Sleep Calculator
90-minute REM cycle math
15 min
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ideal bedtime
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6 cycles (9 hr)
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5 cycles (7.5 hr)
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4 cycles (6 hr)
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3 cycles (4.5 hr)
The 90-minute REM cycle driver
Sleep oscillates between light/deep/REM every ~90 minutes. Waking mid-cycle (especially during deep sleep) causes the groggy "sleep inertia" feeling; waking at cycle-boundary feels natural. 5-6 cycles per night is the sweet spot for most adults.
Related tools
Sleep quality, not just quantity
- Consistency > cycle math: fixed bedtime ±15 min matters more than hitting exact cycles
- Blue-light cutoff: 60 min before bed
- Caffeine half-life: 6 hrs — last coffee by 2-4 pm for a 10 pm bedtime
- Weekend catching-up: ±30 min only — major swing feels like jet lag
- Naps: 20 min power-nap or 90 min full-cycle — never 45 min (mid-cycle grogginess)
Age-based sleep needs (CDC): 18-60 yrs → 7+, 61-64 → 7-9, 65+ → 7-8. Individual variation real — some thrive on 6, others need 9. Use the calculator as a starting framework.