To measure HRV, a sensor records the exact time between your heartbeats and an app turns those gaps into a number, usually RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next. It mainly reflects vagal activity. More about RMSSD . A chest strap that reads your heart’s electrical signal ( ECG ECG A recording of the heart's electrical activity, taken with electrodes on the skin. ECG-based RR intervals are the reference standard for measuring HRV. More about ECG ) is the most accurate home option, while rings, watches and phone cameras that read your pulse with light ( PPG PPG An optical method that shines light into the skin and detects the pulse of blood with each heartbeat. Most watches, rings and phone-camera apps use it to estimate HRV. More about PPG ) come close at rest if you measure the same way every time.
The ways to measure HRV at home
There are four common home options, plus the clinical ECG your doctor might use.
| Method | Sensor | What it detects | When it usually measures | Number shown | Accuracy at rest | Good for |
|---|---|---|---|---|---|---|
| Chest strap | ECG | Electrical signal of each beat | A morning reading you start yourself | RMSSD, sometimes lnRMSSD lnRMSSD The natural logarithm of RMSSD. Taking the log evens out the spread of RMSSD values, which makes day-to-day changes easier to compare and analyze. More about lnRMSSD | The home reference | Precise morning readings, athletes |
| Smart ring | PPG (finger) | Pulse wave at the finger | Automatically, during sleep | Nightly RMSSD-based value | Close to ECG in overnight studies | Passive tracking with little effort |
| Watch or band | PPG (wrist) | Pulse wave at the wrist | During sleep, or spot readings | RMSSD-based value, SDNN SDNN The standard deviation of all NN intervals in a recording, in milliseconds. It measures overall variability, and its value depends strongly on how long the recording lasts. More about SDNN on Apple Watch, or a status label | Close to ECG at rest; varies by model | People who already wear one |
| Phone camera app | PPG (fingertip on the lens) | Pulse wave at the fingertip | A short reading you start yourself | RMSSD or a score | Close to ECG at rest in a small study | Trying HRV with no extra kit |
| Clinical ECG or Holter | ECG | Electrical signal, several leads | In a clinic, or over 24 hours | Several measures | Reference standard | Medical assessment |
Wearables change with each model and software update, so check what your own device shows in its app or manual. Device by device details are on HRV on Oura, Whoop, Garmin and Apple Watch.
What every method measures: the time between beats
All of these methods do the same basic job. They find each heartbeat, time the gap to the next one, and list those gaps in milliseconds.
An ECG picks up the electrical signal that makes the heart contract. Each beat has a sharp spike called the R peak, so the gap between two spikes is the RR interval RR interval The time from one heartbeat to the next, measured in milliseconds between the R peaks of an ECG. HRV is calculated from a series of these intervals. More about RR interval . Because the spike is sharp, its timing is precise.
A PPG sensor shines light into the skin and measures how much bounces back. Each heartbeat pushes a wave of blood through the small vessels, which changes the reflected light. The sensor times the gap between pulse waves instead of electrical spikes. HRV measured this way is called pulse rate variability Pulse rate variability Variation in the time between pulse beats, measured optically by watches, rings and phone cameras. It closely tracks HRV at rest but drifts from it during movement. More about Pulse rate variability (PRV). It is a close stand-in for HRV but not an exact copy. At rest it tends to show slightly more short-term variation than the ECG, partly because breathing affects how each pulse travels through the blood vessels (Schäfer and Vagedes, 2013).
Once the app has the list of gaps, it applies a formula. The lesson on how HRV is calculated walks through it with real numbers.
ECG vs PPG: which is more accurate?
An ECG is the reference. The electrical spike is easy to time and does not depend on blood flow.
PPG works well when you are still. A review of studies comparing the two found PPG accurate enough at rest, though it tends to overestimate short-term variability slightly. Physical activity and some mental stressors made the agreement much worse, sometimes to an unacceptable level (Schäfer and Vagedes, 2013). A loose ring or watch strap adds to the problem, because the sensor loses steady contact with the skin.
That is one reason most wearables measure HRV while you sleep, when you are still for hours. For a fuller comparison, read is wrist HRV as accurate as a chest strap?
Which HRV number you get
The same heartbeats can give quite different numbers depending on the formula.
- RMSSD looks at how much each gap differs from the one before. It is the main time-based measure of the vagal (rest-and-digest) influence on the heart (Shaffer and Ginsberg, 2017). Most rings, bands and watches report a value based on it.
- SDNN is the standard deviation of all the gaps in the recording. It captures slower swings too, and it grows with recording length. Apple Watch stores HRV as SDNN (Apple Developer Documentation).
- lnRMSSD is the natural log of RMSSD. Some athlete apps use it, sometimes multiplied into a 0 to 100 style score.
Because of this, a ring and an Apple Watch worn on the same night can show different numbers with neither being wrong. The lesson on RMSSD, SDNN and lnRMSSD explains each one.
When to measure: overnight or in the morning
There are two main approaches.
Overnight. Rings, bands and most watches measure during sleep and report one value for the night. You do nothing, and the long window smooths out noise. But each maker chooses its own window: the whole night, part of it, or only deep sleep.
Morning. You take a short reading yourself right after waking, usually lying down. This is how many athletes and researchers do it. One large study of app users took 1 to 5 minute readings right after waking, lying still, with either a phone camera or a chest strap (Altini and Plews, 2021).
Both approaches work if you stick with one. Overnight and morning values are not interchangeable, so do not mix them in one trend. Read more in overnight vs morning HRV.
How long a reading needs to be
The traditional standard for a short-term recording is 5 minutes (Shaffer and Ginsberg, 2017). For RMSSD at rest, much shorter readings work. In a study of 3,387 adults, a single 10-second ECG already gave a valid RMSSD, and 2-minute readings matched the 5-minute reference almost perfectly. SDNN needed at least 30 seconds, or several 10-second readings averaged (Munoz and colleagues, 2015).
Shaffer and Ginsberg add a caution: resting readings as short as 1 minute may be enough if errors are carefully removed, but short readings should not be swapped freely with 5-minute or 24-hour values (Shaffer and Ginsberg, 2017). So 1 to 5 minutes is fine for a daily RMSSD, as long as you always use the same length.
Clinical recordings work on a different scale. A Holter monitor records the ECG for 24 hours while you go about your day. Over that length, SDNN is the standard measure doctors use to estimate heart risk (Shaffer and Ginsberg, 2017). A 24-hour SDNN describes a whole day of activity, sleep and stress, so it cannot be compared with a 2-minute morning RMSSD or an overnight ring value.
Go deeper Why SDNN depends so much on recording length
SDNN measures the spread of all the gaps in a recording. A longer recording captures slower swings, such as changes with posture or the body clock, so SDNN keeps growing as the recording gets longer. That is why a 5-minute SDNN, a 24-hour SDNN and a short spot reading on a watch cannot be compared directly. RMSSD looks only at neighbouring beats, so it is far less sensitive to length.
How to take a reliable reading
For a morning reading, keep it simple and keep it the same:
- Measure at the same time, ideally right after waking.
- Use the same position each time, lying or sitting.
- Stay still and silent for the whole reading.
- Breathe normally. Do not try to slow your breathing on purpose.
- Measure before coffee, exercise or checking messages.
- Use the same device, app and reading length.
The lesson on how to take a good HRV reading gives the full routine.
What ruins a reading
Two kinds of error creep into the list of beat-to-beat gaps:
- Technical errors, also called artifacts Artifact An error in a heartbeat recording, such as a missed, extra or mistimed beat, often caused by movement or poor sensor contact. Artifacts can badly distort HRV numbers. More about Artifact . Movement, poor sensor contact or a loose electrode can make the device miss a beat or count one that is not there.
- Physiological errors. Extra beats ( ectopic beats Ectopic beat A heartbeat that starts outside the heart's normal pacemaker and usually comes early. Ectopic beats are common, but they distort HRV unless removed or corrected. More about Ectopic beat ) and atrial fibrillation are real, but they are not the kind of variation HRV is meant to capture.
Both are common, and a single missed or extra beat can change the result a lot. Researchers treat finding and editing these beats as an essential step, and the method used can itself change the numbers (Peltola, 2012). Apps do this automatically, but no filter is perfect. Talking or shifting position during a reading adds movement too, so treat any odd one-off value with suspicion.
How close are wearables to an ECG?
During sleep, the best of them come close. In an independent study, 13 healthy adults wore several devices at night alongside a chest-strap ECG (Polar H10) over 536 nights. For overnight RMSSD, the agreement with the ECG was (Dial and colleagues, 2025):
- Oura Ring Gen 4: concordance 0.99, average error about 6%
- Oura Ring Gen 3: 0.97, about 7%
- WHOOP 4.0: 0.94, about 8%
- Garmin Fenix 6: 0.87, about 11%
- Polar Grit X Pro: 0.82, about 16%
Concordance runs from 0 to 1, where 1 means perfect agreement. The average error is the typical gap between the device and the ECG, as a percentage. This was a small study of healthy adults, and it tested specific models. Apple Watch was not included. The details and other studies are on HRV by device.
Measuring HRV without a wearable
You do not need a ring or watch. Two cheaper routes work at rest:
- A chest strap and a free app. This gives ECG-quality beat timing for a morning reading.
- A phone camera app. You rest a fingertip on the lens and flash. In a study of 29 healthy adults, smartphone camera readings of RMSSD agreed closely with ECG during 5 minutes of rest (Plews and colleagues, 2017). Results depend on the app and on keeping still.
Read more in can a phone camera measure HRV?