You really have an hours leeway, so if you usually take your temp at 7.30am you can get away with doing it anywhere between 7am & 8am without too much discrepency. If you do it outside that hour you may get a bit of variance. And as Nikki said, you need to take it before you move much & before you speak, so you pretty much wake up & tick the thermometer in your mouth before doing a thing.

HTH