system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is happening to your eyes (& brain) when you are thinking about something & you stare into the distance, seemingly oblivious to what is happening in front of your eyes?

I don't know if I'm explaining this properly.  

I'm talking about when you're thinking about something really intensely and you're not really looking at anything in particular, you're just staring and thinking and not really seeing what is happening in front of your eyes.

I've found myself doing that only to "wake up" and realise I've been staring at someone or something without meaning to, simply because I'm been concentrating so hard on whatever I was thinking about.
gpt
Your brain cells require a lot of energy to work at full capacity, and your visual processing center is a fairly large portion of your brain (relative to the amount of space your eyes take up on your body). Put simply, your brain can choose which incoming sensory information is worth dedicating chemical energy to fully process. If you are deep in thought that requires significant frontal lobe usage (for some decision making) or if you're tired and don't have the energy for much of anything, your brain could turn its processing power away from your visual field. You'd still be processing the incoming light waves to an extent, but not much of it would reach your consciousness.