A full sleep cycle — light sleep → deep sleep → REM — takes about 90 minutes. Most adults feel best after 5–6 cycles (7.5–9 hours). The calculator adds your "time to fall asleep" buffer, then steps in 90-minute blocks.
I want to wake up at… — pick your target wake time; it counts backwards to give bedtimes.
If I go to sleep now… — it counts forward from right now and lists the best wake-up times.
The 6-cycle option is highlighted as the recommended night's sleep.