AuraloWhat 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, or HRV, measures how much those gaps vary.
That variation comes largely from your nervous system adjusting your heart from moment to moment. One branch speeds things up when you need energy. The other slows things down when you rest. When the resting side has room to work, the gaps vary more and HRV tends to be higher. When your body is busy with a hard session, a short night or a cold, the gaps get more even and HRV tends to be lower.
That’s why HRV is a good everyday signal of how recovered you are. It isn’t a fitness score and it isn’t a grade. It’s a reading of how much slack your body has today.
Why it changes from day to day
HRV is noisy by nature. It moves with what you did yesterday, how you slept, what you drank and even what time you happened to be measured. Breathing changes it too: the gaps stretch a little as you breathe out and shorten as you breathe in.
So a single reading tells you very little. A reading taken while you were talking or walking around says even less. What matters is the pattern over nights and weeks.
How Apple Watch measures it
Apple Watch takes HRV readings in the background a few times a day, when you’re still. If you wear it to bed with sleep tracking on, you also get readings overnight, and those are usually the most reliable because you’re lying still for hours. Starting a session in the Mindfulness app takes a reading too.
Each reading is saved 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 some days have several and some have only one or two.
Many other wearables report RMSSD, a different formula that focuses on beat-to-beat changes, and they measure at different times of night. Both are valid, but they’re not the same number. Compare each device only with itself.
Your baseline beats any average
There’s no universal good number. HRV tends to fall 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.
The useful question isn’t “is my HRV high?” It’s “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. In the example below, most nights land inside the green band. Two dips stand out, and both have an obvious cause. Neither is a reason to worry on its own.
A baseline needs time. In the first week or two, numbers move a lot because there isn’t much history to compare with. After three or four weeks of nights, the band settles and a dip starts to mean something.
What lowers and raises HRV
Things that tend to lower it
- Short or broken sleep, or a bedtime much later than usual
- Alcohol, especially close to bedtime
- Hard training, especially several hard days in a row
- Illness, such as the start of a cold
- Stress, late heavy meals, heat, travel and time-zone changes
Things that tend to support it
- Regular sleep at roughly the same times
- Regular aerobic exercise, built up over weeks, with easy days in between
- Less alcohol, and none in the last few hours before bed
- Downtime: a walk, slow breathing, anything that helps you switch off
These shifts are gradual. A good week of sleep can nudge your baseline up over a month, not overnight.
How to use it day to day
- Look at nights, not single readings. Overnight values are steadier than daytime ones.
- One low night is normal. Treat it as a hint to take the day a little easier, not as a verdict.
- Several nights 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.
- Check how you feel. HRV is one input. If you feel great and the number is a little low, that’s fine.
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.
Key takeaways
- HRV measures how much the gaps between heartbeats vary. Higher usually means more slack to recover.
- Apple Watch saves SDNN. Other devices often use RMSSD, so don’t compare brands.
- There’s no good number for everyone. Compare with your own last few weeks.
- One low night is normal. Several in a row is worth acting on.
- Sleep, alcohol, hard training and illness move it most.
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 sits 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. If you wear another device, see which wearables share HRV with Apple Health.
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.
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. HRV can’t diagnose anything. If something feels wrong, talk to a doctor.
