If you think too much, it will cause mental fatigue, different from physical. Scientists from Brown University decided to establish what this is connected with.
American scientists conducted an experiment during which they measured specific signaling molecules in the brain of two groups of people. The participants of the first group solved more complex tasks that required serious intellectual loads, while the participants of the other group solved easier ones. While both groups tried to complete the tasks, the scientists measured their levels of the amino acid glutamate, which is a neurotransmitter in the brain. 39 people took part in the experiment.
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A correlation has been found between glutamate levels and mental fatigue. Some believe that fatigue is due to additional intellectual effort, that is, increased metabolism in brain cells, which leads to the accumulation of glutamate. The current study did not support this hypothesis.
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However, it does suggest that glutamate is linked to mental fatigue. This amino acid may be one of the keys to understanding how and why intense thought processes cause fatigue. However, there is also an opinion that glutamate and other metabolic waste products with a low degree of probability provoke fatigue. But their surge may cause an update in the status of our physiological needs such as food, water and sleep. (READ MORE) 
Brown University
Medicine
one of the most prestigious private universities in the US, located in Providence, Rhode Island