Apple Watch - Green lights
What does it mean when there are green lights illuminating on the back of my Apple Watch ?
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Those are heart rate sensors which should not be activated when the Apple Watch is not being worn. Please refer to the "Apple Recommended" answer in this ASC Forum thread: 2 green lights on back of Apple Watch 4 - Apple Community which states that restarting your watch may be helpful. You may also have a third party app which is interfering with the watchOS.