system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do coughs happen during sleep and not sneezes?

Roomie is sick. Coughs though the night, but sneezes during the day. Why the difference? Brain activity level difference?
gpt
You're on the right track.  You sneeze when an *external* irritant enters your nose.  However, the neurotransmitters that send signals to the brain in order to initiate a sneeze are shut down during  REM sleep.  Also, because of reduced movement, you don't encounter as many particulates in the air.  You can sneeze if the irritant is significant enough, but you will wake up to do it  (you can test this on your roomy with some cinnamon).  On the other hand, coughing during a cold is to expel *internal* mucus from your airway, and is instigated by internal stimuli and those nerves are not suppressed the same way while sleeping.