system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are some food considered 'breakfast' food? And some only considered lunch...etc, as a norm?
gpt
You typically eat breakfast on a completely empty stomach, some 12-14 hours after you last ate.  Breakfast food tend to be milder for this reason.  

They also tend be easier to prepare.  It is largely lost in this age of fast food and microwave ovens, but food preparation used to be a full time job.  Breakfast was a fast meal that the cooks didn't have to get up 4 hours early to prepare.