That's a ton of water to lose in just a night. I know I've lost ~8lbs of water weight before, but that's after eating a ton of food and being carbed up + salted up, so I was retaining a lot of water. Special case. I also get up once or twice a night to pee.
That's one reason why I weigh myself when dry fasted. It's a more consistent measurement. My starting weight non dry fasted may have been around 206lbs, but i weighed in at 201.8lbs dry fasted.