All this has been discussed in great detail in this huge thread:
You will find numerous links to trustworthy sources (surprise: Garmin is not part of that 
)
Some initial comments:
- HR from a wrist optical sensor is already not of great quality. HRV from it is worthless.
- HRV over 24hr is meaningless. It will just tell you exactly the same as Total Exercise Time or Load. Exercising is stress inducing, thus 24hr HRV will simply be lower on days that you exercised longer/harder. We already know that so it’s useless. HRV is a response parameter and must be measured at a consistent time, with a consistent protocol and a good quality sensor. Polar H10 connected to a good app like Kubios, EliteHRV, HRV4Training, etc will give you a usable result if you measure in the morning right after waking up. This will show how well your body is coping with the life-style stress and training from the former day. It will show how good you are recovering from it.
- An upwards trending HRV does NOT mean that you are getting fitter. It means that you are better at coping with the actual life style stress, including training. If for some reason, you stop working out, HRV will increase because you will fully recover. But no longer exercising isn’t going to make you any fitter, right.
There are plenty of other remarks, but I’m going to stop here because all the info is available in the above mentioned thread.