Can Thinking Make You More Tired?

Toward the end of your work day, you feel antsy. You find yourself making more mistakes, and you wonder why you're so tired after sitting at your desk. You get home, and rather than take the time to make a healthy meal, you aim for the ice cream. Why does a day at work feel exhausting?

According to a 2022 study in Current Biology, your brain can get tired from too much thinking. The study looked at the region of the brain involved in learning and memory called the prefrontal cortex. The researchers asked participants to perform either hard or easy puzzles, measuring how the brain's messaging system changed according to the level of difficulty. They also used eye scans to measure how their pupils dilated, which is another determinant of brain activity.

Those who had the more difficult tasks showed signs of fatigue in the brain. Over time, their pupils had fewer dilations, which indicated that they were mentally tired. As a result, they chose options that required little mental effort and more immediate rewards — similar to why we might reach for junk food at the end of the day.

The 2022 study published in Current Biology found that the harder tasks required the nerve cells in the brain to release more glutamate, which is responsible for learning and memory. The amount of glutamate in our cells must be properly balanced — it could be toxic if too much is outside the cells in the brain's synapses. During stress and when our brains need to work hard, this glutamate outside of the cells builds up. When these levels are too high, it's difficult to make important decisions, and self-control is more difficult. This is how we experience mental fatigue, and our levels only come back into balance when we rest.

In a news release, study author Mathias Pessiglione said that when we begin to experience mental fatigue, it's a sign for us to stop and take a break. Because we don't always know when our brains are tired, Pessiglione suggested ways for us to detect when too much glutamate is released so that we don't experience burnout or lose our self-control.

According to the Cleveland Clinic, glutamate is an essential messenger in the brain because it signals other nerve cells to fire. It's also needed to help produce gamma-aminobutyric acid (GABA), which has a relaxing effect on the body. Too much glutamate can cause damage to our nerve cells and has been linked to Alzheimer's, amyotrophic lateral sclerosis (ALS or Lou Gehrig's Disease), multiple sclerosis, stroke, and fibromyalgia. It can also result in mental health problems, such as anxiety and depression. Too little glutamate makes it difficult to concentrate.

Keeping our glutamate levels balanced is key. A 2021 review in Frontiers in Human Neuroscience notes that glutamate helps us respond appropriately to everyday stressors. However, chronic stress can overload glutamate levels. Exercise can help keep our brains in balance since it produces both glutamate and GABA. This improves our concentration and ability to relax. Meditation and mindfulness practices also keep glutamate levels in check. A 2013 study in PLOS One took brain scans of experienced meditators and found that these practices help balance levels of glutamate in the brain.

The next time you feel your brain getting tired, be sure to take a break. It will help restore your ability to take in new information, and it might help you refrain from reaching for the ice cream after work.