This feature came to Garmin now, however I still need to verify accuracy. From last night, the Fitbit HRV was 67ms while from Garmin 74ms. Neither may be true but Garmin gives me a little more data so I can try to verify it against other devices:
It looks like HRV overnight is all over the shop and that could be the case for Fitbit, too. Correct or not, Garmin gives me a time domain and actual data from the night (including same graph I can view on the watch right after waking up). I'm sceptical about the accuracy (in general, I consider wrist sensors being of lower accuracy) and will check it against chest strap HRV. I like direction Garmin took that feature and Fitbit again has something to catch up with.
Best Answer@Loboto the HR will be lower during sleep. However, it is a common misconception that HRV also would be lower. It doesn't have to be as it is derived from the different data sets. In fact, R-R intervals may have more variability during rest as there is more time between each beat. If you test HRV during or after activity, you will notice that this is the time when HRV goes way low. It makes sense at the data source but then the RMSSD comes into the picture and the way it is calculated. So no, HRV doesn't have to be lower during sleep. The HRV is calculated within a certain time domain (in Fitbit it is unknown). My night HRV is in fact quite high (60+, sometimes over 80ms). Lower than what Garmin tracks but not by much and it's pretty much in the range of statistical error.
If I compare averages on both platform the trend line shows very similar shape:
The Garmin Connect values are given at the bottom (didn't include in the picture) and they are: 87ms, 75ms, 75ms, 86ms, 73ms, 83ms, and 72ms. Also, for the last two days I have had no rolling average (blue point) because of the new watch (should be back tomorrow). This is in fact a quite good result. It is very unlikely two devices would agree and be both wrong. If results would be much different (looking at the trend line) then it's inconclusive as both devices may be wrong or only one may be right. Right now, my spot HRV (2min domain) is 88ms but that will vary during the day and won't be much different from my sleep HRV (sometimes my sleep HRV is higher).
For the same HR, the HRV may vary a lot. It all depends on the context of taking measurements. Here's a simple example. I took an HRV snapshot with EliteHRV after the cooldown.
For a very similar range of HR, there are two vastly different RMSSD outputs. And yet, in the 5min domain, I sometimes get HRV over 100ms during sleep (last night the highest 5min RMSSD was 120ms).
What does it all mean? It means only that HRV is a very complex metric and understanding it isn't so simple. It isn't the same as resting HR when we know exactly that lower RHR is better, higher is worse. HRV depends on many things and the way it's measured may affect the values greatly. Panic over HRV makes no sense. There is a reason why machine learning algorithms are used in order to analyze the HRV data and draw conclusions from it. Because humans can't do it. Humans look for simplicity and binary answers but HRV won't deliver it.
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Here's another example from Today:
RMSSD is 92.7ms, 2.39ms, 37.47ms.
The middle one is taken right after the 10th interval (1min nearly-all-out + 1min recovery). The conclusion could be that the higher HR, the lower HRV and it makes sense as with high HR there is little room left for R-R intervals. This means that during the night we should be getting the highest RMSSD because when we sleep the heart slows down even more. That's not the case. As much as right after intense exertion the RMSSD will drop, for other states there is no real connection between HRV and HR other than whatever comes out of R-R intervals. The only conclusion I may think of is that if RMSSD was high during intense intervals then that could indicate skipping heartbeats. Anyway, notice the huge drop in RMSSD between pre-workout and post-workout readings. That's why "working out all time" may in fact be counter-productive and get reflected in HRV.
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