A long day at the office can leave you drained of energy and overwhelmed with the desire for the TV and a take-out meal. But you sat all day. So why do you feel as tired as your friends who have physical jobs?
Struggling through your essential to-do list gets more and more exhausting as time goes on at home. Worse still is running into a colleague on the way out who “just wants a minute”.
It might seem obvious that you’re more likely to make impulsive decisions at the end of a long day, but people often succeed anyway.
A recent study that scanned people’s brains at different times during their working day found that high-demand tasks that require intense and constant focus can lead to the buildup of a potentially toxic chemical called glutamate.
Normally used to send signals from nerve cells, high amounts of glutamate impair the performance of a region of the brain involved in planning and decision-making, the lateral prefrontal cortex (lPFC).
Science has shown time and time again that mental fatigue has real effects. Numerous studies show that court decisions can depend on the degree of fatigue of the judge.
For example, after a long day in court, judges are more likely to deny parole (which is considered the safest option). Studies show that clinicians are more likely to prescribe unnecessary antibiotics at the end of a tiring clinical session.
The new study, from the Paris Brain Institute (ICM), investigated whether cognitive functions such as concentration, memory, multitasking and problem solving can cause lPFC fatigue, which influences decisions that we take when we cross things off our list.
The brain is the command center of the body, regulating circulation, respiration, motor function and the nervous system. The brain coordinates these activities at the expense of enormous energy consumption.
Nerve cells break down nutrients to release energy (metabolism). But this process accumulates by-product molecules called metabolites. Glutamate is a type of metabolite. The brain eliminates this toxic chemical waste while you sleep.
The authors of the Paris study wanted to see if prolonged cognitive tasks depleted the brain’s nutrient supply. They also tested whether this type of highly focused demand accumulates a greater concentration of toxic substances in the lPFC than in other parts of the brain.
In this case, the authors compared the PFC to the primary visual cortex, which receives and processes visual information.
To test their hypothesis, the authors divided their 40 participants into two groups. The two groups sat in an office in front of a computer for six and a half hours. A group had to perform difficult tasks that required their working memory and constant attention.
For example, letters were displayed on a computer screen every 1.6 seconds and participants had to sort them into vowels and consonants or, depending on the color of the letter, into upper or lower case. The second group performed similar but much simpler tasks. Both groups achieved an average correct answer rate of 80%.
Scientists used magnetic resonance spectroscopy (MRS) to scan participants’ brains and measure metabolite levels. The authors took readings at the beginning, middle, and end of the day.
They found markers of fatigue, such as an increased concentration of glutamate, but only in the high demand group. Accumulation of toxic chemicals was observed only in the lateral prefrontal cortex [lPFC]) and not the primary visual cortex.
After the high- and low-demand cognitive tasks, both groups had decision tests. This included choices about their willingness to exert physical effort (cycling at different intensities), cognitive effort (performing harder or easier versions of cognitive control tasks), and patience (how long they were willing to wait to receive a greater reward).
Rewards ranged from €0.10 to €50 (about US10¢ to $50). Reward receipt times ranged from cash immediately after the experience to wire transfer after one year.
Rethinking the working day
The authors found that the high demand group, which had a high level of metabolites in the lPFC, preferred the less demanding choices. The pupils of these participants were less dilated (dilated pupils suggest excitement) and took less time to make decisions, indicating that they experienced this part of the experience as undemanding.
Thus, the Paris study also raises questions about whether the working day is structured in the best format.
According to the results of the study, we should break down high-demand cognitive control tasks that require working memory and constant attention and consider that performance suffers at the end of the day. Some professions may require very different structuring given these results.
During their shift, air traffic controllers only guide planes for a maximum of two hours, followed by a half-hour break. But bus drivers, clinicians and pilots would also benefit from regular, mandatory rest.
Our brain has many different areas that are active during different tasks, such as speaking, hearing, and planning. Not all our decisions can therefore be explained by the results of the Paris study.
Considering interactions throughout the body, a 2006 US study suggested that new information may be better processed in a state of hunger. But hunger makes it more difficult to store newly learned information. Satiety means fuels are available to build neural circuits to store long-term memory.
Decisions about a third party, for example a judge rendering a verdict on a defendant, may be better in a state of satiety while tasks that involve fine motor functions, such as surgery, may be compromised. This is because after a meal the self-interest in survival is diminished because we don’t need to search for food.
This allows us to judge our environment more objectively. But satiety is a time when the body needs to rest to process food, which is why complex fine motor skills aren’t at their best in this state.
The next time you have to make a tough decision at the end of a long day, know that you’ll be inclined towards low-effort actions with short-term rewards.
If possible, you should sleep on it.
Zoltan MolnarProfessor of Developmental Neurosciences, University of Oxford and Tamas Horvath, Professor of Neurobiology and Obstetrics/Gynaecology, Yale University
This article is republished from The Conversation under a Creative Commons license. Read the original article.
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