Heart rate variability is the tiny variation in time between one heartbeat and the next, measured in milliseconds. A healthy, rested nervous system doesn't beat like a metronome. It speeds up and slows down constantly, and more of that variation is generally a good sign. Wearables track HRV overnight and use it as a proxy for how recovered you are.
Why it matters
HRV is a window into your autonomic nervous system: the tug-of-war between the stress side and the rest-and-recover side. Hard training, bad sleep, illness, alcohol, and life stress all tend to suppress it. Solid recovery tends to restore it. That makes it one of the few numbers that reflects total load on your body, not just what happened in the gym.
One thing first, though. HRV is wildly individual. Your friend's 90 and your 45 can both be perfectly healthy baselines, so comparing scores across people is pointless. And a single low morning means almost nothing. You slept weird, you measured at a different time, the sensor slipped. The signal lives in your multi-day trend against your own baseline. Nowhere else.
How to use it in training
Wear your device consistently for 3 or 4 weeks to establish a baseline before trusting anything it says. After that, treat a sustained dip (several days running below your normal range) as useful information: an easier session, an earlier night, one less drink.
Don't let it become the boss. If you feel strong and HRV says you're wrecked, go train. Feel and performance outrank the metric, which works best as a tiebreaker on days you're genuinely unsure.
Related terms
Go deeper
Want the studies on HRV-guided training? Read our full breakdown: Heart rate variability and training research.