My wife has a fitbit charge HR and loves it. While the sleep tracking is fine, the silent alarm is the big feature. Sleep tracking is always going to be a bit wonky when you can only track restlessness.
My guess is that it isn't 100% accurate down to the single step or heartbeat, but likely nothing is when you have to be unobtrusive to the wearer. It's a really good glance at your day to see if you're doing what you need to be doing, and for most people that's enough.
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