Hi, It's funny, my daughter (19 months) is doing the EXACT same thing! I am actually having to change her diaper in the middle of the night when she wakes up otherwise she will have soaked through by morning. She always has a sippy of water available, but lately has been drinking it all and then getting up wanting more during the night. I was up with her on and off from 1-3:30 last night. With Zoey, it seems to coincide with teething.
She is not a fan of water during the day and will often have a fit if she doesn't get milk instead (her favorite). But, at night the girl drinks like a camel.
I guess I don't have any real advice, but wanted you to know you aren't alone. I'm looking forward to finding out if anyone else has any ideas...