system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do you have back and neck aches for days if you "sleep wrong" one night?
gpt
Especially if you are heavy and/or out of shape, if you sleep in a bad position it can put pressure on local nerves. This causes pain and inflammation (swelling). The inflammation can have the effect of stressing the nerve further, making the problem worse in a ~~negative~~ ~~positive~~ feedback loop. (**edit**: it's a feedback loop of some kind).

If this happens, you can take a strong dose of ibuprofen or other anti-inflammatory medicine and (properly) use ice to reduce inflammation in the affected area. Usually, once the inflammation  abates, the pain does as well.

You should see a doctor if the problem is recurrent or doesn't respond to these treatments. (I am not a doctor.)