What is HRV and why does it matter?
HRV (Heart Rate Variability) is the small variation in time between consecutive heartbeats, measured in milliseconds. Higher HRV generally indicates better cardiovascular fitness, lower stress, and good recovery. Lower HRV can signal stress, illness, overtraining, or poor sleep. Most modern wearables (Whoop, Oura, Apple Watch, Garmin, Fitbit) measure HRV continuously, and it's become the single most useful metric for daily readiness.
What HRV actually measures
Your heart doesn't beat at exactly the same interval — there's a tiny variation between beats controlled by your autonomic nervous system. When you're relaxed, your nervous system is flexible and HRV is high. When you're stressed, sick, or recovering from hard exercise, HRV drops.
What's a "good" HRV number?
HRV is highly individual — comparing your number to someone else's is meaningless. What matters is your personal baseline and the trend. A 7-day rolling average is more useful than a single-night reading. Most adults fall between 20-80ms (RMSSD measure).
How wearables use HRV
- Whoop Recovery score — primarily HRV + RHR
- Oura Readiness — HRV is the biggest input
- Garmin Body Battery — HRV + activity
- Apple Watch — measures but doesn't heavily score
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