Heart rate variability (HRV) is how much the time between your heartbeats changes from one beat to the next, measured in milliseconds. A healthy heart doesn't tick like a metronome, and the size of that variation tells you how your autonomic nervous system is doing and how well you've recovered.
How HRV Is Measured
The time between two heartbeats is called the RR interval, and it shifts constantly. At a resting heart rate of 60 bpm, your beats don't land exactly every 1,000 ms. One interval might be 940 ms, the next 1,060 ms, the next 980 ms. HRV puts a number on that spread.
Sport science and most consumer wearables use RMSSD (Root Mean Square of Successive Differences). To get it, you take the difference between each pair of consecutive RR intervals, square those differences, average them, and take the square root. RMSSD is highly sensitive to parasympathetic activity and is widely regarded as the most reliable beat-to-beat HRV metric for judging recovery.
Two branches of your autonomic nervous system pull HRV in opposite directions. When the parasympathetic side (rest-and-digest) is in charge, variability rises, which means your body is calm, recovered, and ready for stress. The sympathetic side (fight-or-flight) lowers it, so low HRV reflects more sympathetic tone from hard training, poor sleep, illness, psychological stress, or alcohol. After a stressful week at work and a couple of short nights, your HRV can sit below your usual range even though you've barely trained.
HRV also tends to fall with age (Voss et al., 2015), but people of the same age vary widely and devices measure it differently, so there's no single chart your number should match. Trained endurance athletes often have higher resting HRV than untrained people their age, because of aerobic training's effect on the parasympathetic nervous system.
Apple's Health app has long reported HRV as SDNN, a different measure, so don't judge it against RMSSD numbers from other devices. See what a good HRV on Apple Watch is for how to read it.
How HRV Relates to Training
Athletes use HRV in three main ways. The first is daily readiness. You compare your overnight or morning HRV with a rolling 7-day and 30-day personal baseline. Elevated HRV is a green light for intense training, while suppressed HRV means you're under systemic stress and should reduce intensity or rest.
The second is spotting overtraining. HRV that stays below your personal baseline for several days can be an early sign of accumulated fatigue, especially when sleep, resting heart rate, or performance are slipping too. It isn't a reliable overtraining test on its own, since resting HRV often changes little when athletes overreach (Bellenger et al., 2016).
The third is long-term fitness. Months and years of aerobic training raise your baseline HRV as the parasympathetic nervous system adapts, so an upward trend over a training block is a sign of real aerobic adaptation.
If you take manual readings, the standard routine is:
- Measure immediately upon waking, before getting out of bed
- Remain lying down (supine) throughout the measurement
- Measure for 3–5 minutes at the same time each day
- Avoid caffeine, food, or significant movement prior to measuring
With FITIV you can skip this routine: by default it reads HRV from your main sleep each night, so there's nothing to measure when you wake up.
Either way, one reading on its own tells you little. Act on the trend over 7–30 days.
How FITIV Uses HRV
FITIV Pulse reads HRV passively from your Apple Watch through Apple Health, using the readings your watch records while you sleep by default. It can also read overnight HRV from a Garmin or Fitbit you've connected under the More tab, so there's no manual measurement session. You can have FITIV calculate RMSSD from the watch's beat-to-beat data, or use the HRV value Apple Health stores.
HRV is a major input to your daily Recovery score, compared against your own typical range, alongside resting heart rate, your Sleep score, and other overnight vitals. Recovery then feeds Training Readiness:
- Readiness 75–100 (High or Prime): ready for hard training
- Readiness 50–74 (Moderate): a balanced, moderate session is appropriate
- Readiness below 50 (Low or Poor): light activity or rest recommended
Where you'll see it:
- Vitals card: last night's HRV appears on the Today tab, labeled Typical, Above, Below, Well Above, or Well Below compared with your own typical range.
- Vitals Summary: tap the card for your typical range, a 30-day chart, and your 7-day and 30-day averages, so you can track how aerobic training is shifting your parasympathetic baseline over time.
- Recovery: HRV has its own row under Contributors on the Recovery detail screen.
Frequently Asked Questions
Is higher HRV always better?
In general, yes, within an individual's personal range. A higher HRV relative to your personal baseline indicates greater parasympathetic dominance and better autonomic recovery. However, extremely high HRV can sometimes indicate cardiac issues in clinical contexts. For healthy athletes, trending upward within your personal range is the goal. Do not compare absolute values to other athletes; HRV is highly individual.
What is the difference between HRV and resting heart rate?
Resting heart rate (RHR) measures how many times the heart beats per minute at rest, a measure of cardiac efficiency. HRV measures the variability between those beats, a measure of autonomic nervous system balance. Under stress, RHR tends to rise while HRV falls, and both improve with aerobic fitness, but HRV responds faster to daily changes in recovery state and is a more sensitive short-term marker. RHR is better for tracking long-term aerobic fitness trends.
Does stress at work affect HRV?
Yes, significantly. Psychological stress activates the sympathetic nervous system and suppresses HRV through the same pathway as physical training stress. Athletes under significant occupational or personal stress will often see HRV decline even during reduced training periods. FITIV's readiness score reflects total systemic load (not just workout-derived fatigue), which is physiologically accurate: your body does not distinguish the source of the sympathetic activation.
What is a good HRV score?
HRV is highly individual and not directly comparable between people. It tends to fall with age, but people of the same age vary widely, and different devices measure it in different ways. What matters most is your personal baseline: a reading well below your usual range tells you more than the absolute number.
