I downloaded "Sleep as Android" application. It's not just an alarm clock. It has a censor which records its user's sleeping patterns and rings at the best time before the time set.
The "best" time means the time when the user's sleep is the lightest. I've used the application's free trial version for two weeks. I could get up more comfortably than before.
If you don't have your battery of your smart phones run out of electricity during your sleep, this application will be one of your best alarm clocks!
