But you could argue that we aren't doing as much in our sleep so using less fluid, when awake that's a different matter as we are using everything, including our brains.

Edit: just re-read, yes I agree with you. Thirst is just a sign you are starting to get low and need water, if you ignore that then very likely you will have dehydration

Think of it this way, the brain is 80% water. So just as much as 2% dehydration can really effect its performance and cause issues.