Sampling rate

A sensor does not watch your heart continuously. It takes snapshots, many times a second, and the sampling rate is how many. A device can only place a beat on one of its snapshots, so the time between snapshots limits how precisely it can time each beat.

Quigley and colleagues (2024) give the numbers for an ECG 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 : at 1000 Hz the R peak can be timed to about ±1 ms, at 250 Hz only to about ±4 ms. They recommend 1000 Hz and warn that lower rates add error, “especially for HRV metrics”. Laborde, Mosley and Thayer (2017) set a lower floor of 125 Hz for research, with 500 Hz as a safer choice when the breathing swing is small.

Why it matters for HRV. Timing errors land in the intervals, and RMSSD RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next, in milliseconds. It mainly reflects vagal activity. More about RMSSD is built from the change from one interval to the next, so it picks up the error directly. The smaller your real beat-to-beat change, the more a fixed error matters. Phone cameras record at about 25 to 30 frames a second, so each beat is rounded by up to 20 ms. Software can win some of that back by interpolating between samples, which works when the beat’s shape spans several samples, as a pulse wave does.

Try it below: set the rate to 25 Hz and RMSSD reads about a third too high; switch to the pulse wave and turn on interpolation to see most of the error disappear.

How finely each beat is timed

The teaching recording, resampled

Gap before each beat, first 60 seconds 780–980 ms; dashed is the true timing

Timing resolution

±0.5 ms

RMSSD, measured

33.4 ms

RMSSD, true

33.4 ms

SDNN

28.7 ms

pNN50

8.2 %

At 1000 Hz each beat is timed to the nearest millisecond. RMSSD matches the true value.

Sampling rate 1000 Hz, one sample every 1 ms 10005002501005025

Shape of each beat

ECG spike (narrow) Pulse wave (wide)

Interpolate between samples to refine each beat's time

The controls work when JavaScript is on.

A line of the gap between beats over the first 60 seconds of the teaching recording, with the true timing dashed and the device's timing solid. Readouts give the timing resolution in milliseconds, RMSSD as measured and as it truly is, SDNN and pNN50. A slider sets the sampling rate from 1000 down to 25 Hz, two radio buttons choose a narrow ECG spike or a wide pulse wave, and a checkbox turns on interpolation between samples. At 1000 Hz the numbers match; at 25 Hz without interpolation RMSSD reads about a third too high.

For more on how watches and cameras time a pulse, see the PPG guide and can a phone camera measure HRV?