BBT help!

Francesca • I`m 28 and TTC #2 with my fiancée! Lots of losses & my first arrived at 29 weeks so trying to stay positive!
Does anyone else do this? I bought a thermometer that does it properly and took it yesterday but forgot today before I stood up! Does it absolutely have to be as you wake up after a three hour straight sleep? My daughter still wakes up a lot in the night so three hours beforehand in never guaranteed! Any advice?