When should I wake up to feel rested?
90-minute sleep cycles + sleep-debt model. Find the next 5 cycle-aligned wake-times, see how habits cut your effective sleep, compare interventions side by side.
How the math works
Three calculations stacked on simple sleep-science consensus.
- Cycle alignment: 90-minute sleep cycles (AASM consensus). Cycle-aligned wake-times = bedtime + 14 min fall-asleep + N × 90 min. Waking mid-cycle (especially in deep sleep) feels groggy.
- Effective sleep: scheduled hours × (1 − habit penalties). Screen time within 60 min of bed and afternoon caffeine each apply small linear deductions to estimated sleep quality.
- Productivity loss: ~12% cognitive impairment per hour short of need (Walker 2017, Van Dongen 2003). Capped at 40% to avoid extrapolation.
Why 90-minute cycles? The American Academy of Sleep Medicine standard cycle: light → deep → REM. The number is an average — real cycles range 70-110 min and shift across the night. The model is directional, not clinical.
Scenario comparison shows the effective-sleep impact of three single-variable interventions (earlier bed, no afternoon caffeine, no pre-bed screens) so you can see which lever moves the most.
Math runs locally. Inputs never leave your browser. Persistent sleep issues warrant a sleep specialist, not a calculator.