I had similar problem with my gear sport, during the day 15% of battery consumed, in the night over 30% (8 hours of sleep vs 16 hours rest of the day - 4x higher consumtion during the night). I realized, that it is related to flight mode. If I put my watch to flight mode before sleep, watch battery starts draining very fast (I tested and if I enable the flight mode during the day, the watch can barely last one day). This is very wierd, because if I turn of all connections in settings (bluetooth, wifi, NFC), then there is no battery drain. So, I have to switch all these settings every eveneing and every morning to get normal battery life (3 days). This is annoying, and so I have no idea, what else is flight mode doing, that it consumes so much battery. I would appreciate the switch button in quick settings for bluettoth (there is a shortcut for NFC and wifi). Btw, I have heart rate monitoring set to every 10 minutes, and it has minimal impact on battery life. But, I switched of the activity detection, because it was annoying (it was spaming my workout log), not sure about the impact of this on battery life. Hope this helps, and hope Samsung will fix the flight mode.
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