system
Given the following post, respond as if you're commenting on Reddit:
human
What causes our voice to be different when tired or just after waking up?
gpt
When you sleep, fluids accumulate in the tissues of your throat,  changing the way your vocal cords vibrate for a little while.  It's also the same reason you get puffy eyes in the morning, until gravity can do its thing and help the fluids leave. 
If you sleep with your mouth open,  your vocal chords can dry out too. That also changes the way you sound.