system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can't we see well after waking up ?
gpt
Rheum is the fluid that lubricates and coats your eyeballs.  When you're awake, normal blinking action keeps that stuff moving and working itself out via ducts.  When you sleep, though, it just sorta sits there and thickens up a bit, maybe gathers a little dust, etc.  In the corner of your eyes where it dries completely, that's what the crusty stuff is, too.

Anyway, once you're awake for a bit, your blinking clears it up again.