BBT question... Please someone answer

Hi ladies! I have noticed that on the weekends my bbt is way higher than usual, even before ovulation. I take my bbt pretty late (cuz i love to sleep until late on weekend). Does taking the temp at a different time give you higher values? My temp last sat and sunday was on the values of my post ovulation. Then, on monday i took it at a regular time and it was lower again. Any suggestion?