Q. Why a 90-minute cycle?
Human sleep moves through light → deep → REM stages roughly every 90 minutes. Waking at the end of a cycle (during light sleep) feels far more refreshing than being yanked out of deep sleep.
Q. What's the 15-minute buffer?
It takes the average adult about 15 minutes to actually fall asleep (sleep latency). The calculator factors this in so the cycle math runs from your real sleep onset.
Q. Why am I more tired after 7.5h than 6h?
7.5h = 5 full cycles, so it usually works. If those extra 30 minutes land mid-deep-sleep, you may actually feel groggier than completing 4 cycles at 6h.
Q. Do naps follow the same rule?
Short 20-30 minute naps stay before deep sleep — fine. Longer naps should complete a full 90-minute cycle. The awkward 60-minute nap is the worst.
Q. Same hours, different friends feel different — why?
Individual cycles vary between 80 and 100 minutes. Find yours by trying slightly different wake times for a few days and noting which feels best.
Q. Are weekend lie-ins OK?
Up to 1-2 extra hours can repay sleep debt. Beyond that breaks your weekday rhythm.
