At rest, and especially during sleep, a good wrist or ring sensor can come close to a chest strap, but a chest strap is still more accurate, and the gap grows as soon as you move. A chest strap reads the heart’s electrical signal directly, while a wrist sensor works out each beat from the pulse in your skin, which is easier to disturb.
Why the two sensors differ
HRV is built from the time between heartbeats, so everything depends on timing each beat exactly.
- A chest strap picks up the heart’s electrical signal, like a simple 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 . Each beat makes a sharp spike, which is easy to time.
- A wrist or ring sensor shines light into the skin and measures how much comes back as blood pulses through ( 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 ). The pulse wave is rounder than an ECG spike, so its exact timing is harder to pin down.
The pulse also has to travel from the heart to your wrist before the sensor can see it. A review of studies comparing pulse sensors with ECG found that pulse-based readings tend to slightly overestimate short-term variability, which the authors linked to the way breathing and the circulation interact. The same review found good accuracy at rest, but movement and some mental stress made agreement much worse, often to an unacceptable degree.
What the studies show
During sleep
A 2025 study followed 13 healthy adults (mean age 33) at home for 536 nights, with a chest strap as the reference. For overnight 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 , the ring and wrist devices tested had average errors from 5.96% (Oura Gen 4) to 16.32% (Polar Grit X Pro), with WHOOP 4.0 at 8.17% and Garmin Fenix 6 at 10.52%. So at night some optical devices were close to the strap and others were further off. The study was small and only included healthy adults.
Sitting still
A 2021 study of five healthy young adults compared several devices with a multi-lead ECG during seated recordings, over 148 trials in total. A chest strap paired with a phone app had the smallest error for RMSSD, 4.10%. A finger ring was next at 6.84%. A different chest-strap system did worse, at 11.27%, and it used three-minute recordings instead of five. The authors concluded that the length of the recording had a bigger effect on accuracy than the type of sensor.
Moving and exercising
Chest straps hold up best here. In a study of 10 healthy adults doing activities from easy to hard, a chest strap timed 99.6% of beats correctly overall, and 99.4% during high-intensity exercise, judged against the raw ECG trace. The authors suggested a simple chest strap could serve as the reference for beat timing during hard exercise.
Which should you use?
- For an overnight trend, a ring or watch you wear every night is a reasonable choice. Compare it only with itself, and see the HRV by device guide for what each brand reports.
- For a morning reading, either works if you stay still. A chest strap gives the closest match to an ECG, and a longer recording helps.
- During or right after exercise, use a chest strap. Optical sensors struggle with movement.
- If you switch sensor type, expect your numbers to shift and give yourself a few weeks of new data before comparing.
For a fair reading from any sensor, wear it snugly, keep still and measure at the same time each day. The how HRV is measured guide covers the methods in more detail, and can a phone camera measure HRV? covers the cheapest option.
When to talk to a doctor
Neither a sports chest strap nor a wrist sensor is a medical ECG, and HRV cannot diagnose a heart condition. Talk to a doctor if you notice palpitations, a racing or uneven heartbeat, dizziness, fainting, chest pain or shortness of breath.