Alarm for amount of sleep instead of specific time

Suggestion: please set up an alarm that tracks the amount of time I’m actually asleep and alarms only after I have slept my optimal amount. I can set a one time alarm for my sleep amount, but if I don’t fall asleep for an hour I won’t get a full night’s sleep.

I want to press a button to start a sleep session that will end and alarm after I have slept 8 hours rather than at a specific time. This would be nice for those days where i get to sleep in, but getting to much sleep sometimes leaves one fuzzy. So, getting eight hours of sleep even though I woke up for 30 minutes in the middle of the night would be nice.