system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do some people wake up with puffy faces after a night of sleep? What is going on inside that makes this happen?
gpt
Gravity makes the fluids in your body (the ones in your blood vessels and the ones between cells, outside of your blood vessels, called interstitial fluid) to redistribute evenly through your body when you lie down. So when we're standing up most of the day most of the fluids distribute downwards, but when we lie down there technically isn't any force to promote that...


If there's enough force in the blood vessels (called hydrostatic pressure), such as when more blood rushes into them, but the diameter of the vessel stays the same, then the pressure will increase within it and promote fluids to leak out of the vessel into your tissues (becoming part of your interstitial fluid). So this makes your face swell up. 


That's why people recommend putting ice packs or cold spoons on your eyes to get rid of the puffiness. It constricts blood vessels, redirecting the accumulated fluid in your tissues back to the rest of your body.