Brain Drain: Scientists Look at Why Mental Exertion Triggers Exhaustion

It’s a familiar feeling on a Friday evening. After finishing a gruelling day’s work, you finally agree with friends on where to meet for a night out. But by the time you have figured out what to wear and where you left your keys, a night on the sofa begins to sound more appealing than one on the tiles.

Now, scientists think they may be able to explain why you feel so weary before you have even reached the bus stop: your brain has slowed down to manage the strain. The brain could suffer from something similar to the painful buildup of lactate in muscles during physical exercise. This could be why hard mental yards – and resisting the temptation to give up throughout the day – feel equally taxing.

Prolonged mental activity leads to the accumulation of a potentially toxic neurotransmitter in the prefrontal cortex, according to a study published in Current Biology. The researchers suggest the brain slows down its activity to manage the buildup, offering an explanation to why we feel tired.

“Even when you resist scratching an itch, for example, your brain is exerting cognitive control,” said Antonius Wiehler of the Paris Brain Institute, the first author of the study. Repeated demands on cognitive control functions can lead to fatigue, he said. The prefrontal cortex is the region of decision-making and cognitive control, which is applied when the brain overrides an impulse or fights any kind of temptation.

The team monitored the brain chemistry of 40 participants while they completed repetitive tasks on a computer. They formed two groups, who performed either hard tasks or easy tasks for over six hours. The researchers measured levels of a neurotransmitter in the prefrontal cortex. They found a greater accumulation of glutamate in participants who were given the harder tasks.

Work that involves a lot of thinking requires the brain to repeatedly resist the temptation to do something less demanding. Unsurprisingly, this can leave people feeling tired, but the brain chemistry behind it has remained unclear. Now, researchers suggest cognitive control may lead to the accumulation of glutamate in the brain – of which high levels can be harmful because it overexcites neural cells.

“We found that glutamate was accumulating in the region of the brain which controls the tasks we set participants,” said Wiehler. “Our understanding is that the brain has some kind of clearance mechanism to counteract this, which may slow down activity.” The researchers posit that mental fatigue could be linked to recycling the glutamate that builds up during neural activity. “The accumulated glutamate needs to be cleared away, which we think is likely happening during sleep,” said Wiehler.

When participants were asked to report their level of fatigue, no definitive link between glutamate and fatigue was found – the groups performing hard and easy tasks recorded the same tiredness. Researchers said this could be due to fatigue being subjective, and those doing the easy task were unaware of the difficulty of the other.

“The fact that glutamate levels don’t track the reported fatigue is slightly disappointing, but not surprising because there is often a dissociation between biological features and self-reported fatigue,” said Dr Anna Kuppuswamy from the Institute of Neurology at University College London, who was not involved in the study.

The researchers monitored only glutamate but suggest other related substances could be linked to fatigue. “The study measures a single neurotransmitter in a very specific part of the brain, so we have to look at it more globally,” said Kuppuswamy.

But the results were encouraging, she added. “We know that during physical exercise lactate accumulates in the muscles, leading to muscle fatigue. It is kind of intuitive that something similar happens in the brain and this is good first evidence to suggest that.”

By: Sascha Pare

Source: Brain drain: scientists look at why mental exertion triggers exhaustion | Neuroscience | The Guardian

Critics:

Mental exhaustion is a feeling of extreme tiredness, characterized by other feelings including apathy, cynicism, and irritability. You may be mentally exhausted if you’ve recently undergone long-term stress, find it hard to focus on tasks or lack interest in activities you usually enjoy.

Mental exhaustion often happens as a result of overuse, like physical overuse injuries. Even though you can’t point to it, it has more in common with repetitive stress injuries, like carpal tunnel or tennis elbow. Rather than overstressing a muscle group, mental and emotional exhaustion come from overstressing your mind.

Mental exhaustion is completely possible and is probably more common than it should be. After a long period of stress or time of intense emotions, mental exhaustion is bound to happen. Just like our bodies show symptoms after we push too hard, our minds are bound to display signs of mental exhaustion if we don’t take proper care.

Mental exhaustion can be caused by many things. Typically, though, people feel mentally tired after experiencing long-term stress. This is especially true if the stressors increase a person’s cognitive load or reduce their resources.

For example, you may be responsible for completing a challenging project with many moving parts and tradeoffs. This would require a high level of project management skills and political savvy (e.g., increased load).

Another example would be traveling for work. Constantly changing time zones would leave you feeling jet-lagged (reduced resources). Many stressors involve both reduced resources and increased load.

Work travel to an unfamiliar country where you don’t speak the language amps up the cognitive load. Taking care of a sick family member may involve coordinating medical care and interpreting unfamiliar terms while managing emotions (increased load). But you may also be getting less sleep (reduced resources). Over time, increased responsibility and stress plus poor self-care can result in mental exhaustion.

Though a wide variety of stressors can cause you to feel mentally drained, we’ve boiled down the 7 most common causes of mental exhaustion below.

1. Chronic stress

This is the most frequent cause of mental exhaustion. Chronic stress keeps your brain — and body — on high alert all the time. Over time, this begins to wear away at your well-being. Chronic stress can also lead to empathy or compassion fatigue. It can become difficult to muster an emotional response to the constant strain.

2. Uncertainty

The human stress response was designed to work efficiently in the face of short-term stress (think fight-or-flight). However, it’s a much less effective response to a constant, nagging feeling of uncertainty. Unfortunately, uncertainty has become far too normal of a feeling since the start of COVID-19 pandemic. This has made mental exhaustion more common than ever.

3. Work stress

Stress at work can take many forms. It can arise from a values mismatch, difficulty managing tasks and priorities, or a high-demand, risk-oriented job. Some jobs (or programs of study) involve a lot of new learning. They could also require processing and making sense of a lot of information. Whatever the reason, it’s not always possible to leave work at work. Left unchecked, workplace stress can even evolve into burnout. Your work stress could bleed into your weekends and ultimately, develop into a bad case of the Sunday scaries.

4. Family issues

Few things are more stressful than worrying about a family member. Being a caregiver for young children, sick relatives, or aging parents can be mentally taxing. Even if everyone’s healthy, families can bring all kinds of stressors. Divorce, disagreements, and estrangements have a way of following you into all areas of your life. Ultimately, family troubles can be a big cause of mental exhaustion.

5. Juggling multiple commitments

In addition to caring for family, many people have other commitments on their plate — those commitments come with details, schedules, logistics, and challenges. Balancing an intensive school or training program, a second job, or a freelance business can leave you feeling like you’re never “off.” If you’re not able to, or don’t know how to, manage your priorities, you’re at risk of becoming mentally drained.

6. Emotional stress

There are dozens of things that can cause emotional stress. No matter the cause, the experience is similar. Constant negative feelings, events, and circumstances can make it difficult to relax. This emotional exhaustion can quickly lead to mental fatigue.

7. Poor self-care

Without gas in the tank, you won’t get very far. When you get busy or you’re feeling a bit down, it’s easy to neglect self-care. However, over time this will affect your ability to be resilient in the face of stressful situations. If you have a chronic illness, like multiple sclerosis or chronic fatigue syndrome, brain fog may be a side effect. Rather than pushing through, it’s important to be especially diligent about self-care.

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