Comment by Izkata

There's an Android app called "Sleep" where you set an alarm window and put your phone on the corner of your bed, and the alarm only goes off when it detects you're a little active already and not in deep sleep (using the accelerometer to detect movement). If it doesn't detect enough activity it'll go off regardless at the end of the window.

It has a bunch of other features as well (rating how well you slept and detecting patterns to give advice, a snore detector, etc) but that alarm is the one I use it for.