RECOVERY
Sleep Calculator
Calculate optimal bed and wake times based on 90-minute sleep cycles.
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Result
Sleep duration ladder
More cycles increase total sleep opportunity. Five or six cycles is the strongest window for most adults.
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). Fewer than 4 cycles (<6 hours) consistently impairs muscle protein synthesis, growth hormone release, fat oxidation, 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 (6 hours/night for 2 weeks) has been shown in research to reduce strength gains and muscle recovery equivalent to the effect of 24 hours of total sleep deprivation.
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. 4-6 complete cycles typically results in 6-9 hours of sleep, which is the recommended range for most adults.
What do the ratings mean?
Ideal = 5-6 cycles (7.5-9+ hours), Good = 4 cycles (~6.5 hours), Short = 3 cycles (~5 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.
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