Glossary
PPG
Also called photoplethysmography, optical heart rate sensor
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.
Photoplethysmography (PPG) is the light you see flashing on the back of a watch or inside a ring. The light shines into the skin, and a sensor measures how much comes back. Each heartbeat pushes a little more blood into the small vessels under the skin, which changes how much light is absorbed. The result is a wave with one bump per heartbeat. Pulse oximeters in hospitals work on the same principle.
To get HRV, a device times the gap from one pulse bump to the next. Strictly, that gives 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), not heart rate variability: it times the pulse wave in the skin, not the heart’s electrical signal. The two are close but not identical.
A review of studies comparing PPG with 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 found the agreement good enough when people are at rest, although PPG tends to slightly overestimate short-term variability. Physical activity and some mental stressors made agreement worse, sometimes to an unacceptable degree.
This is one reason many wearables measure HRV during sleep or ask you to keep still for a spot check. When a PPG reading looks odd, think about whether you were moving before reading anything into it.
The lesson Why your devices disagree on HRV and the question Is wrist HRV as accurate as a chest strap? go further. The lesson How HRV is measured: from beats to a number shows where PPG fits in the measurement chain.