The Sleep Cycle alarm clock is a bio-alarm clock that analyzes your sleep patterns and wakes you when you are in the lightest sleep phase. Waking up in the lightest sleep phase feels like waking without an alarm clock - it is a natural way to wake up where you feel rested and relaxed. Since you move differently in bed during the different phases, Sleep Cycle uses the accelerometer in your iPhone to monitor your movement to determine which sleep phase you are in. 

 Have you ever woken up feeling completely wrecked when the alarm clock goes off, despite the fact that you have slept "enough" hours? When this happens you have probably been awakened during a deep sleep phase, and your whole day can turn into one long zombie marathon. 
Other days you spring out of bed with a smile on your face, feeling completely rested even though you shouldn't. As the alarm clock goes off, chance seems to play a big role in how your day will become. But does it really have to be that way? This is where the Sleep Cycle alarm clock application comes into play.

During the night you go from light sleep to deep sleep, occasionally entering into a dream state which is called REM-sleep. These are things that your normal alarm clock does not care about, and will go off at the set time regardless of whether you are in a light sleep phase or in the deepest sleep. However, since you move differently in bed during the different phases, the Sleep Cycle alarm clock is able to use the accelerometer in your iPhone to monitor your movement and determine which sleep phase you are in. Sleep Cycle then uses a 30 minute alarm window that ends at your set alarm time and wakes you in your lightest sleep phase. 

  http://www.mdlabs.se/sleepcycle