Bustle reviews some of the most popular sleep tracking smartwatches on the market.

When it comes to sleep tracking, the Fitbit Versa 2 is a cut above the rest. It tracks sleep by monitoring restlessness as well as your heart rate, which varies depending on which stage of sleep you’re in. Based on that, the accompanying app gives you an overall sleep score, as well as a graph that displays how much time you spent in light, deep, and REM sleep, plus any time lying awake in bed.

This watch is also the only pick that has a built-in SpO2 sensor that tracks your estimated blood oxygen levels throughout the night — a tool that can be helpful for managing sleep apnea. (Of course, this is not a substitute for professional medical help.)

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