A full night of sleep runs through several roughly 90-minute cycles of light, deep, and REM sleep. Waking up at the end of a cycle, rather than in the middle of one, tends to feel noticeably less groggy, even with the same total time asleep.
Each sleep cycle runs about 90 minutes. This calculator adds a 15-minute buffer for the time it typically takes to fall asleep, then counts back or forward in 90-minute blocks (6, 5, 4, and 3 cycles) to suggest bedtimes or wake times that land at the end of a cycle rather than in the middle of one.