It can and does happen that heart rate can be lower during the day if you are very relaxed, even after a good night's sleep and low resting heart rate.
To be more clear - I agree with
@gregmcc that resting heart rate is best measured when you wake up, and average during sleep should be lowest unless you have a bad night's sleep or nightmares etc. but that does not mean it will never drop lower than that at times during the following day for a short while.
Not long after I made post #45 my heart rate dropped to lower than what my resting heart rate was for a few seconds (I had Garmin connect open at the time and was very chilled).
It is not uncommon for this to happen, but I will not take that as my resting heart rate for the day, it is just interesting to me when it does and shows what my state of mind/rest was at the time.
Only almost 10 years, that's real rookie numbers...