Resting HR goes up during the day

Your resting HR can be easily affected by a number of things (caffine, sleep, stress, etc..). Unless we knew what your daily habits, food intake, stress levels, sleep schedule, there's no way for us to know if it's the watch or a change in your day to day activities.

My resting heart rate fluctuates based upon the intensity of my workouts throughout the week. It can vary a lot when I've ramping up or tapering down.

I also have a Fenix 3HR. I have been using it for more than a year and I did not see the resting HR going up during the duration of the day. I do not mean the instant reading of the watch but the value for the resting HR from the dedicated widget.

I have had that happen a few times as well. Check in the morning an my RHR is "low" (lets say 50), and then later in the day when I check again it it "higher" (lets say 53). It confused me as well. I don't know how RHR can go up during the same day. That would be like your maximum heart rate going DOWN during the day.

I only noticed it a couple of times so I don't bother with it...

If you wear the watch overnight whilst you sleep, then it should calculate the RHR during sleep time (for better or worse). If you don’t wear the watch whilst you sleep, then it calculates RHR from ‘rest’ periods during the day. I have found that this means the RHR can change during the day as more data is gathered; so going up is entirely possible.

"RHR is determined as the lowest average reading over a one minute time period during the day" This is a quote from the link provided by Crispin_Ellisdon. Acording to it the resting HR can not go up! Period!

Indeed. But at the start of the day there is still a reading for RHR, even if the watch hasn’t had time to get a lowest average reading over one minute in that new day. I can only assume it displays the RHR from the previous day (or some other unknown past source) until that new lowest average over one minute is obtained. In this case the number could go up from that displayed at the start of the day. My experience is usually the other way (when I haven’t worn the watch overnight), in that the RHR figure decreases as the day progresses (so it does what the text says), but I have seen it rise and the only explanation I can think for this is given above.

I will check to see what is displayed as rest HR as soon as I power on the watch and to check the behaviour.

Hmmm. That you actually power the watch off overnight may also be a factor.

I really do not know what to do...

You could try raising a Garmin Customer support service ticket (via the website or phone them). But if you are on the latest public beta software, a far more direct route to have your concerns addressed is to wrap up your observations in a logical way and email it in to the Fenix beta team (email address in the first post of every beta software sticky thread at the top of this forum). That is by far the quickest way to get issues you observe fed in to the software development. Don’t take it personally if they don’t reply to you; they do read all of the emails, but only reply if they need follow up info.

Edit: Just another thought; where are you reading the current day resting heart rate from? The only place I can see to derive this value on the watch is to look at the last dot on the 7 day graph for the RHR and ‘best guess’ what value that dot is; the number at the top of this graph is for the 7 day average and not the current day. The only place I can find to actually see a RHR number for the current day is in Garmin Connect Mobile. I also remember that the Fenix 5 series had improvements to the HR sensor over the 3HR, which means the 5 series pole HR every second (even out of activity) and also allowed for the 24 hr stress measurements. So it is no surprise you would see difference in behaviour between the 2 watches. All of this aside, I agree it still doesn’t explain why the RHR goes up for you during the day; as long as you are looking at the right number.