They are both basically trying to tell you how well your body is coping and recovering from stress (BOTH physical and emotional).
Garmin with its ‘Body Battery‘ (which I have on my MARQ Expedition) does it over the whole day.
Polar with its ‘Nightly Recharge‘, per the unambiguous name, focuses on your nightly ‘recharge’ — with the emphasis on the early part of your sleep.
Polar’s ‘Nightly Recharge’ definitely gives you more data than I get with the Garmin, e.g., breathing rate, heart rate variability and beat-to-beat interval. (See images below).
Click images to ENLARGE.
Garmin’s ‘Body Battery’ feedback on Garmin Connect.
Polar’s ‘Nightly Recharge’ status on their APP.
As you can see from above, there are TWO aspects that are taken into consideration: autonomic nervous system (ANS) recovery and how well you slept.
The Sleep feedback which is not that dissimilar to what you can get with a Garmin.
It appears I am doing OK. I could have told you that without either the Garmin or Polar! SMILE. I sleep like a LOG. To say I sleep like a baby would be wrong. As a father of four I know that babies are fussy sleepers. I am not. I fall asleep fairly easily and when I do, I am GONE. I sleep deeply as far as I am concerned.
There is a very good reason as to why I sleep well. I am dead tired by the time I go to bed … usually after 1am in the MORNING. I also don’t get as much sleep as I would like. 8 hpurs would be a dream. 7 would be nice. I usually end up with something less than 7.
As long as I get up at my usual time, which is past 8am, I wake up RESTED and ready to go.
So, both the Polar and Garmin are basically confirming this. I sleep OK and am recharged when I get up.
Would be interesting to see what others get in terms of readings.