I already own a Garmin Fenix 3 wich I use to track my triathlon activities.
Because for sleep it only tracks "awake", "light sleep" and "deep sleep" I switch it to my right hand when going to bed.
This way it realises my wakings pretty clearly because of the movement of writting down in my DJ and I see where my dreams (and REM) are ending.

If this REM reading is accurate it may be worth another investment, though!?