HRV Guide

How to Measure HRV

Heart rate variability can be measured with a clinical ECG, a chest strap, a smartwatch, a ring, or even your phone's camera. But not all methods are equal, and how you take the reading matters as much as what device you use.

The Two Sensing Technologies

Every HRV device uses one of two fundamental approaches to detect heartbeats. Understanding the difference matters because it directly affects accuracy.

Electrocardiography (ECG) detects the electrical signals that trigger each heartbeat. Chest strap heart rate monitors like the Polar H10 use this method, as do clinical Holter monitors. ECG is the gold standard because it detects the precise electrical R-wave, giving you true RR intervals with millisecond accuracy.

Photoplethysmography (PPG) uses light to detect blood volume changes in your blood vessels as each pulse of blood passes through. Wrist-worn devices (Apple Watch, Garmin, Fitbit), smart rings (Oura), and phone camera apps all use PPG. What they measure is technically Pulse Rate Variability (PRV), not HRV, because they detect the pulse at the periphery rather than the electrical heartbeat at the source. Under resting conditions in healthy individuals, PRV closely approximates HRV. During exercise, cold exposure, or in certain health conditions, the two can diverge.

Device Options

ECG Chest Strap

Polar H10, Garmin HRM-Pro, Wahoo TICKR X

A chest strap with ECG electrodes remains the most accurate consumer option for HRV measurement. The Polar H10 in particular has been extensively validated in peer-reviewed research and is the sensor we use in our own HRV study. Chest straps capture true RR intervals and work reliably during both rest and activity.

True ECG accuracy. Extensively validated in research. Works during movement. Affordable.

Requires wearing a chest strap. Less convenient for overnight or habitual daily use than wrist devices.

Smartwatch (Wrist PPG)

Apple Watch, Garmin, Samsung Galaxy Watch, Fitbit

Modern smartwatches measure HRV automatically, typically during sleep or on demand. Apple Watch uses green LED PPG and reports SDNN from overnight readings. Garmin and others report various metrics. Wrist PPG accuracy is good at rest but degrades with motion, and can be affected by how tightly the watch is worn, skin tone, tattoos, and wrist hair.

Always on your wrist. Automatic overnight readings. No extra device needed. Good enough for trend tracking.

PPG, not ECG. Lower accuracy than chest strap. Motion artefacts. Metric reported varies by brand.

Smart Ring (Finger PPG)

Oura Ring, Ultrahuman Ring, RingConn

Finger PPG benefits from better blood flow at the fingertip compared to the wrist, which can improve signal quality. The Oura Ring in particular has been validated in several studies and reports RMSSD as its primary HRV metric. Rings are comfortable for overnight wear, making them well-suited to habitual daily tracking.

Comfortable for 24/7 wear. Good finger PPG signal. Oura specifically well-validated. Reports RMSSD.

Still PPG (not ECG). Sizing can be an issue. Battery life varies. Not all rings equally validated.

Phone Camera

HRV4Training, Welltory, Camera HRV

Some apps use your phone's camera and flash to measure PPG from your fingertip. HRV4Training is the only app that has been independently validated for this method. It works by detecting colour changes in your fingertip as blood pulses through. Quality depends heavily on holding still and consistent finger pressure.

No extra hardware. Free or low-cost. HRV4Training specifically is well-validated.

Requires holding perfectly still. Sensitive to finger pressure. Not all apps equally reliable.

How to Take a Good Reading

The most common mistake with HRV is not the device. It is the protocol. An accurate device used inconsistently produces meaningless data.

Measure at the same time each day

First thing in the morning, within a few minutes of waking, before coffee or significant activity. HRV varies dramatically throughout the day, so timing consistency is non-negotiable.

Use the same position

Sitting or lying down: pick one and stick with it. Sitting adds a mild orthostatic stressor that can make the reading more sensitive to changes, which is useful for athletes. Lying down gives a higher baseline reading.

Breathe naturally

Do not try to control your breathing during the measurement. Forced deep breathing directly modulates HRV and will distort the reading. Just breathe normally and stay relaxed.

Record for 1-2 minutes minimum

Shorter recordings are less reliable. Longer is fine but not necessary for RMSSD. The European Society of Cardiology recommends at least 5 minutes for research-grade short-term recordings.

Track the trend, not individual readings

A single HRV reading is a snapshot influenced by dozens of variables. Look at your 7-day rolling average. That is where the signal lives.

What we use at Vagal

Our HRV study uses the Polar H10 chest strap connected via Bluetooth to our web-based HRV monitor. We calculate RMSSD, SDNN, pNN50, and Poincare metrics (SD1/SD2) in real-time, with data relayed to our cloud infrastructure for before/during/after analysis of vagus nerve stimulation sessions. This gives us research-grade HRV measurement alongside live tVNS, something no consumer wearable currently offers.