Apple Watch combines an optical heart sensor, accelerometer, gyroscope and, on newer models, temperature and blood oxygen sensors to build a picture of daily activity and health trends.
These sensors are generally reliable for trend tracking — resting heart rate over weeks, sleep consistency, workout intensity — but are not a substitute for clinical-grade medical devices.
Features like fall detection and irregular rhythm notifications are designed as early warning signals, prompting the wearer to seek proper medical evaluation rather than to self-diagnose.