For clarity: The Apple Watch does have a built-in Alarm which will wake you by Vibrating on your wrist. If you've muted the watch (swipe up from the watch face and tap the Mute icon) then you won't get a corresponding alarm noise.
I like the suggestions for #1 and #2 though. Not sure how you'd track light vs REM sleep.
a. Accelerometer: As I understand it, people don't move at all when in REM sleep, but I don't think it follows that people are guaranteed to move often (or at all) during light sleep, so I'm not sure that the accelerometer could be used to determine that.
Source: http://www.brainfacts.org/about-neuroscience/ask-an-expert/a...
b. Pulse monitoring: Apparently pulse rate is steady during light sleep but variable during REM sleep or when awake. Again, I'm not sure that's enough to reliably determine things.
Source: http://healthysleep.med.harvard.edu/healthy/science/what/cha...
I like the suggestions for #1 and #2 though. Not sure how you'd track light vs REM sleep.
a. Accelerometer: As I understand it, people don't move at all when in REM sleep, but I don't think it follows that people are guaranteed to move often (or at all) during light sleep, so I'm not sure that the accelerometer could be used to determine that. Source: http://www.brainfacts.org/about-neuroscience/ask-an-expert/a...
b. Pulse monitoring: Apparently pulse rate is steady during light sleep but variable during REM sleep or when awake. Again, I'm not sure that's enough to reliably determine things. Source: http://healthysleep.med.harvard.edu/healthy/science/what/cha...
Perhaps combining the two?