I did quite a bit of research on this subject when a group of patients was trying to choose a device that we would all wear and compare results. An important point is the technology used to track this data. The watch-only devices use an optical measurement (a light measuring blood flow in the wrist), while the chest-strap units record electrical activity and are much more accurate for HR. But, they are a pain to wear all the time. I tried wearing my Polar chest strap unit 24/7 for a while and it not only was not very comfortable but it irritated the skin.
Last I checked, the ones that you wear on your wrist can only calculate heart rate (HR), but not heart rate variability (HRV). Only the ones you strap to your chest can calculate both HR and HRV.
We actually bought Fitbit wrist units (the new Inspire 2--$100). We got a free year of their premium package and it is does track HR, HRV, breathing, sleep (detailed graphs showing both time asleep, awakenings and sleep stages), steps and it is supposed to detect the type of exercise activity you are doing.
I used to have a polar with chest strap. I liked it, but the strap was annoying.
Very annoying! But super accurate. Note all these HR monitors average your HR over several beats--so they do NOT give a beat-by-beat report. What you see on your display is an average over several seconds. For most purposes this is fine. If you are trying to track an arrhythmia, it can be slightly misleading.
Thanks @Pyrrhus ! As you can tell, this is all new to me. So - it sounds like you need a chest strap in order to calculate HRV and a smartwatch alone can't do it - is that correct?
Nope, some wrist units will track both.
So, after about 6 months of using the Fitbit Inspire 2, here is how it has performed (I'm guessing that all the various models of Fitbit will give similar results).
HR: it is off by about 3 BPM--I know this because I have a pacemaker that is set for an exact rate and the Fitbit reads 3 beats faster.
HRV: The graph display is not as detailed as the garmin vivosmart 4 that
@keepswimming posted about. I have no way to know if the HRV data on the Fitbit is accurate though.
Breathing: Not a detailed graph and I don't have a way to check its accuracy.
Sleep: great graphs but not so accurate. Sometimes it reports that I am asleep when I am awake and vice-versa--but it does give a pretty good general picture.
Steps: not totally accurate but pretty good ballpark. (I have counted my steps for short periods to compare).
Detecting exercise: I have been trying the occasional short, outdoor bike ride. It recognizes this, times it and gives my average HR for the ride--but it only recognizes it about 2/3 rds of the time.
Another consideration is EMFs. They all emit some but smart phone type tracker seem to emit more. Most trackers of all sorts use Bluetooth.
Hope this helps.