https://wccftech.com/latest-update-allows-galaxy-watch-to-track-rem-sleep-without-heart-rate-monitoring/

Samsung’s Galaxy Watch is arguably the best Android smartwatch available on the market right now. It has received a few updates since its release and as of the latest update, the Galaxy Watch can now track REM sleep at night without needing continuous heart rate monitoring. Before the latest update, REM sleep data was only recorded when the watch was always set to monitor heart rate. The dependency of REM sleep tracking on heart rate monitoring has now been removed.

Now, the Galaxy Watch has a separate toggle for enabling or disabling REM sleep recording after the latest update. Continuous heart rate monitoring gives you a better idea of how much REM sleep you’re getting each night because the heart beats faster during REM sleep, so it is unclear as to how the watch will measure REM sleep without a heart rate measurement. The only downside of setting the heart rate monitoring function to trigger every ten minutes is increased battery usage.