Every alarm clock app I could find allows you to choose when you want to wake up, but not how long you want to sleep. There's evidence that humans need a particular amount of sleep per day. Why not use that amount to set your alarm when you're going to bed?
With that in mind, I created Warpclock, an Android alarm helper. You can configure your preferred amount of sleep in the app UI. Then, with a single button press, it will set an alarm to wake you up after that amount of time has passed. It also provides a widget, allowing you to set alarms directly from your home screen.
The app has no ads, no analytics, and no in-app payments. You can download it on F-Droid for free!