system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can't sick people breathe properly when they lay down?

Why is it that when you have a cold/cough etc., you have to sit up all the time because it's hard to breathe laying horizontal?
gpt
When you're ill, your lungs get inflamed and start producing extra mucous which in turn blocks the bronchi and alveoli from moving oxygen efficiently. When you sit up, the mucous moves to the lower parts of the lung, freeing up the passageways. Same when you cough up phlegm, it clears them more and you can breathe a bit easier. 

AKA, you're still getting oxygen, it's just not being used efficiently making you feel out of breath.