system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why, when I have a cold, is my nose stuffy when I'm sitting down, but clears up when I'm walking?
gpt
It helps to explain the source of the stuffiness: blood capillaries that are leaky and cause the sinus tissues to swell and "stuff" your nose.  When you become active (like walking), your sympathetic (fight or flight) nervous system becomes slightly activated and some epinephrine (adrenaline) is released into the blood stream.  This epinephrine works to constrict blood vessels and raise your blood pressure. The blood vessel constriction in your nose stops the capillaries from leaking so much, and the swelling in your nasal tissues decreases.

This also helps explain why your nose gets more stuffy when you lie down, because it increases the blood flow to your nose, and the capillaries leak. Leaky vessels + blood pooling there => nasal congestion. If you're congested, sleep sitting up to help clear your nose.

Pseudoephedrine is a common medication that stimulates the blood vessels to constrict and stop being leaky, which explains why it works as a decongestant.