watchOS 9 will know what heart rate zones you're in during sleep and workouts
 

As expected, Apple has introduced watchOS 9 at WWDC. The new Apple Watch platform includes a big upgrade to fitness features with enhanced metrics that include distance and time intervals. You'll know when to slow down for a recovery, for instance. There are also heart rate zones (for both sleep and workouts), multi-sport workouts and personal bests that can help you improve performance over time. You won't always need the wristwear, though— Apple has also promised to offer fitness features on the iPhone.

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