Why I become tired, when I write a blog

I often make a bike ride. When I ride with an easy speed and the distance is not long, afterwards I feel relaxed. When I ride a long distance with a fast speed, I feel tired at the end. We think it’s normal that making a great physical effort makes you tired. It is because your muscles become filled with lactic acid, which must be cleared up in order to feel fresh and rested again.
Once a week I write a blog. When I have finished it, I feel tired, for usually it is a hard job. So, afterwards I want to relax, for instance by making an easy bike ride. But why do I feel tired? Most of the time I sat down in my chair and only my fingers moved a little bit in order to type my blog and to do a few internet searches. Nonetheless, I feel tired. Even more, mentally tired people who must take decisions, like chess players, are going to make mistakes, just because they are tired; types of mistakes they wouldn’t make with a fresh mind. How come?
Several explanations have been given why thinking makes you tired. Actually, you are not tired but bored, some say. You want to do something else and so your brain tells you that you are tired. Or, another explanation, the deeper you go into a problem, the more complicated it seems, so that’s why you are going to make mistakes. It’s a way to explain mistakes made by chess players. At the beginning of a game they can use their routine, but later on they must rely on their insight and then they can fail. Other explanations for mental fatigue have been suggested as well. However, a group of French researchers didn’t find the existing explanations very satisfying, so they developed their own approach. Let’s look in the brain and see what happens if people get mentally tired, they thought, and they developed relevant tests. If you make a light physical or mental effort, you don’t become tired, but you do if your effort is hard. Maybe, you become mentally tired because some stuff piles up in your brain when you are thinking, which makes that you feel fatigue; just as in the case of a physical effort like cycling. Therefore, these researchers developed a light and a difficult version of the same mental task and watched the brains of their test persons and looked what happened; moreover, they looked at the reaction of the pupils. The task chosen lasted several hours. For watching into the brains of the test persons, they used an MRS scanner (MRS = magnetic resonance spectroscopy). With such a scanner you can see what happens with the blood currents and the chemistry in the brain, when the test person is active or passive.
What did they find? Indeed, becoming tired when doing a mental task is not a mental affaire, but it is as physical as becoming tired in your legs when cycling. There were clear differences between the test persons that did a light task (and didn’t become tired) and those who did a hard task (which made them tired). The researchers saw signs of fatigue, including reduced pupil dilation, only in the latter group. Those in that group also showed in their choices a shift toward options proposing rewards at short delay with little effort. But most important, they had higher levels of glutamate in synapses of the brain’s prefrontal cortex. The prefrontal cortex is the part of the brain in the forehead where decisions are taken. These results made the researchers conclude, together with earlier evidence, that it is likely that glutamate accumulation makes further activation of the prefrontal cortex more costly, such that cognitive control is more difficult after a mentally tough workday. In other words, glutamate poisons your brain and that’s why it disturbs your thinking. Only after it has been cleared up, you feel fresh and rested again. Actually, there is not much difference between feeling physical and mental fatigue. The difference is in the details and the solution is also the same, and actually not really surprising: take a rest or take a nap. And if you are really tired, go to bed. The conclusion feels like kicking open an open door. Nevertheless, it isn’t. People often think: I am tired, the task must be done. And they go on till the job has finished. Maybe, they even think that feeling mentally tired is an illusion, and anyway that it’s not that bad (think of doctors, at the end of a workday). Maybe, there are other reasons to go on, but from a psychological point of view it’s a bad idea: It’s almost sure that mistakes will be made. Fatigue is an objective real phenomenon with a physical base. So, avoid making important decisions when you are tired.
The knowledge from this study (and from future research) can also help to tackle mental problems. For example, the researchers say, monitoring of prefrontal metabolites could help to detect severe mental fatigue. Such an ability may help adjust work agendas to avoid burnout. In future studies, the researchers hope to learn why the prefrontal cortex seems especially susceptible to glutamate accumulation and fatigue. They’re also curious to learn whether the same markers of fatigue in the brain may predict recovery from health conditions, such as depression or cancer.
So, now you understand why I am tired, when I have written a blog.

Sources
- “Why thinking hard makes you tired”, Cell Press (here and there above literally quoted or paraphrased)
- Antonius Wiehler et al. “A neuro-metabolic account of why daylong cognitive work alters the control of economic decisions”