Yes, you place the phone on the corner of your mattress. So when you move in bed, it can detect that because your mattress shakes. And how much you move in bed is related to your sleep cycle (it's called Actigraphy). I tried it last night and i remember i woke up twice and checked the time. The graph produced by the app matches these times. The alarm simply went off at the set time, but it says that the app needs 2 nights to calibrate to your bed and your sleep cycle, so i can't say how well that works.