Interesting, I've got a bunch of old iPhones in a drawer and would be interested in doing some sleep tracking. What app are you using/any idea how old of an iPhone would still work for this?
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!