Heart rate variability. Resting heart rate. Body battery. Stress score. Recovery advisor. Your smartwatch produces a daily readiness assessment in various forms, and most runners have no idea what to do with it.

Some of these metrics contain real signal. Some are noise. Knowing the difference is the difference between data-informed training and data-distracted training.

Heart Rate Variability (HRV)

HRV measures the variation in time between consecutive heartbeats. A healthy, well-recovered nervous system produces irregular spacing — beat-to-beat intervals of 0.82s, then 0.91s, then 0.85s. A stressed or fatigued system produces more regular spacing.

Higher HRV generally indicates better autonomic nervous system balance — your body is in a parasympathetic (rest and recover) state. Lower HRV suggests sympathetic dominance — your body is still in fight-or-flight mode, whether from training stress, poor sleep, illness, or life stress.

What the research says: In elite endurance athletes, sustained HRV suppression (7+ days below baseline) correlates with overreaching and underperformance.[1] A single day of low HRV means almost nothing. The trend matters. The absolute number doesn’t.

What your watch does: It takes a single or averaged measurement and presents it as a daily score. The quality of this measurement varies enormously between devices — medical-grade chest straps are accurate, wrist-based optical sensors less so. The algorithms that convert raw HRV into a “readiness score” are proprietary and unvalidated against any published research.

Resting Heart Rate

Resting heart rate is simpler and more reliable. Your heart rate when fully at rest (measured first thing in the morning, before getting out of bed) follows predictable patterns:

As you get fitter: Resting heart rate decreases. Your heart pumps more blood per beat, so it doesn’t need to beat as often.

When you’re overtrained or getting sick: Resting heart rate increases — often 5-10 beats above baseline. This is one of the oldest and most reliable signs of accumulated fatigue.

Day to day: Expect 3-5 beat variation from normal factors like hydration, sleep quality, and caffeine timing.

Resting heart rate is the one readiness metric that has decades of coaching evidence behind it. It’s simple, any device measures it accurately, and a sustained elevation (3+ days above your personal baseline) is a meaningful warning sign.

Body Battery / Stress Score / Recovery Rating

These are proprietary composite scores that combine HRV, resting heart rate, sleep data, and activity data into a single number. Garmin calls it Body Battery. Whoop calls it Recovery. Apple calls it various things depending on the feature.

The problem: you can’t see how the score is calculated, you can’t validate it against your actual readiness, and the correlation between these scores and running performance is too noisy to be actionable at the individual level.

Your watch might say you got “poor recovery” when you feel great. Or “fully recovered” when you’re exhausted. This isn’t because the technology is broken — it’s because reducing a complex physiological state to a single number requires assumptions that don’t apply equally to every person.

Why Pacewright Doesn’t Use These Metrics

Pacewright stores HRV data from Garmin but doesn’t use it in any algorithm calculation. This is a deliberate design decision, not a technical limitation.

The reason: RPE already captures the downstream effect of all readiness factors. If you slept badly, your RPE will be higher for the same effort. If you’re getting sick, your RPE will climb. If you’re overtrained, your RPE will trend upward across sessions.

Training load — calculated as duration × RPE — inherently accounts for readiness because it measures how hard the work actually felt, not how hard a watch predicted it should feel. A “bad readiness” day that produces normal RPE on an easy run doesn’t need intervention. A “good readiness” day where your easy run feels like RPE 7 needs attention — regardless of what your watch says.

What to Actually Do With This Data

Track resting heart rate. It’s the most reliable, most validated readiness metric available to recreational runners. Take it every morning. Note your baseline. If it’s elevated 5+ beats for 3+ days, consider an easy day or a rest day.

Watch HRV trends, not daily numbers. A single HRV reading is noise. A week of suppressed HRV is signal. If you’re going to use HRV, use a 7-day rolling average and only react to sustained changes.

Ignore composite scores for training decisions. Use them as interesting background information if you want. Don’t use them to decide whether to run today. Your RPE during the first 10 minutes of running is a better readiness indicator than any pre-run watch score.

Trust your body over your device. If you feel great and your watch says you’re not recovered — run. If you feel terrible and your watch says you’re fully recovered — take it easy. The subjective signal is more reliable than the objective measurement for training purposes, because the subjective signal integrates everything — physical, mental, emotional — while the watch measures a subset of physical markers.