HRV value messages store the heart rate variability measured during the night, every 5 minutes.
timestampDateTimeMoment the message refers to, in UTC. FIT stores it as seconds since the FIT epoch (December 31, 1989, 00:00:00 UTC); decoders usually expose it as a regular date.
Heart rate variability (RMSSD) over 5 minutes.
mesgNumStringGlobal message number identifying the kind of message. Decoders use it to know which profile to apply to the data.