How to read your Polar Nightly Recharge

Nightly Recharge includes ANS charge, on a scale from −10 to +10, where 0 is your usual level over the last 28 days.

Polar measures it over roughly the first four hours of sleep, then gives an overall status from “very poor” to “very good”.

Polar’s developer API reports a night-time HRV average and beat-to-beat intervals for Nightly Recharge. We have not confirmed from Polar which HRV formula that average uses, so we do not label it RMSSD or SDNN here.

HRV type: Night-time, first ~4 h of sleep (formula not confirmed).

Compare a night with your own usual range over the last weeks, not with another person or another device. A single low night is most often a reaction to something specific — a short night, alcohol, an infection, stress or hard training. Several days below your range together with a higher resting heart rate and feeling unwell are a reason to rest.

Scores such as Readiness, Recovery, Nightly Recharge or HRV Status are each maker’s own algorithm. Two devices on the same night can show different HRV numbers because they measure at different times and with different formulas; neither is “wrong”.

Polar has a public API (AccessLink). We plan a direct connection, but it is not available yet and we cannot give a date.

Through Apple Health

According to Polar’s own support page, Polar Flow does not write HRV to Apple Health, so your Polar HRV will not reach ONDA that way. If your device is not Apple Watch, ONDA can still measure your pulse with the iPhone camera, along with a breathing-rate estimate. The camera gives pulse, not HRV.