Researchers say wearable activity trackers have become increasingly accurate at measuring dozens of different metrics from heart rate and heart rate variability to blood oxygen and skin temperature. For example, the Apple Watch measures heart rate more frequently than some competing devices, making it better suited to capturing brief spikes in heart rate. “Everything else we’re doing based on heart rate, movement, temperature, all that is trying to estimate the brain.” A resting heart rate of 80 beats per minute, for example, may be perfectly normal for the general population but represent a significant change for someone whose usual resting heart rate is 60. Instead, companies’ privacy policies and state laws largely dictate how wearable data can be collected, shared and used.