Sleeptracker is a watch that, using a built-in accelerometer, monitors your movements as you sleep to determine how deep of a sleep you are in at any moment during the night. You set a "window" (0, 10, 20, or 30 minutes) in which the Sleeptracker will decide to wake you up when you are in the most responsive (awake) state so that you wake up refreshed. For example, if you would normally get up between 6.30am and 7am, the Sleeptracker will find a time, say 6.47am, when you are the most awake and will play a gentle alarm.

Added from www.sleeptracker.com