After being involved in hrv research and using it regularly with athletes for almost four years I would give a cautionary tale about relying on the apps only too much. These decisions are not as simple as it would appear and definitely are not infallible.
While the outline @stevemz provides “can” be useful and would not likely do harm, it can be misleading and very simplistic if you are looking to optimize training. Far more needs to be taken into consideration. Something as simple as what has been eaten can affect hrv and throw things out of whack. Same with fluids, sleep auality and quantity, and breathing.
Readings taken minutes apart can give completely different suggestions. Not breathing naturally or breathing in a controlled manner can alter results as well. Not just a little either. Even readings between two separate apps moments apart can give completely different recommendations. I now only use two at a time but at one point was using six apps in the morning to determine accuracy and reliability with research subjects. I can assure you it is not as simple as we would like to think and neither are the recommendations from the apps.
As an example, having had an “easy” recovery week as per a TR plan and then starting into a new plan the app suggested that recovery was needed by one app and the other app suggested proceed as usual. Then later in the week the exact opposite were the suggestions from the two apps.
I use the inner balance app regularly for meditation and breathing and it is surprising how much breathing, breath rate and thought patterns affect hrv.
To illustrate this, I did a sample reading (see below) with controlled breathing and because this is able to increase my hrv above my baseline it gave me a suggestion to tak it easy due to a higher than usual reading despite positive trending subjective scores. This was moments after a “good” score. The same thing happens when someone is relaxed and focussed on heart breathing. Moments later with normal breathing (not thinking about it) and anxious thoughts can produce exactly the opposite results. Then throw in what happened physically within the day’s or weeks proceeding this, as well as nutrition or hydration, and you can receive suggestions that aren’t necessarily optimal.
Breathing can change reccomendation examples:
Hrv while “can” be helpful, it should not be the only thing taken into consideration. I have tons more examples if people are interested but this should highlight the potential pitfalls of using hrv alone. Long term trends are far more useful than the day to day readings typically.