Mental fatigue after a though workday

You’re sitting there - 8 hours into your workday.

Your friends just texted, asking when you’re joining tonight, when suddenly, your boss asks you to prepare a short presentation for tomorrow.

It’s just five minutes, sure. But as an ambitious young professional, you know that even a brief window where your bosses are listening to you, can be more valuable than it seems.

So, you want to be sharp, engaged and ready to produce the best 5 min presentation in the history og your company.

But you’re tired. You haven’t been productive for the past hour, and your brain feels like it’s fighting an uphill battle.

What’s actually happening in your brain and body after a full day of mental effort?

Today, we will look at how your nervous system shifts, how your brainwaves change, and the physiological reason for why “pushing through” just might not be a strong enough tactic.

Today’s insights is mainly drawn from the scientific article: 

“Mental fatigue caused by prolonged cognitive load associated with sympathetic hyperactivity”

And incorporates insights from:

“The impact of mental fatigue on brain activity: a comparative study both in resting state and task state using EEG”

Today's newsletter

• Takeaways:

• The study in a nutshell:

• What they did:

• What the authors found out:

  • Autonomic Function Changes:

  • EGG:

• What can we learn from this study?

• Quick Answers to Your Top of Mind Questions

Takeaways:

🔬 A standard workday can result in measurable changes in your autonomic nervous system

Our main study today showed that just 8 hours of cognitively demanding work was enough to produce significant shifts in participants’ autonomic nervous system state.

🧠 Meanwhile, brainwave activity can change after just 100 minutes of mentally demanding work

The other study featured in today’s newsletter showed that a 100-minute session of mentally demanding work led to measurable changes in the resting brainwave characteristics of the participants.

The study in a nutshell:

Today’s main study examined how prolonged cognitive workload affects the autonomic nervous system in healthy individuals.

The authors used an 8-hour period - resembling a standard workday - for participants to complete mentally demanding tasks while researchers tracked changes in autonomic nervous system activation, and compared these results to the same participants reactions to an 8-hour relaxation session.

The findings showed a significant increase in sympathetic (stress-related) activity and a reduction in parasympathetic (relaxation-related) activity.
-If you need a quick overview of the nervous system, I highly recommend this video.

In the other study, 20 young males had their EEG signals measured before, during, and after a 100-minute mentally fatiguing math session – both while resting and while actively engage in the mental work.

It was shown that mental fatigue led to changes in brainwaves both while doing the fatiguing task and at resting state.

What they did:

In today’s main study each participant completed two separate sessions divided by 4 weeks:

1. Fatigue session – Eight hours of mentally demanding tasks.
2. Relaxation session – Eight hours of passive activities such as reading, watching movies, or casual conversation.

The activation of the autonomic nervous system was measured using accelerated plethysmography (APG), which provided insights into heart rate variability (HRV) and autonomic balance. Key measures included:

•  High-frequency (HF) power – An indicator of parasympathetic activity.

• Low-frequency (LF) power – Reflecting both sympathetic and parasympathetic activity.

• LF/HF ratio – Used to assess the dominance of sympathetic activity.

Additionally, subjective fatigue levels were measured using a Visual Analogue Scale (VAS).

In the other study participants completed one session of 100 minutes mental work, divided into four blocks.

Between the blocks, EEG was measures both at resting state, and while actively engaged in the mental work.

What the authors found out:

Autonomic Function Changes

After the relaxation session, not surprisingly the autonomic function did not change significantly. But after the fatigue session, some interesting changes were observed:

• Parasympathetic activity (HF power) significantly decreased, which indicates a reduced ability to relax and recover.

• Sympathetic activity (LF/HF ratio) significantly increased, which indicates a heightened stress response.

• Total autonomic power declined, reflecting an overall reduction in autonomic function efficiency.

Subjective Fatigue and Correlation with Autonomic Changes

• Participants reported a significant increase in subjective fatigue after the fatigue session.

• Statistical analysis showed a strong correlation between increased sympathetic activity (LF/HF ratio) and higher self-reported fatigue levels.

EGG:

In the EGG study, the authors found evidence that:

• Delta rhythm decrease across all brain regions, indicating lower deep-processing activity.

• Theta rhythm increased in the central region, which is a marker often associated with fatigue.

• The authors split the Alpha-wave into Alpha1 and alpha2 rhythms, which exhibited significant changes across all brain regions.

• Beta rhythm showed a notable decrease in the temporal region, further suggesting reduced cognitive engagement.

Importantly, resting-state EEG showed more pronounced fatigue-related changes than task-state EEG, which could indicate that EGG-fatigue assessment may be more effective when measured at rest.

What can we learn from this study?

Viewed together, these studies clearly show that mental fatigue induces physiological changes in both the brain and body.

The autonomic nervous system doesn't work as well, and the brain's activity changes.

I find these insights truly important because, at least for me, they help illuminate the concept of mental fatigue in a concrete way. Pushing through massive amounts of work doesn’t necessarily lead to the advantageous outcomes.

Hard mental work strains the body, just like intense physical effort. Most people understand that after running a tough 10K, they’ll likely feel fatigued - and if they attempt heavy squats right after, they may not get the same benefits as they would if they waited until the next day.

The same principle applies to cognitive effort.

What’s interesting is how we navigate this.

I don’t believe in excessive relaxation, either. If we want to achieve our goals, consistent effort is necessary.

But there’s space to find a meaningful balance. Perhaps by detecting mental fatigue and implementing strategic, intermittent recovery periods.

This is something we’ll explore further in the coming posts.

Quick Answers to Your Top of Mind Questions

🙋‍♀️   How does an 8-hour demanding workday affect your nervous system?

A full day of mentally demanding work increases sympathetic (stress-related) activity and reduces parasympathetic (relaxation-related) activity, making it harder to recover and relax.

🙋‍♂️ What brainwave changes occur after mental fatigue?

Mental fatigue reduces deep-processing delta waves, increases theta waves (linked to fatigue), and alters alpha and beta activity, especially at resting state.

_{Disclaimer: The above is based on the article “Efficacy of the Mental Health App Intellect to Reduce Stress: Randomized Controlled Trial With a 1-Month Follow-up” by Sean Han Yang Toh, BA; Jessalin Hui Yan Tan, BA: Feodora Roxanne Kosasih & BA; Oliver Sündermann, PhD 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.}