Sleep Calculator
Wake up between cycles, not in the middle of one: bedtimes for your alarm — or alarms for your bedtime — built on the 90-minute rhythm, with its honest error bars attached.
| 🛏 9:45 PM | 6 cycles · 9 h asleep | full night |
| 🛏 11:15 PM | 5 cycles · 7.5 h asleep | solid |
| 🛏 12:45 AM | 4 cycles · 6 h asleep | short |
| 🛏 2:15 AM | 3 cycles · 4.5 h asleep | nap territory |
Includes ~15 min to fall asleep. Cycles average ~90 min but range 80–110 by person — treat these as good starting guesses, and let how you feel at each option calibrate yours.
Spotted a mistake or have an idea? — errors get fixed and credited publicly.
Last reviewed: July 20, 2026 · Formulas verified by automated tests before every release.
What time should I sleep to wake up at 7 AM?
Working back in ~90-minute cycles plus 15 minutes to drift off: fall asleep by 9:45 PM for six full cycles (9 h), 11:15 PM for five (7.5 h), or 12:45 AM for four (6 h). The 5-cycle row is the sweet spot for most schedules. Cycles vary 80–110 minutes by person — use these as calibrated starting points, not commandments.
The rhythm and its honest limits
bedtime = wake − (cycles × 90 min) − 15 min · cycles: 6 = 9h · 5 = 7.5h · 4 = 6h
The cycle heuristic optimizes where your alarm lands; it cannot replace enough sleep — 7–9 hours remains the adult recommendation, which is why the 4-cycle row is labeled damage control. Building the evening around a target bedtime? The countdown timer and Pomodoro help land the runway; the TDEE calculator covers the other pillar of feeling human.
Sources, assumptions and limitations
- Formula: ~90-minute average adult sleep cycle (sleep-medicine literature); +15 min average sleep-onset latency
- Assumptions: Healthy adult sleep architecture; consistent schedule. Individual cycles range ~80–110 min — stated on-page by design.
- Not included: Insomnia, sleep apnea, shift-work and jet-lag strategies, chronotype differences, children's sleep (different cycle lengths). Persistent poor sleep is a medical conversation — this is an educational heuristic, not medical advice.
- Last reviewed: July 30, 2026 by the CalcNotebook team
Frequently asked questions
What time should I go to bed?
Count backwards from your wake-up time in ~90-minute sleep cycles, plus about 15 minutes to fall asleep. To wake at 7:00 AM: bed by 9:45 PM (6 cycles, 9h), 11:15 PM (5 cycles, 7.5h) or 12:45 AM (4 cycles, 6h). Waking at a cycle boundary tends to feel easier than being pulled out of deep sleep mid-cycle.
Why 90-minute cycles?
A full sleep cycle — light, deep, then REM sleep — averages about 90 minutes in adults. Alarms that land near a cycle's end catch you in lighter sleep, which is why 7.5 hours can feel better than 8. But it's an average: individual cycles run roughly 80–110 minutes, so treat the boundaries as starting guesses, not physics.
Is 6 hours (4 cycles) enough sleep?
For most adults, not routinely — sleep-medicine consensus recommends 7–9 hours. Four cycles is a better bad night (waking between cycles beats mid-cycle) — it is damage control, not a plan. If 6 hours is your normal, that's worth addressing before optimizing alarm times.
What if I don't fall asleep in 15 minutes?
Fifteen minutes is the average for people without insomnia; yours may differ. If you reliably take 30 minutes, add the difference. Regularly lying awake much longer — or waking exhausted despite full nights — is a conversation for a doctor, not a calculator.