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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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.
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