We have alarms to wake you at a set time (allowing for a smart wakeup period). And we have an alarm to prompt us to go to sleep to achieve that wakeup time.
How about being able to set an alarm to wake you up after your set sleep duration goal has been achieved, from whenever you press ‘start sleep tracking’? It could even have an option to extend the time based on any sleep deficit you have (with a max limit set on how much is ‘reclaimed’ e.g. 20 mins). Of course, if you have a credit rather than deficit, then no time is deducted form the alarm.