Lately I've been using an alarm clock that determines what phase of sleep you are in (light or deep) so that it waits until you are at a lighter sleep phase before waking you up.
It does this by using the microphone and motion sensors to determine if you are moving around or not.
I was wondering if there were a more accurate method of determining this and I was thinking that maybe the person's heart rate would have a noticeable difference between the different sleep phases, is this correct?