You should decide at the beginning of BBT temperature charting what you consider the beginning of a new day, either after midnight or when you wake up. Both choices are OK.
There is sometimes a question when you should consider the beginning of a new day. Do you go by the calendar and does the new day begin after midnight? If you make love at 1 minute after midnight, is that a new day or the previous day? Or does a new day begin in the morning, when you wake up?