Sleep Calculator
Bedtimes that fit whole 90-minute sleep cycles, so the alarm doesn't land mid-cycle.
Why 90-minute cycles
Sleep runs in cycles of roughly 90 minutes through light, deep and REM stages. An alarm that fires during deep sleep produces that groggy, hit-by-a-bus feeling (sleep inertia); one that lands near a cycle boundary feels far gentler. The times here add about 15 minutes to fall asleep, then whole cycles: 5 cycles is 7.5 hours of actual sleep, the sweet spot for most adults.
Honest caveats
90 minutes is an average; real cycles run 70 to 110 minutes and drift through the night, so treat this as a good heuristic rather than physiology-grade precision. What matters more: consistency (same wake time daily anchors your rhythm) and total volume, since most adults need 7 to 9 hours. If you routinely need the 3-cycle row, the problem isn't the arithmetic.