I always weigh in the morning at roughly the same time, and after I've been to the loo, not because I'm lighter then, but because I think that gives the least variables i.e. I'll vary less with water weight, with how much I've eaten that day (as I won't yet have eaten anything), with how much has emptied that day (as I'd think I'd have digested everything overnight and got rid of the waste - sorry if tmi!).