The one thing your app is missing to make it the best option to buy is the additional option before start sleep tracking is pushed, is sleep factors.
The option to add preset, and make your own, things that affect sleep for that night. Such as, caffeine, smoke, drink, medicine, ate late, pain, excersice, sick and so forth. Check out Sleep Monitor app.
Not everyday is the same and always affected by something. It would be nice to know on that day you slept like crap, why you might of slept like crap.