Why Thinking Hard Wears You Out

A workday full of mentally demanding tasks can make you feel drained. A relaxing evening of streaming TV shows is more likely to be what you choose after a long day of thinking. According to a new study, thinking hard can lead to a build up of chemicals in the brain.

Scientists haven't been able to find an explanation for why our mental resources deplete. Researchers theorize that long periods of mental effort lead to a lack of resources for the brain that needs energy. Experiments in the 2000s showed that people who drank a sugar drink had a reduction in blood sugar. Work didn't reproduce those findings. There has been no effect if you look at all of the studies together.

A study published in 2016 shows that long periods of mentally effortful tasks make people more likely to choose immediate gratification over waiting for a bigger reward. The change in behavior was accompanied by a decrease in brain activity in theLPFC. The team didn't know what was causing the change in brain activity.

The new study was published in Current Biology on August 11 and involved 40 volunteers. The lab was the equivalent of six and a half hours of work for participants. The "N-back" task asked people to remember a letter that appeared on a screen before. One group was assigned a difficult version of the tasks, while the other was given a simpler version. Only those who had been given the harder task were more likely to choose to take home an immediate reward rather than wait for a larger cash-out at a later date.

To find out what was happening, the team used magnetic resonance spectroscopy, a form of magnetic resonance that can be used to detect chemicals in the brain. The investigators found that people who did the harder task had higher concentrations of the neurotransmitter. They found that the molecule was moving faster in the difficult group and that it was building up outside the cells.

When the researchers looked at the primary visual cortex, they didn't find any changes. Matthew Apps, a cognitive neuroscientist at the University of Birmingham in England, was not involved in this work. The accumulation of materials in the brain may stop it from functioning properly, and that could be what leads to the consequences of fatigue on your behavior.

There are a number of areas where the findings might be useful. There is a person at work. In a field like surgery, people in jobs that require a sustained intense focus can be at risk of being burned out. The ability of these individuals to sustain attention for a long period of time may be boosted by the use of drugs aimed at reversing the build up of glutamate. Chronic fatigue syndrome is a condition in which fatigue is a symptom. Patients who struggle with exhaustion might be helped by the presence of glutamate.

It is a challenge for the field to define fatigue. He says there are different ways to measure fatigue. Whether the same brain mechanisms underlie different types of fatigue is an open question. The same brain region appeared to be involved in the build up of fatigue after a physically demanding task according to a study published last year. It's clear that there's going to be physical fatigue in your muscles, but we don't know where and if it's actually separated once you get to a certain level in the brain

The team observed that subjective feelings of fatigue and more objective measures of mental exhaustion may be different. Anna Kuppuswamy was not involved in this work, but she was interested in the difference between subjective and objective fatigue measures. In patients with post-stroke- or multiple-sclerosis-related fatigue, the level of self- reported fatigue does not always match up with the severity of the disease. Many in the fatigue field have come to realize that there is no single physical factor that can account for the perception of fatigue.

There are a lot of open questions for the person. One limitation of the new study was that it focused on two areas of the brain, so it's not clear how cognitive effortful tasks affect other areas. It's not known why the balance of glutamate is restored after rest or why theAccumulation of glutamate in theLPFC is a problem. There is a chance that toxins are flushed out from the brain. He says there is a lot of research to come.