This is a simple application I slapped together for the heart rate data that I captured here from the Apple Watch.

Third party apps do not have direct access to the heart rate sensor.

In accordance with Appleʼs approval of the study

onWristA value that indicates whether the watch is on the user’s wrist.

The sensors an app can read.

You can use HealthKit and have your app request access to the Blood Oxygen data (HKQuantityTypeIdentifierOxygenSaturation)You can find out more about the HealthKit framework here: https://developer.apple.com/documentation/healthkit

as far as I know there is no way to access the raw PPG from the watch

Now, as part of my regular practice, I spend about five minutes out of each hour exercising with this package. This almost always reveals things to me that change at least the slant of my approach during the next hour, and often stimulates a relatively significant change in my short-range plans.

Other smartwatches force you into nightly charging.

In terms of hardware, before Apple Watch debuted in 2014, there were a lot of rumors about the wearable including a multitude of health sensors that could track things like blood glucose or blood oxygen.