For about six months I weighed myself first thing in the morning, after pee, and last thing at night before bed. I have a Tanita scale, so also took my bodyfat%.
My weight was always lowest in the morning, but body fat was higher, with the reverse at night. As others have mentioned this reflects hydration levels as the Tanita scale is affected by that.
The main thing is not to worry too much about actual weight, but changes, and by recording the results, you can see what the trend is over a period of time, not just a quick snapshot of a given time.
I still use the same system, but not quite as stringently.
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"...perhaps the world needs a little more Canada" - Jean Chretian, 2003.