Health & Fitness
Sleep Cycle Calculator
Sleep Cycle — sleep Cycle Calculator works in roughly 90-minute sleep cycles, counting back or forward from your chosen wake or bedtime and allowing about 15 minutes to fall asleep. This page runs that same calculation on your numbers, in your browser: no signup, no uploads, nothing stored.
Sleep Cycle Calculator helps you time sleep around 90-minute cycles. Enter a reference time, choosing bedtime or wake-up and it returns the best times to fall asleep or wake instantly, right in your browser with nothing uploaded.
About this calculator
Sleep Cycle Calculator works in roughly 90-minute sleep cycles, counting back or forward from your chosen wake or bedtime and allowing about 15 minutes to fall asleep. That is the whole of the calculation — the mistakes happen in the inputs. It matters when you want a quick screening number before deciding whether a habit, plan or measurement deserves a closer look. Worked example: Falling asleep at 23:00, five 90-minute cycles (7.5 h) plus ~15 min to drift off suggests waking near 06:45 feels most rested. Use the calculator with your own figures; it runs entirely on your device.
How to use
- Choose to plan a wake time or a bedtime.
- Enter the fixed time you are working around.
- Press Calculate to see cycle-aligned times.
- Choose an option that completes full cycles.
Why use Sleep Cycle Calculator
Fast
Calculations run in your browser. No round trip to a server.
Private
Your inputs never leave your device. Nothing is uploaded.
Free
No signup, no paywall, no ads-in-results, no watermarks.
Educational
Uses common estimation formulas for educational purposes. Results are not medical advice and should not replace professional guidance.
Mobile-ready
Optimised for phones, tablets, and desktops alike.
Transparent
Clear about formulas used and their limits, with disclaimers where they matter.
Common uses
Sleep Cycle to wake between sleep cycles
Use it when planning a nap
Use it for planning a bedtime that feels rested
Use it for shift-work and travel timing
Use it for early-start planning
Use it when helping kids' bedtimes
Technical notes
Sleep Cycle Calculator works in roughly 90-minute sleep cycles, counting back or forward from your chosen wake or bedtime and allowing about 15 minutes to fall asleep.
A general sleep-timing guide based on an average cycle length; everyone differs, so adjust it to how you actually feel.
All arithmetic runs in your browser using double-precision floating point, so extreme inputs may show tiny rounding differences.
FAQ
Why are sleep cycles about 90 minutes?
A full cycle through light, deep, and REM sleep averages around 90 minutes, so waking near a cycle boundary tends to feel easier than waking mid-cycle.
What happens if I wake mid-cycle?
Waking during deep sleep can cause grogginess known as sleep inertia, which is why cycle-aligned wake times often feel more refreshing.
Is 90 minutes exact for everyone?
No — cycles range from roughly 80 to 110 minutes and vary by night, so the suggestions are approximate guidance rather than a guarantee.
Does the Sleep Cycle Calculator send my data to a server?
No. The Sleep Cycle Calculator runs entirely in your browser — your inputs never leave your device, and nothing is uploaded or stored.
Worked example
Falling asleep at 23:00, five 90-minute cycles (7.5 h) plus ~15 min to drift off suggests waking near 06:45 feels most rested.