system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why our face looks slightly different after we wake up from a sleep?
gpt
During the day you generally keep your head up at the top of your body, and your body gets used to having to pump blood way up there, and you look normal.

At night, you position yourself horizontally and stay that way for hours.  Blood flows much more easily to your head and that extra blood in there (it's not pooling up, there's just more in there as it's moving around) makes your face a little puffy n' stuff.

Then once you get yourself upright again after a bit everything settles back down under gravity's constant tug, and you look normal again.