You also lose a lot of water from breathing. Your respiration has moisture in it. Every breath (on a mirror, for example) leaves moisture.
So if you weigh yourself before bed, don't even get up to pee at all or sweat much, you will still have lost a noticeable amount during the night. Try it.
But yes, the minute you chug a gallon of water (dangerous and stupid), you will weigh 8lbs more... but nobody would do that because you are flushing all your electrolytes out of your system and making yourself a worse athlete/runner. Don't over drink water. It is dumb. Drink to thirst and maybe a cup extra.