tasks, the lateral prefrontal cortex slows down. In an effort to regulate the changes and find balance, the brain has to engage in a cost-benefit analysis: Is this activity really worth the energy?2 

In most cases, an overworked brain is more likely to see the costs of exercising (time, energy, physical demand) rather than its benefits (long-term health).11 This neurological process results in prioritizing short-term rewards (going home and ordering a pizza) over activities with long-term gains (a mile on the elliptical).

How mental fatigue affects physical endurance

Let's consider what might happen if you do make it to the gym after a mentally challenging day at work. It's likely the workout will feel more physically demanding than usual. You might feel exhausted more quickly and be tempted to call it quits halfway through.10-12

Research describes this as “perceived exertion.” 

Mentally fatigued people don’t show physical signs of intense exertion during exercise, such as low oxygen or elevated blood lactate levels.12 In fact, one study found that the heart rates of mentally fatigued participants are lower during exercise, indicating they might be unconsciously downgrading a vigorous workout to a moderate one.10 

This is the brain's energy resource allocation at work. A workout that doesn't feel rewarding is more likely to feel tiresome, and you're likely to pull back your effort.

Thinking and stress

A perceived threat or stressor – like a research paper due by midnight – can prompt the brain to release cortisol, a stress hormone that affects nearly every system in the body. Muscles tense, breathing speeds up, blood vessels dilate, and the bowels might spasm.13 Even short periods of highly demanding mental work might impair your well-being the next day.14 

Most healthy people can bounce back from periods of acute stress. Planning a wedding or meeting a tight deadline at work might feel overwhelming in the moment, but typically leaves no long-term psychological residue. 

But chronic stress, which comes from the accumulation of periods of acute stress, is another story. Chronic stress can arise even when there are no real threats, only perceived ones.6 If your fight-or-flight response is provoked daily by stacks of paperwork or conflict management, your energy is at risk of further depletion. That's because stress can wreak havoc on sleep routines, exercise habits, and eating patterns – lifestyle choices that play a big role in energy management.

Plus, chronic stress increases your risk of many ailments, including headaches, digestive problems, depression, and heart disease.15 Many of these physical problems feature fatigue-like symptoms.

Managing the cycle

It's clear that cognitively taxing activities can zap your energy. So, what actionable steps can you take to break this cycle and effectively address mental fatigue?

Several things, according to experts.

  • Avoid mentally draining tasks before a high-endurance physical activity.10-12 If you want to conserve your energy to help a friend move in the afternoon, then don't spend the morning on your laptop creating a family budget.
  • Create rewards to mitigate cognitive fatigue during work. Studies show that physical markers of fatigue in the brain can be reversed when participants are offered money to keep working.2,16 Finding your own brain-boosting incentives, like a piece of dark chocolate or a funny video on the internet, can help break up long periods of work.
  • Take breaks. Avoid an accumulation of mental strain by prioritizing periods of rest.1 Mind-body activities, such as meditation or progressive muscle relaxation, can help counteract mental fatigue. If you're nodding off at your desk, try going for a brisk walk to refresh your energy level.17
  • Don't buy a car on your lunch break. In other words, don't make important decisions when you're mentally fatigued. A tired brain can warp your sense of priority and make cost-benefit analysis swing too heavily in the direction of low-effort, high-reward choices.
  • Embrace self-care. The three big domains of physical self-care – eating, moving, and sleeping – can't be underestimated as predictors of wellbeing. Eat healthy foods, get 150 minutes of moderate exercise each week, and sleep 7-9 hours each night. These lifestyle habits have a profound influence over your energy level and your brain's ability to respond to mental fatigue.

The bi-directional relationship between cognitive fatigue and physical well-being will continue to become clearer from future research. As of now, we know that feeling unmotivated and tired after a mental task is the result of neurological processes rather than a sign of laziness. It's your brain doing you a favor by prioritizing its own integrity. 

You can help your brain by taking a holistic approach to the workday by focusing not only on productivity but on your well-being, too.

A word from Thorne

You can also support your efforts with nutritional supplements that support cellular energy production and brain function, such as Brain Factors or Daily Greens Plus.*

References

  1. Wiehler A, Branzoli F, Adanyeguh I, et al. A neuro-metabolic account of why daylong cognitive work alters the control of economic decisions. Curr Biol 2022;32(16):3564-3575.e5 
  2. Darnai G, Matuz A, Alhour HA, et al. The neural correlates of mental fatigue and reward processing: A task-based fMRI study. Neuroimage 2023;265:119812. 
  3. Kaposzta Z, Stylianou O, Mukli P, et al. Decreased connection density and modularity of functional brain networks during n-back working memory paradigm. Brain Behav 2021;11(1):e01932. 
  4. Pihlaja M, Peräkylä J, Erkkilä EH, et al. Altered neural processes underlying executive function in occupational burnout – Basis for a novel EEG biomarker. Front Hum Neurosci 2023;17:1194714. 
  5. Gavelin HM, Neely AS, Dunås T, et al. Mental fatigue in stress-related exhaustion disorder: Structural brain correlates, clinical characteristics and relations with cognitive functioning. Neuroimage Clin 2020;27:102337. 
  6. Joshi S, Li Y, Kalwani RM, et al. Relationships between pupil diameter and neuronal activity in the locus coeruleus, colliculi, and cingulate cortex. Neuron 2016;89(1):221-234. 
  7. Jung FU, Pabst A, Rodriguez FS, et al. Perceived stress of mental demands at work, objective stress and resilience – an analysis of the LIFE-Adult-study. J Occup Med Toxicol 2023;18(1):20. 
  8. Stress effects on the body. American Psychological Association. https://www.apa.org/topics/stress/body. [Accessed Jan 17, 2024.] 
  9. van der Wel P, van Steenbergen H. Pupil dilation as an index of effort in cognitive control tasks: A review. Psychon Bull Rev 2018;25(6):2005-2015. 
  10. Brown DMY, Bray SR. Effects of mental fatigue on exercise intentions and behavior. Ann Behav Med 2019;53(5):405-414. 
  11. Harris S, Bray SR. Effects of mental fatigue on exercise decision-making. Psych Sport Exerc 2019;44:1-8.
  12. Van Cutsem J, Marcora S, De Pauw K, et al. The effects of mental fatigue on physical performance: A systematic review. Sports Med 2017;47:1569-1588. 
  13. Joshi S, Li Y, Kalwani RM, et al. Relationships between pupil diameter and neuronal activity in the locus coeruleus, colliculi, and cingulate cortex. Neuron 2016;89(1):221-234. 
  14. Chronic stress puts your health at risk. Mayo Clinic. https://www.mayoclinic.org/healthy-lifestyle/stress-management/in-depth/stress/art-20046037. [Accessed Jan 17, 2024.] 
  15. Blasche G, Khanaqa TAK, Wagner-Menghin M. Mentally demanding work and strain: Effects of study duration on fatigue, vigor, and distress in undergraduate medical students. Healthcare (Basel) 2023;11(12):1674. 
  16. Matuz A, Darnai G, Zsidó AN, et al. Structural neural correlates of mental fatigue and reward-induced improvement in performance. Biol Futur Published online October 27, 2023.
  17. Hoover CS, Ragsdale JM, Ayres TB. An experimental test of resource recovery from physical and relaxation work break activities. Stress Health 2022;38(3):477-489.