Yes you have to take your temp at the same time every day, I do mine at 6am, and when my son gets up later than that I still do it at 6am but try and go back to sleep. It can be a pain in the backside to be honest, but once you get used to it it becomes second nature. I temp vaginally as the temps are much more accurate, and if you ever take your temp earlier or later then I dont think it's a problem, as I've done this before and still pinpointed Ovulation. You need to make sure you use a basal body thermometer which is .00 as a normal thermometer (like one you'd take your kids temp of for instance) wouldn't be sensitive enough.