Same problem here. After one night a good reading (REM and deep sleep), the following night the heartrate was inconsistent and the third night it even was a flat line. Even resetting the watch did not help. Sent the watch back and got a new one. The first night the sleep tracking was fine, consistent heartrate with REM sleep and deep sleep. Today, the second night it measured the heartrate all night! But sleeptracking says there is no consistent heartrate for the sleeptracking?? Ones more looked at the heartrate and saw there were two times missing from the 10 min reading between 5 and 7. I will keep trying. It's an expensive watch, 320 euro's, it should work, if it keeps these problems the watch is going back.
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