Waking up in the middle of a deep sleep cycle is what makes you feel wrecked even after eight hours in bed. Sleep runs in cycles of roughly 90 minutes, so the trick is to aim for a whole number of them and to leave time for actually falling asleep. Pick which end you know — the alarm you have to hit, or the time you are getting into bed — and the calculator lists the matching times, with the 7.5-hour and 9-hour options marked as the ones most adults do best on.
How it works: each option is your target time shifted by a whole number of cycles — bedtime = wake-up time − (cycles × 90 min) − time to fall asleep, and wake-up time = bedtime + time to fall asleep + (cycles × 90 min). Six cycles is 9 hours of sleep and five is 7.5 hours, which is why those two are flagged; four cycles (6 hours) is a short night, and three (4.5 hours) is a last resort.