Heart Rate During Rest: Surprisingly Reliable
Resting heart rate is the one number most smartwatches get consistently right. The optical sensors on the underside of the band, which use green LED light to detect blood volume changes in your wrist, work well when your arm is still. A 2019 study published in the Journal of the American Medical Informatics Association tested six consumer wearables and found resting heart rate accuracy within 5% of ECG readings across all devices tested. For detecting trends over weeks, whether your resting rate is creeping up during a stressful month or dropping as your fitness improves, this number is genuinely useful. The caveat: darker skin tones can reduce optical sensor accuracy. A 2021 paper in npj Digital Medicine found that pulse oximetry and photoplethysmography sensors perform measurably worse on people with Fitzpatrick skin types V and VI, which covers a large portion of Indian users. Resting heart rate is still the most dependable metric on the list, but it is not equally reliable for everyone.
