Have a look into heart rate variability and your mind will boggle.
Mine drops to 28 in the day and has dropped to 26 at night.
I do have a heart condition though that causes this!
Its funny but I have started wearing a bracelet when I ride that says heart rate very low, don’t give me atropine or put a pacing wire in!
I worked in cardiology until recently and put a pacing wire in a women whos heart rate was dropping below 30, she arrested twice. I told the registrar who was showing me how to put the wire in that mine dropped to 28. He didn’t believe me so afterwards we put an ecg on me (It was the middle of the night, we were bored) he was pretty shocked at what he saw and that it didn’t seem to affect my performance!
I also received a shot of atropine in Burton A+E whilst working as a med student after I passed out whilst having my toenails removed post marathon (don’t ask). Hence the atropine bracelet, it was not a fun experience. They even had the defib pads on me, scary!