Heart rate variability (HRV) is the beat-to-beat variation in time between heartbeats. A higher HRV indicates that your autonomic nervous system is able to flexibly respond to changing demands — a sign of good cardiovascular fitness, efficient recovery, and low physiological stress. HRV declines with fatigue, illness, overtraining, poor sleep, and alcohol. Monitoring it over time reveals how well your body is adapting to the stress it faces.

rMSSD (root mean square of successive differences) is the most widely used HRV metric for short-term recordings and is the value reported by most consumer wearables — Garmin, Oura, Whoop, Polar, and Apple Watch all measure it. Take your reading first thing in the morning before getting out of bed, after at least 5 minutes of lying still, for the most consistent baseline.

Healthy adults show a wide range of rMSSD values. Normative data from Nunan et al. (2010) — a systematic review of over 20,000 participants — shows median rMSSD values of approximately 42 ms in people under 30, 38 ms in their 30s, 32 ms in their 40s, 27 ms in their 50s, and 22 ms in their 60s. Athletes typically score 20–40% higher. Your personal trend matters more than a single absolute reading: a consistent rMSSD that is rising (or stable) is a stronger positive signal than any one number.

The 7-Day Trend mode calculates a composite recovery score from your full week of morning rMSSD readings. It evaluates three things: (1) your average relative to age-adjusted norms; (2) whether your HRV is trending upward (last 3 days vs. first 4 days), stable, or declining; and (3) how consistent your readings are — high day-to-day variability in HRV itself is a sign of an unsettled nervous system. Together these signals give a more reliable picture of your recovery state than any single morning reading.