system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What happens when your mind drifts, and your eyes can't focus?
gpt
You are employing more focus to a task (usually a mental one like daydreaming) and as a result less focus on the information around you, as your brain requires more processing power than what is required by the wasted effort of your senses. 

This can happen during extreme focus as well, on an enjoyed or important task your vision can "narrow" (solving a puzzle but not realizing someone entered the room)

You can only really focus on one thing at a time, but to counter this our brains sift through the data and bring the important shit to our attention: it's hot->drink water, she's frowning->what's wrong, etc.

This informs our conscious actions, we take the "relevant" data and act on it. Waking up and brushing your teeth in the morning is nothing more than a series of many small steps all informed by what you process. When processing power is diverted to thinking, other systems have to be side-lined.