Hello @Kspo and welcome to the help forums. REM is the time you spend dreaming.
Laurie | Maryland
Sense 2, Luxe, Aria 2 | iOS | Mac OS
Take a look at the Fitbit help site for further assistance and information.
Best AnswerAdding to @LZeeW 's good explanation, the term comes from an acronym for Rapid Eye Movement, which your eyes do when dreaming.
Best Answer