What Constitutes FIRST Morning Urine?

My bladder is the size of a thimble, so I pee several times every night. Now, I know FMU is supposed to be the holy grail of hpt, so I want to do it right. Should I avoid going to the bathroom all night before testing, should I just make sure I don't pee for, say, 4 hours before I wake up, or does it not matter at all?