This should be so easy to implement. you already have the wrist worn software and algorithm ala vivofit.
it should already be apart of the F1/2/T features while acting as a daily fitness monitor "DFM" ala indoor mode.
Now for those that charge their Fenix/Tactix at night? Develop algorithm that monitors sleep by placing watch on corner of bed. (iPhone and Android can already do this)
Just an idea.