Enter when you need to wake up (or when you go to bed) to get bedtimes that end on a full sleep cycle.
A sleep cycle lasts about 90 minutes, and most people need about 15 minutes to fall asleep. Bedtime = wake time − cycles × 90 min − 15 min.
To wake at 6:30: 6 cycles (9 h) means bed at 21:15, 5 cycles (7.5 h) at 22:45, 4 cycles (6 h) at 00:15.