system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does the brain create nightmares?

It doesn’t make sense to me that it would induce a state of terror in our most vulnerable state.
gpt
Our brains are really good at telling stories based on insufficient information.

When you sleep, the plaques in your brain get flushed, resulting in random excitement of neural pathways.  This means you trigger a random combination of sensations and memories.  When you are in REM sleep, the part of your brain that manages narrative is active and not in maintenance mode.  This means it can string together sensations being triggered into a story you experience and sometimes remember.

These can be positive or negative, depending on all sorts of factors including hormonal balance, general mental state, lucidity, strong neural pathways, repressed memories/fears/hopes, and much more.