What causes our brain to get “tired” and lose focus when thinking hard for extended periods of time like studying or thinking through a difficult concept?
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Thinking hard takes resources – time, energy, attention. Our brain evolved in an environment where the most productive activities involved physical or social effort, and so our emotions and urges are made to encourage those activities.
Working quietly on abstract tasks in a climate-controlled room is not a situation our brain evolved for. Our brain wants us to instead do things that make it feel productive, like eating, exercising, or socializing. These all would be the most important things for our pre-civilization ancestors to do, but many modern tasks do not require that sort of activity.
Not an expert; there is some kind of „waste“ chemical that is building up when your brain is used. In our sleep this „waste“ is flushed out which is why after waking up it’s like a „reset“.
The build up of adenosine in your brain during your waking hours causes you to feel tired. This is called “sleep pressure”. Adenosine is a metabolic byproduct. I don’t know the finer details, but it may be related to the metabolic use of ATP (adenosine tri-phosphate) by your brain.
Caffeine keeps you awake because caffeine occupies but does not activate the adenosine receptors in your brain, preventing adenosine from fitting those receptors and causing you to feel sleepy. But when the caffeine wears off, all that built up adenosine can then cause a caffeine crash, where you quickly get really tired from the built up adenosine.
When you sufficient sleep, your brain clears out the extra adenosine in your blood and resets the whole thing. (For a detailed treatment of this, I recommend the book “Why We Sleep” by the sleep researcher Matthew Walker.)
Since adenosine builds up throughout the day due to brain activity, I speculate (and please note, this is just my educated speculation) that when you concentrate really hard on something and do a lot of brain-intensive work, the pace of production of adenosine should increase with that increase in brain activity, and that increase in adenosine concentration in your brain may give you that tired feeling.
The brain uses a large amount of energy to operate. Everything in your body can get tired. Your digestive system. Your muscles, your ligaments, your eyes. If you use your brain it also will lower in performance throughout the day.
I’m very much not any kind of expert so take it with a grain of salt but I recall reading about some experiments suggesting that it is to do with glucose depletion, and eating some helps powering through
[Veritasium on YouTube has a good video](https://youtu.be/UBVV8pch1dM) on this topic that explains the different types of thinking and the energy they take.
Partly, it’s that brain work burns a lot of blood sugar. Willpower has been linked with blood sugar, and willpower is a large part of focus. Eat regularly when you’re studying, and eat things that will keep your blood sugar from spiking (e.g. nuts, beef jerky, maybe fruit if you don’t overdo it).
(Source: “Willpower” book by Baumeister and Tierney)
The brain has a high caloric demand. Often times when we focus on studying or doing a task, we put off eating or noticing we are hungry. Snack a bit during sessions that require focus and hard thinking.
I think glutamate buildup was recently implicated (might have been mentioned in another sub recently) https://www.science.org/content/article/mentally-exhausted-study-blames-buildup-key-chemical-brain
When you get bored of something, your brain decided that it’s not important and devoted less resources to it.
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