ELI5 Activity Trackers - how do they detect deep sleep?

ELI5 Explain Like I'm 5: Bite sized answers to stuff you should know about - in a mini podcast

What are the creative ways people fool their FitBit? How does my activity tracker differentiate between steps and my tapping to a rhythm? Does it use GPS to calibrate? How does it know what stage of sleep I'm in?

... We explain like i'm five!

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Thank you to the r/explainlikeimfive community, and specifically the following reddit users whose questions and responses formed the basis of this conversation: failtuna, fieldpeter, devilishgainz, srslylovesgames & groundhogcakeday

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