The app uses the iPhones accelerometer to measure your movements during NREM sleep and you non-movements during REM sleep. It graphs this data for you as well. The app uses this data to wake you up just after REM sleep and as close to the alarm you set.*

For us lucid dreamers we can use this app to find out when we are usually in REM sleep so we can set our own alarm just before it. Or so that's the theory. Just got the app today will try and graph my sleep pattern tonight and will post results.*