Is your BBT lower if you wake up early?

Woke up at 6am and thought I would likely stay awake and get up - My BBT  was 36.37 but then fell asleep until 8.45 and took again - it was 36.65 - Which temp should I use? Looks like my temp is always lower the earlier I get up. To confuse matters I also had a positive opk this am (faint line yest)