system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Is there a reason why it's so hard to run in dreams?

For a few days I've been having dreams that involve a lot of running. Sometimes for sport sometimes away from something and almost every single time it feels like my legs weigh 200 pounds each and I'm trying to drag them through water.

So I was wondering if there's a specific reason why it's so difficult to run in dreams? I can understand things like clocks and mirrors being messed up but if you can walk why can't you run?
gpt
When you go to sleep, your brain chemistry changes to prevent you from moving much in your sleep. So when you run in a dream, your legs don't move. Without that feedback, you brain conveys to you that you're not moving. So it feels like you're, as you say, running with 200 pound weights on your leg.