does anyone know why we have to drink so much water?

I'm struggling to drink so much water while pregnant. Is it so serious that our amniotic fluid supply is affected by our water consumption? I try not to drink too much at night bc I don't want to be up all night in the toilet however I'm the most thirsty at night times. I'm probably not doing the best thing but isn't sleep more important than water? I don't know! When are you the thirstiest?