I once used a mobile alarm clock app that would try to be smart about waking you up. You put it on your bed and it would measure your sleep cycles by your movem

I once used a mobile alarm clock app that would try to be smart about waking you up. You put it on your bed and it would measure your sleep cycles by your movement and try to maximise your sleep, but wake you up after a whole cycle had completed. The theory being that being woken from deep sleep is worse than being woken from the REM stage of sleep, and that it is more natural to wake up after a full cycle. I can't remember if it actually helped that much, and I've since gone back to normal alarms.