What time should I go to bed if I wake up at 7:00 AM?

I count back from 7:00 AM in 90-minute blocks, then leave 15 minutes to fall asleep. Pick the amount of sleep that fits tonight.

Four bedtimes for a 7:00 AM alarm

90-minute cycles, 15 minutes to fall asleep
  1. 6 cycles, 9 h 0 min
  2. 5 cycles, 7 h 30 min
  3. 4 cycles, 6 h 0 min
  4. 3 cycles, 4 h 30 min