The app I use is called "sleep cycle"; it is old. I don't know if i'd be able to download it now from the app store, honestly probably not, I haven't tried, but

The app I use is called "sleep cycle"; it is old. I don't know if i'd be able to download it now from the app store, honestly probably not, I haven't tried, but I already had it downloaded onto the phone. basically, the app judges your current phase of sleep by sensing your movements, and purports to wake you up at the optimal moment. It seems to work for me, which is good enough for me, after all. YMMV? But if you're unable to download it now, I don't have a good solution for you, sorry. I hope you figure it out!