Is this a feature that some trackers offer? When I worked at Fitbit I noticed this correlation (along with resting heart rate) and developed a model that could

Is this a feature that some trackers offer? When I worked at Fitbit I noticed this correlation (along with resting heart rate) and developed a model that could predict onset of illness a few days in advance with fairly good accuracy. But unless the accuracy is perfect it can be difficult to offer it as a product feature without a lot of validation and experiments showing that your health related claim is accurate.