A low HRV number on your Apple Watch is usually less alarming than it looks. Apple Watch measures a different kind of HRV than most rings and bands, it takes short readings at random moments, and one value on its own says very little. Here is how to read it.
Apple Watch measures SDNN, not RMSSD
Heart rate variability (HRV) is the small variation in time between heartbeats. Devices summarise it in different ways:
- •Apple Watch reports SDNN — the spread of beat-to-beat intervals — from short readings of about a minute, taken in the background a few times a day and during Breathe or Mindfulness sessions.
- •Oura, Whoop and Garmin report RMSSD, usually averaged over the night, when the body is most relaxed and HRV is at its highest.
Short daytime readings run lower than a night-time average, and SDNN and RMSSD are different numbers. So an Apple Watch HRV of 35 next to an Oura value of 55 does not mean one of them is wrong, or that your recovery is worse. Why your HRV differs on every device explains this in more detail. Newer watches also show a separate Recovery HRV; see Recovery HRV vs Overall HRV.
What is a normal Apple Watch HRV for your age?
HRV falls with age. Our SDNN ranges below are built from a large study of healthy adults measured with 5-minute ECG recordings (Voss 2015):
| Age | Typical range (25th–75th percentile) | Median |
|---|---|---|
| 18–34 | 35–60 ms | ~46 ms |
| 35–44 | 32–54 ms | ~42 ms |
| 45–54 | 27–44 ms | ~34 ms |
| 55–64 | 22–39 ms | ~29 ms |
| 65+ | 20–35 ms | ~26 ms |
These are population anchors, not targets. Apple Watch’s one-minute readings scatter more than a lab recording, so compare your weekly average, not a single value. You can check one number against these ranges in our HRV by age calculator — choose Apple Watch so it uses the SDNN table.
Why your reading might be low
The most common reasons are ordinary and temporary:
- •When the reading was taken. A reading during a busy afternoon, after coffee or while moving is often lower than one at rest.
- •Short sleep or a late night. HRV tends to dip after poor sleep.
- •Alcohol. Even a couple of drinks can lower HRV that night and sometimes the next day — see how much alcohol lowers HRV.
- •Hard training. HRV often drops for a day or two after a heavy session and recovers with rest.
- •Illness, stress or dehydration. HRV often falls before or during a cold or a stressful period.
- •A loose watch. Readings are less accurate if the watch moves on your wrist.
HRV also differs a lot between people of the same age. Some healthy people simply have lower HRV, so your own trend matters more than how you compare with others (Shaffer & Ginsberg 2017).
How to get a fairer picture
- •Look at the weekly average in the Health app, not single readings.
- •Wear the watch snugly, and wear it to sleep if you can, so more readings come from rest.
- •Compare like with like: the same device, at similar times.
- •If the trend is low, the usual levers are regular sleep, less alcohol, regular aerobic exercise and slow breathing — see how to raise HRV naturally.
When to talk to a doctor
A low HRV number on its own is not a diagnosis. Talk to a doctor if your HRV stays well below your usual level for more than a couple of weeks together with symptoms such as unusual tiredness, a racing or irregular heartbeat, dizziness or breathlessness. If you have chest pain, fainting or severe shortness of breath, contact emergency services now.
How ONDA helps
ONDA reads HRV and resting heart rate from your Apple Watch through Apple Health and builds your personal baseline from about two weeks of data, so you can see whether today’s number is unusual for you rather than for an average person. It is a wellness tool, not a medical device.