Suggested bed/wake times aligned to full 90-minute sleep cycles.
Sleep progresses through cycles of roughly 90 minutes, and waking near the end of a cycle (rather than in the middle of deep sleep) generally feels less groggy. The calculator offers 3 to 6 full cycles rather than an arbitrary number of hours.
It's a built-in estimate of how long it typically takes to fall asleep after getting into bed, so the suggested bedtimes (or wake-up times) already account for that gap rather than assuming you fall asleep the instant you lie down.
The calculator flags the options that fall within 7–9 hours of total sleep (5 or 6 cycles), since that's the commonly cited range adults generally need per night.