(ORDO NEWS) — A long day at work can leave you exhausted and make you want to watch TV and eat. But you’ve been sitting all day. So why do you feel as tired as your friends who do manual labor?
As time goes by, you get more and more stressed out with your to-do list. Even worse, if along the way you run into a colleague who “just wants to give you a minute.”
It may seem obvious that you’re more likely to make impulsive decisions at the end of a long day, but people often do it anyway.
A recent study that scanned people’s brains at different times of the day showed that tasks that require intense and constant focus can lead to the accumulation of a potentially toxic chemical called glutamate.
Commonly used to transmit signals from nerve cells, large amounts of glutamate alter the functioning of an area of the brain involved in planning and decision making, the lateral prefrontal cortex (lPFC).
Science has proven time and time again that mental fatigue has real consequences. There are many studies that show that court decisions can depend on how tired the judge is.
For example, after a long day in court, judges are more likely to deny parole (which is considered a safer option).
Studies show that doctors are more likely to prescribe unnecessary antibiotics at the end of a tedious clinical session.
A new study by the Paris Brain Institute (ICM) examined whether cognitive functions such as focus, memory, multitasking and problem solving can cause lPFC fatigue, which affects the decisions we make when we cross things off the list.
The price of opportunity
The brain is the command center of the body, regulating blood circulation, respiration, motor function and the functioning of the nervous system. The brain coordinates this activity at the expense of huge energy costs.
Nerve cells break down nutrients to release energy (metabolism). But during this process, by-product molecules known as metabolites accumulate. Glutamate is one type of metabolite. The brain clears this toxic chemical from waste products during sleep.
The authors of the Paris study wanted to find out whether long-term cognitive tasks deplete the brain’s nutrient stores.
They also tested whether this type of need for high concentration creates a higher concentration of toxic substances in the LPFC than in other parts of the brain.
In this case, the authors compared the lPFC to the primary visual cortex, which receives and processes visual information.
To test their hypothesis, the authors divided 40 participants into two groups. Both groups sat in front of a computer in the office for six and a half hours. One group had to perform complex tasks that required working memory and constant attention.
For example, letters appeared 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, uppercase and lowercase.
The second group performed similar but much simpler tasks. Both groups managed an average of 80 percent correct answers.
The 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 elevated glutamate levels, but only in the high exercise group. The accumulation of toxic chemicals was observed only in the lateral prefrontal cortex [lPFC]), but not in the primary visual cortex.
After completing high and low need cognitive tasks, both groups completed decision making tests. They included choices about willingness to exert physical effort (whether to ride a bike at different intensities), cognitive effort (whether to perform harder or easier variants of cognitive control tasks), and patience (how long they were willing to wait to receive more rewards).
The reward ranged from 0.10 to 50 euros (about 10 cents to 50 US dollars). Delays in receiving rewards ranged from immediate cash after the experiment to bank transfer a year later.
Rethinking the working day
The authors found that the high-demand group, which had elevated LPFC metabolites, preferred to choose the less burdensome options.
The pupils of these participants were less dilated (dilated pupils indicate arousal), and they took less time to make decisions, indicating that they perceived this part of the experiment as undemanding.
Thus, the Paris study also raises the question of how optimally the working day is structured.
According to the study, we should separate high-load cognitive control tasks that require working memory and constant attention, and take into account the fact that productivity declines at the end of the day. Some professions may need a completely different structure given these results.
During their shift, air traffic controllers only direct aircraft for two hours, followed by a half-hour break. But bus drivers, doctors and pilots would also benefit from regular, mandatory rest.
There are many different areas in our brain that are active during various tasks such as speaking, hearing and planning. Therefore, not all of our decisions can be explained by the results of the Paris study.
Considering whole-body interactions, a 2006 US study found that new information can be best processed while fasting. But hunger makes it difficult to store newly learned information. Being full means having the fuel to build circuits of neurons to store long-term memory.
Decisions concerning a third party, such as a judge giving a verdict to a defendant, may be better when satiated, while tasks that involve fine motor functions, such as surgery, may be compromised. This is because after eating, the interest in survival is reduced, since we do not need to look for food.
This allows us to more objectively assess the environment. But satiety is a time when the body needs rest to process food, so complex fine motor skills in this state are not up to par.
The next time you have to make a tough decision at the end of a long day, know that you will lean towards low-effort, short-term rewards.
Get some sleep if possible.
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