It may not be just water weight. Not all of that food is fully digested by the very next morning, that food weighs something too. I think all of us have experienced a large rise in the scale a day after a really bad meal.
I find it more useful to weigh every single day because then I become aware of my body's fluctuations (e.g. noting that the 7 pounds is now gone like 2 days later). Consequently I dont get as upset or defeatist about a few extra pounds. However, that gain gives me a mini "wake-up call" which freaks me out enough to be good at least for the next few days, lest those pounds of water "turn into fat".
When the scale starts fluctuating out of my usual 3 pound range (its probably a larger range for a larger person), I know I've either lost fat (if I hit a *new low*) or gained actual fat and not water (if I know I've been "bad" and now I'm fluctuating one or more pounds higher than before--which is indeed the case for me now).
Also, if you choose a day as you're weigh-in day where you happen to be at the high end of your normal fluctuation, you may get unduly upset that you "lost nothing", when the very next day you may fluctuate below your normal and actually MISS that NEW LOW.
Personally for me weighing every day has given me the reality check I needed to maintain my weight. Its a struggle and a biatch managing your food intake somewhat based on the scale (I've been doing this for at least 20 yrs), but for people like us who have to watch our weight it can be worth it in order to maintain (you hear a lot of stories about people re-gaining). Of course you can also do this with a once a week weigh-in but it may not be as quick to catch an upward trend. To each his/her own I guess.
).
