Mental fatigue - causes and effects

My goal with this newsletter is to better understand mental energy - with the goal of optimizing my career without sacrificing mental well-being.

But to create meaningful strategies, we first need to know what we’re dealing with.

That’s why this March, I’m focussing on mental fatigue - the opposite of mental energy.

What drains us, how does that impact performance, and what can we do about it.

Today’s insights is drawn from the scientific article: 

“Understanding mental fatigue and its detection: a comparative analysis of assessments and tools”

Today's newsletter

• Takeaways:

• The study in a nutshell:

• Definitions and causes:

• Effects of Mental Fatigue:

• Detection of Mental Fatigue:

• What can we learn from this study?`

• Quick Answers to Your Top of Mind Questions:

Takeaways:

🧠 You don’t always realize you’re mentally fatigued.

Self-reported fatigue assessments are unreliable. Mental fatigue can impair decision-making and focus before you even notice it, leading to mistakes in high-pressure situations.

🚀 Pushing through fatigue won’t make you more productive

When your cognitive load exceeds capacity, performance declines—not improves. Breaks, structured recovery, and smart workload management are essential for long-term success.

The study in a nutshell:

In this study, the authors examined mental fatigue and compared different methods to detect it.

Mental Fatigue is a state of cognitive exhaustion caused by prolonged mental effort, that impairs executive functions, reduces focus, decision-making ability, and reaction speed.

Traditional self-reported questionnaires are widely used as a detection method but has some limitations. The authors therefore looked into more objective alternatives, including brain wave monitoring (EEG), heart rate variability (HRV), salivary cortisol levels, and saccadic eye movements.

What I find very interesting is, that among these, tracking saccadic eye movements - rapid, involuntary shifts in gaze - showed to be the most promising method.

This was a method that I personally was not familiar with before doing this research, but one I find very interesting for young ambitious professionals like us.

Definitions and causes:

Mental fatigue is a state of cognitive exhaustion resulting from prolonged periods of demanding mental activity.

When cognitive resources are overused, the prefrontal brain area gets less effective in asserting top-down control, leading to slower reaction times, impaired decision-making, and increased likelihood of mistakes.

EEG studies show that mental fatigue increases slow brain waves, especially theta and alpha waves.
This indicates that the brain struggles to keep up, which leads to slower thinking and reduced alertness.

Mental fatigue is often triggered by excessive cognitive load, which occurs when the brain processes more information than it can handle. This can result from:

• Prolonged cognitive tasks – Activities requiring sustained attention, such as studying for long hours or making complex decisions, deplete mental energy.

• High-pressure environments – Surgeons, pilots, and air traffic controllers are particularly vulnerable due to the intense focus required for extended periods.

• Lack of recovery time – Continuous cognitive engagement without sufficient breaks prevents the brain from replenishing mental resources.

• Stress and emotional strain – While stress and mental fatigue are related, they are not the same. Stress is the body’s response to external challenges and an activation of the “fight-or-flight” system. Mental fatigue, on the other hand, builds gradually due to prolonged cognitive overload and leads to a sense of mental exhaustion rather than heightened alertness or anxiety.

Effects of Mental Fatigue:

When mentally fatigued, one can experience a range of disadvantageous effects—especially for young, ambitious professionals like us:

• Reduced attention and focus – Mental fatigue weakens the brain’s ability to filter distractions, making it harder to concentrate on important tasks.

• Impaired executive function – The ability to plan, make decisions, and suppress impulsive responses is significantly weakened.

• Memory deficits – Fatigue affects working memory, which will make it harder to retain and recall information accurately.

• Increased risk of errors – Miscalculations in financial reports, mistakes in sales calls and mess ups in business deals. All of these are likely outcomes from pushing through mental fatigue.

Detection of Mental Fatigue:

Traditional self-reported questionnaires have been widely used in research to detect mental fatigue. But they suffer from subjectivity and response bias. Today’s study evaluated several alternative methods that offer more objective ways to assess mental fatigue.

Attention Network Test (ANT)

• Is a cognitive test that evaluates attention by measuring reaction times to different visual stimuli.

• This could for example be the Stroop Test and the PVT test which you can try on our resource site.

• Mentally fatigued individuals typically show slower responses and reduced accuracy.

Electroencephalogram (EEG) Monitoring

• Is a technique that measures brain wave activity. Identifying changes in some brainwaves can be used as a reliable measure of mental fatigue.

• Mentally fatigued individuals show increase in what is called “theta wave activity” in the frontal cortex, associated with reduced alertness and cognitive slowing.

Heart Rate Variability (HRV)

• Measures variations in the time intervals between heartbeats, which reflect autonomic nervous system activity.

• Mental fatigue often leads to a decrease in HRV, indicating reduced ability of the nervous system to regulate stress and cognitive effort.

• Some studies have linked increased heart rate and lower HRV to cognitive overload, but results are inconsistent.

Salivary Cortisol Levels

• Cortisol is known as the “stress hormone”.

• Salivary cortisol levels tend to increase during cognitive exertion, but research is inconclusive on whether this reliably distinguishes mental fatigue from general stress.

Read more about cortisol and stress here.

Saccadic Eye Movements (Most Promising Method)

• This method tracks rapid, involuntary eye movements when shifting focus – for example by monitoring eye movement on a screen via webcam.

• Mental fatigue slows saccadic eye movements, increases blink frequency, and reduces pupil dilation range.

• The “antisaccade” task (where individuals must look away from a visual target instead of toward it) effectively measures inhibition control, which declines with mental fatigue.

• Because saccades are directly linked to brain activity in the frontal lobe, this method is a highly objective and non-invasive way to detect mental fatigue in real time.

What can we learn from this study?`

I believe it is super important to be aware of the effects of mental fatigue, as it serves as yet another reminder that pushing through massive workloads and feeling highly productive isn’t always the smartest choice.

I know we don’t always have the luxury of choosing when to slow down, but remember that mental fatigue can impair our ability to make sound decisions and suppress impulsive reactions!

Imagine having to finalize a crucial negotiation for your startup in this state - it’s not ideal.

Throughout our lives and careers, I believe we have these few pivotal moments where our decisions have multiplying effects…

I know I want to show up to these moments with the best possible mental clarity.

That’s why I believe so strongly in leveraging science-backed techniques and technologies to detect fatigue before we realize it ourselves.
By taking the right approach, we increase our chances of making the best decisions and ultimately achieving our goals.

Quick Answers to Your Top of Mind Questions:

🙋‍♂️ Can mental fatigue affect physical performance too?

Yes! It doesn’t just slow thinking—it affects reaction time, coordination, and even balance, increasing the risk of errors and accidents.

🙋‍♀️ Is stress the same as mental fatigue?

Not exactly. Stress triggers a fight-or-flight response, while mental fatigue builds over time due to cognitive overload, leading to exhaustion and poor decision-making.

_{Disclaimer: The above is based on the article “Understanding mental fatigue and its detection: a comparative analysis of assessments and tools” by Kaveena Kunasegaran, Ahamed Miflah Hussain Ismail, Shamala Ramasamy, Justin Vijay Gnanou, Brinnell Annette Gaszo & Po Ling Chen and aims to provide key takeaways and a condensed overview of its content. While the essence is drawn from the original article, some parts have been simplified or rephrased to enhance understanding. Please note that we at, OptiMindInsights or any other potential writers or contributors to our summaries, do not accept responsibility for any consequences arising from the use of this summary. The information provided should not be considered a substitute for personal research or professional advice. Readers are encouraged to consult the original article for detailed insights and references. The summary does not include references, but they can typically be found within the original publication. Always exercise due diligence and consider your unique circumstances before applying any information in your personal or professional life. We refer to the creative commons for reproducibility rights.}