system
Given the following post, respond as if you're commenting on Reddit:
human
What is happening in the brain when we try and 'think hard'?
gpt
A priority of brain resources are routed to the prefrontal cortex. This is a "newer" part of the brain, evolutionary speaking and is why some of us are better at concentrating than others. 

Fun note, this also leads the brain to produce extra waste in the forms of proteins. As this protein builds up, it makes focusing harder, just like when muscles produce lactic acid as we exercise. Only sleep "flushes" these out. 

So, also, thinking really can hurt.