system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do smart watches and fitness trackers know when you've fallen asleep?
gpt
You don’t move much anymore and your heartbeat goes down. Both metrics such a device tracks. 

It tracks your heartbeat over the day and thus knows your “at rest, but awake” value and the sleep value typically is even lower. 

Possibly it also notices that you are laying down, which is another clue for sleeping. 

So the combination of ‘lower than average heartbeat frequency’, ‘doesn’t move much’ and ‘is laying’ would be a good indicator for ‘is sleeping’