system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When trying to sleep, how can one position be so comfortable when you first get into it, but then uncomfortable after 5 minutes?
gpt
What's nice for your muscles and what's nice for your bones and ligaments isn't always the same thing, especially if you have posture problems.

You might have a tight shoulder for example, and being in a particular position might relieve that. But the same position could put stress on your spine. So you lie down and feel shoulder relief, but a few minutes later your spine starts hurting.

If you have persistent problems with sleeping comfortably, you may want to get checked out by a physiotherapist. Especially if it's back or joint pain.