Recovery
Sleep Calculator
Calculate optimal bed and wake times based on 90-minute sleep cycles.
Result
Five 90-minute cycles: 7 h 30 min asleep, 7 h 45 min in bed.
Bedtimes by number of cycles
| 9:15 PM | 6 cycles · 9 h | 7 h or more |
| 10:45 PM | 5 cycles · 7 h 30 min | 7 h or more |
| 12:15 AM | 4 cycles · 6 h | Under 7 h |
| 1:45 AM | 3 cycles · 4 h 30 min | Under 7 h |
Sleep duration ladder
Hours asleep for each number of cycles. Five or six meet the 7 hours adults need.
How to use it
- Enter your required wake time or target bedtime. The calculator outputs optimal sleep start times based on 90-minute sleep cycles — waking mid-cycle (not at cycle end) causes sleep inertia and prolonged morning grogginess.
- Target 5–6 complete cycles (7.5–9 hours asleep): adults need 7 hours or more. Four cycles (6 hours) or fewer, night after night, impairs recovery and cognitive performance.
- If you sleep the recommended hours but still wake tired, you likely have a sleep quality issue: alcohol within 3 hours of bed fragments deep sleep, screen light delays melatonin onset by 30–60 minutes, and inconsistent timing disrupts circadian rhythm.
- Sleep schedule consistency matters as much as duration. Varying bedtime by more than 60 minutes across the week degrades sleep quality even at adequate total hours — prioritize a consistent wake time above all else.
- During high-intensity training blocks, aim for 8–9 hours. Chronic short sleep adds up: in a laboratory study (Van Dongen et al., 2003), two weeks of 6 hours a night impaired attention about as much as a full night without sleep.
Questions people usually ask
What is a sleep cycle?
A sleep cycle is approximately 90 minutes and includes different stages: light sleep, deep sleep, and REM sleep. Your body naturally completes cycles in roughly 90-minute intervals.
Why does 90 minutes matter?
Waking up at the end of a complete cycle (rather than in the middle) can help you feel more rested. Five or six complete cycles give 7.5 to 9 hours of sleep, inside the 7 to 9 hours recommended for most adults; four cycles (6 hours) fall short.
What do the ratings mean?
Ideal = 5-6 cycles (7.5-9 hours asleep, meeting the 7 hours or more adults need). Short = 3-4 cycles (4.5-6 hours). Aim for ideal when possible, but any complete cycle is better than waking mid-cycle.
How do I account for the time it takes to fall asleep?
This calculator includes a fall-asleep time (default 15 minutes). If you typically fall asleep faster or slower, adjust the slider to match your experience for more accurate results.
Can I use this if I work night shifts?
Yes! The calculator works for any 24-hour time. Just enter when you want to wake or sleep, regardless of whether it's day or night.
Is this medical advice?
No. This tool provides general sleep optimization guidance based on the 90-minute cycle hypothesis. Sleep needs vary by age, health, and lifestyle. Consult a healthcare provider for personalized sleep advice.
Is this tool free and private to use?
Yes. AI Fit Hub tools are free, no-signup browser tools. Inputs stay in your browser unless you choose to share a URL.
Do these tools replace medical guidance?
No. These outputs are general fitness estimates — not medical advice.
Can I switch between metric and imperial units?
Yes. Every tool supports both systems and remembers your preference in localStorage.
The formula, worked through
- Formula Sleep Cycle Formula (90-Min)
90-min sleep cycle planning: bedtime = wake_time − (cycles × 90 min) − 15 min sleep latency. 4-6 cycles per night. Waking at cycle end reduces sleep inertia.
- Formula Sleep Debt Formula
Sleep debt formula: rolling 14-day deficit vs need. Van Dongen 2003 showed performance decline at <6h for 14 days = equivalent to 24h total sleep deprivation. AASM 7-9h adult need.
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Background on the Sleep Calculator
- Guide How to Improve Sleep for Better Recovery
How to improve sleep for Better recovery: optimize your sleep habits for enhanced physical and mental recovery.
- Comparison Morning vs Evening Workouts
Compare the pros and cons of morning vs. evening workouts for recovery, performance, and sleep. Find your ideal workout time with AI Fit Hub's balanced.
- Alternatives Oura Ring Alternatives (2026)
Oura Ring alternatives in 2026: the Ultrahuman Ring PRO, WHOOP, and Garmin compared on sleep accuracy, subscription cost, and who each tracker fits.
- Examples Sleep Calculator Examples
Explore practical sleep calculator examples for diverse scenarios like shift work, startup life, and athletic training to optimize recovery and performance.
- Checklist Post-Workout Recovery Checklist
Optimize your post-workout routine with this actionable checklist. Learn to refuel, repair muscles, and enhance performance for faster, more effective recovery.
- Statistics Sleep Statistics: Performance, Recovery, and Health Data
Evidence-based sleep statistics: prevalence of insufficient sleep, effect on testosterone and muscle protein synthesis, athletic performance impact.
Articles
- 8 min read Oura Ring 5 vs WHOOP 5.0 vs Galaxy Ring 2026: Compared
Oura Ring 5 vs WHOOP 5.0 vs Galaxy Ring in 2026: verified prices. Oura tracks sleep best, WHOOP coaches recovery, Galaxy Ring skips the subscription.
- 7 min read How to Improve Sleep for Recovery
Improve sleep for recovery: stage architecture, evening blue-light load, caffeine half-life, sleep-restriction, and what actually moves HRV the next day.
- 10 min read Sleep Debt vs Training Stress: The Interaction
Sleep debt and training stress compound nonlinearly. The TRIMP-adjusted recovery model and what 5 hours of sleep does to next-day output.
By goal
Use it from code
The same calculation as a JavaScript module you can import. It runs where you import it, with no request, key or rate limit.
import { compute } from "https://aifithub.io/engines/sleep-calculator.js";Input and output contract, TypeScript types and the guide for agents.