RHR and HRV: two numbers that actually mean something

You have two cheap, useful signals about how your body is handling training: resting heart rate (RHR) and heart rate variability (HRV). Both come from the same place — your autonomic nervous system, the part of you that runs in the background.

RHR is the simpler of the two. It's just your heart rate at rest, ideally measured in bed before you stand up. As your aerobic fitness improves, RHR tends to drop. Sleep loss, illness, alcohol, hard training, and life stress all push it back up. So a few days where your morning number is 5–10 beats above your usual is a real signal — your body is dealing with something.

HRV measures the tiny variations in time between consecutive heartbeats. Counter-intuitively, more variation is better. High HRV means your parasympathetic nervous system — the "rest and recover" side — is doing its job. Low HRV means you're in a more stressed, sympathetic-dominant state.

The honest caveat: a 2021 systematic review found that letting day-to-day HRV decide your workouts didn't dramatically outperform a well-designed fixed plan for fitness gains — the advantage was small, though it did show better-preserved vagal tone. So HRV isn't a magic dashboard. It's a useful sanity check, especially across multiple days.

What to do with this: track both for two or three weeks to learn your normal range. Don't react to single days. Watch for multi-day trends — RHR creeping up 5+ beats and HRV trending down together is your body asking for an easier day, not a single rough night's sleep.

Source: Bellenger et al., systematic review — HRV-guided training (PMC)