system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do I always feel dehydrated when I wake up no matter how much water I have the night before?

When I wake up my throat is always dry and my pee is always a yellow colour yet I drink plenty of water the night before. 
EDIT: I'm pretty sure I don't have diabetes because I have a couple of diabetic friends but I'm going to my GP soon to get checked out about something else so I'll mention this to them to see if anything is actually wrong with me, thanks for all the answers guys, I'm not 100% sure what it is but I've got a rough idea from you guys and I will see what my doctor says.
gpt
You don't. Dehydration is a particular condition that is a result of an *excessive* deficiency of water in the body resulting in metabolic changes. It wouldn't take place over 8 hours of sleeping, especially if you drank water before bedtime.  

More likely, you simply feel thirsty because you haven't had anything to drink for ~8 hours (or however long you slept). But you're not dehydrated. 

If you're experiencing a dry throat, a likely source of that could be nasal congestion. If your nasal passages are congested, this will cause you to breath through your mouth while you sleep, drying out your throat. 

And urine is naturally yellow.