What HRV is
Your heart doesn't beat like a metronome. Even at a steady 60 beats a minute, the gap between one beat and the next keeps changing by a few milliseconds. Heart rate variability (HRV) measures how much those gaps vary.
That variation comes largely from your nervous system adjusting your heart from moment to moment. When you're rested, HRV tends to be higher. When your body is dealing with a hard session, a short night, alcohol or illness, it tends to be lower. That's why HRV is a good everyday signal of how recovered you are.
How Apple Watch measures HRV
Apple Watch samples HRV in the background while you're still, including overnight if you wear it to bed. It saves the result to Apple Health as SDNN: the standard deviation of the time between normal heartbeats, in milliseconds. How many readings you get depends on how still you are, so nights usually give the most reliable picture.
Other devices often report RMSSD, a different formula that focuses on beat-to-beat changes, and they may measure at different times of night. SDNN and RMSSD are both valid, but they're not the same number. Don't compare HRV across brands. Compare each device only with itself.
What's a good HRV?
There's no universal good number. HRV falls with age, and it differs a lot between people because of genetics, fitness, body size and how it was measured. A healthy 25-year-old and a healthy 55-year-old can be tens of milliseconds apart, and so can two healthy people of the same age.
So the useful question isn't "is my HRV high?" but "is my HRV normal for me?". That means comparing today with your own recent baseline: the range you usually sit in over the last few weeks.
What lowers HRV
- Poor or short sleep, or a late, irregular bedtime
- Alcohol, especially close to bedtime
- Illness, such as a cold or fever
- Hard training, especially several hard days in a row
- Stress, late heavy meals, heat, travel and time-zone changes
Regular exercise, consistent sleep and lower stress tend to support a higher baseline over weeks and months.
How to read your HRV trend
- Look at nights, not single readings. A reading taken while you were moving or talking says little.
- One low night is normal. Treat it as a hint to take the day a little easier, not as a verdict.
- Several days below your usual range is the signal worth acting on: sleep more, keep training easy and look for the cause.
- Read it with resting heart rate. HRV down and resting heart rate up together is a clearer sign than either alone.
- Give it a few weeks. A baseline needs time. Early numbers move a lot.
How Auralo uses your HRV
Auralo reads the SDNN your Apple Watch saves to Apple Health. Each night's HRV is the average of the samples inside your main sleep. It's compared with your own 30-day baseline: the median of the previous 30 days, not counting today. Auralo works in log space because HRV is skewed, and it never uses population thresholds.
- A baseline starts after 5 nights and counts as established from 14. Until then, the app says it's still learning.
- HRV is the largest part of the recovery score, at 45%.
- Days you tag as travel, illness or injury are left out of the baseline, so a cold doesn't reset your usual.
- Everything is worked out on your iPhone. Your HRV isn't uploaded. See the privacy policy.
HRV is best with a Watch worn overnight. Here's how to track sleep with Apple Watch. Wondering how Auralo compares with a dedicated wearable? See Auralo vs WHOOP, Oura and Bevel.
FAQ
What is a good HRV?
There isn't one number that's good for everyone. HRV varies a lot with age, genetics and fitness, so two healthy people can be far apart. A useful HRV is one that's in your own usual range. Compare today with your recent baseline, not with someone else.
Can I compare my Apple Watch HRV with WHOOP or Oura?
Not directly. Apple Watch reports SDNN, while some other devices report RMSSD, and they measure at different times. The numbers use different formulas, so compare each device only with itself.
Why did my HRV drop overnight?
Common reasons are a short or broken night, alcohol, a hard training day, a late meal, stress, travel or the start of a cold. One low night is normal. A drop that lasts several days is worth paying attention to.
Is higher HRV always better?
Usually a value at or above your own baseline is a good sign, but more isn't always better. An unusual jump can also come with illness or a disturbed night. Look at the trend and at how you feel.
How does Auralo use HRV?
Auralo averages the HRV samples your Apple Watch records during your main sleep and compares that with your own 30-day baseline. HRV is the largest part of the recovery score, at 45%. There are no population thresholds.
Try it at launch
Auralo turns your HRV, resting heart rate and sleep into a morning verdict and a plan for the day. It's free, with no account. Join the waitlist and we'll email you once, on launch day.
Sources
- Shaffer F, Ginsberg JP. An Overview of Heart Rate Variability Metrics and Norms. Frontiers in Public Health, 2017.
- Apple Developer. heartRateVariabilitySDNN (HealthKit documentation).
- Apple Support. Monitor your heart rate with Apple Watch.
- Apple. Using Apple Watch to measure heart rate, calorimetry, and activity (PDF, 2024).
Auralo gives wellness information, not medical advice. For medical concerns, contact a health professional.