Heart rate variability

Heart rate variability is the variation in the length of time between
heartbeats, measured in milliseconds.1 It is different from heart
rate.

Garmin calls it body battery and my latest Garmin watch measures it.
I've been ignoring it until just recently I've noticed that I have a
much better bike ride when I have a high HRV.

My Garmin Battery rating was 100% when I rode sixty miles May 16th. I
had a great ride and came home feeling great.

I had a miserable night last night, woke up with a headache, a higher
than normal temperature and blood pressure. Garmin said my body
battery was at 60 this morning.  At twenty miles out on my ride today
I knew I should turn around and go back. I came home totally beat.