system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Brain fatigue

Why can't I study for hours on end? Why does my brain feel weary and unable to learn after hours of work, and why does taking a break help?
gpt
While it is working, your brain (and the rest of you, but especially the brain) produces adenosine, a chemical which slows down your thinking, among other things. This forms a homeostatic feedback loop that keeps physical and mental activity at a reasonable average level so that you don't overwork and injure yourself. Your body breaks down adenosine *almost* as fast as it is produced, which causes it to build up throughout and make you tired until you go to sleep, at which point you break it down faster than you produce it. This is why stressful physical or mental work causes you too feel dazed and sleepy. As a side note, caffeine stops adenosine from working but **not** it from being produced, which is why caffeine will not make you more awake when you are already tired and why you feel very tried all of a sudden when the caffeine wears off.