Ok, I can go along with this, but why do you gain it back during the day? You are still breathing. Your house is still the same temp, with the same humidity level. Did I miss something? One time when I was super obsessive about weighing, I found out that if I weighed myself when I first got up and then again around 10 AM, I weight a bit less at 10 AM. And that was also the time when my weight would start to rise for the day.