not always always charting bbt at same time every day

I have a 8mo baby and I can only get sleep randomly, usually like 6 hours max. so I just check my bbt after I know I've gotten my max amount of sleep (meaning I slept 6 hours straight no interruptions). sometimes i check it during the day or sometimes it's at night depending on when I got my "nap" in. is this still reliable? 😥