Renee,
Its suggested that you take your BBT after at least 3 hours of solid sleep, i guess it depends on how solidily you sleep in those 2 hours as well as the half an hour, if your laying there just doing nothing, not a round of house work then as sarah said i wouldnt worry too much about it, as overall in charts its looking for patterns not exact temperatures.
there is also the temp. corrector as well if your a VIP member on FF, or some lovely girl on here to help out.