Most sleep calculators just spit out times with no explanation. SleepCycler shows you exactly why each bedtime works — based on 90-minute sleep cycles, your age group, and a 15-minute fall-asleep buffer. Features: bidirectional calculator (wake time or bedtime), Sleep Now button, Sleep Debt Calculator with 7-night tracking and recovery plan, and a full sleep science blog. Free, no signup, no ads.