This app helps you keep your sleep duration the same every night.
Set an alarm that rings after a certain duration or a timer that ends at a specific time.
Start them with a tab on the widget before you go to bed.

Wake up easier during weekends and holidays, if you work from home or in shifts, travel through different time zones or after the summer/winter time change.

You bedtime might vary, but why should the length of your sleep? Find your own rhythm.

This is a simple but efficient app.
It doesn't have it's own alarms or timers, instead it is compatible with most clock apps and will extend their functionality.
Free, no adds and no unnecessary permissions.