system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What causes the muscles in your neck to hurt for hours when you "sleep wrong"?
gpt
When deep in sleep, your head can fall into a position that puts unnecessary stress upon your neck muscles. After a bit, your neck can get agitated. Your sleeping brain might do you a solid and command your body to reposition itself so that your neck gets a little relief. Alternatively, your brain may just ignore the agitation, exacerbating the strain.

Basically long exposure to slight strain builds up overnight if you remain in a less than ideal position.