How do smartwatches provide heart-rate monitoring without sacrificing battery life?

28 Tem 10:23

Smartwatch heart-rate monitoring is basically a game of “measure often enough, but not too often.” Most devices use photoplethysmography (PPG) – a green LED and a photodiode that shine light into the skin and read the reflected signal. Green works best because hemoglobin absorbs it strongly, giving a clear pulse waveform while keeping the LED power relatively low. Some newer models add infrared or dual-wavelength sensors to improve accuracy in low-light or motion-heavy situations, but that adds a bit more power draw. On the firmware side, manufacturers rely on adaptive sampling and signal-processing tricks. The watch will run a high-frequency scan (e.g., 1 Hz) when you’re actively exercising or when the app requests continuous tracking, but it drops down to a low-frequency “background” mode (often 0.05–0.2 Hz) when you’re idle. The processor does a quick Fourier or peak-detection algorithm on the raw data, discarding noisy samples and only storing a smoothed value. Combined with aggressive duty-cycling of the LED (a few milliseconds on, then off) and low-power sensor hubs, this keeps the battery from draining too fast. In my Galaxy Watch 5 I’ve noticed that the heart-rate sensor stays off most of the night unless I enable “sleep tracking,” and the watch still makes it through a full day with a single charge. The exact trade-off is usually decided by the OEM’s target battery life – they’ll set the baseline sampling rate to hit a ~24-hour runtime, then let users opt into higher-resolution monitoring when they need it, accepting the extra drain.