system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how does a fit bit know when you are asleep and what stage of sleep you are in? How accurate is it?
gpt
it tracks your movement while you sleep.  If the wearable has a heart rate monitor, it uses that as well to further it's accuracy.  Generally speaking, the more you move in your sleep, the less accurate the readings will be.  But overall, some studies were conducted on sleep trackers, like the Jawbone and others, and it was concluded that they're pretty accurate with sleep tracking.

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Also, to this point, no matter what your sleep tracker  says, it cannot accurately tell you about your REM, or deep sleep, stages.  For this, it needs more sophisticated equipment like eye tracker, brain waves, etc...They try to do as accurate a job as they can with movement and heart rate to measure REM sleep but it's not really that accurate.