Health & Fitness
Sleep Cycle Calculator
This clock model assumes 14 minutes to fall asleep and four, five or six fixed 90-minute blocks. Actual sleep cycles vary, so the calculated times do not identify measured sleep stages or guarantee refreshed waking. Read the sleep duration beside each option.
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
This clock model assumes 14 minutes to fall asleep and four, five or six fixed 90-minute blocks. Actual sleep cycles vary, so the calculated times do not identify measured sleep stages or guarantee refreshed waking. Read the sleep duration beside each option.
How to use
- Choose to plan a wake time or a bedtime.
- Enter the fixed time you are working around.
- Press Calculate to see times for the stated duration assumptions.
- 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
Use it to compare stated clock-time and sleep-duration assumptions.
Use it to check the time available in an overnight schedule.
Use it to check the time available in an overnight schedule.
Use it for shift-work and travel timing
Use it for early-start planning
Use it to check the time available in an overnight schedule.
Technical notes
This clock model assumes 14 minutes to fall asleep and four, five or six fixed 90-minute blocks. Actual sleep cycles vary, so the calculated times do not identify measured sleep stages or guarantee refreshed waking. Read the sleep duration beside each option.
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?
NHLBI describes cycles recurring about every 80 to 100 minutes. Ninety minutes is a rough arithmetic assumption, not a fixed duration that applies to every cycle.
What happens if I wake mid-cycle?
This calculator does not measure the stage in which you will wake. Its fixed blocks cannot guarantee a more refreshed awakening or identify the cause of grogginess.
Is 90 minutes exact for everyone?
No. Sleep cycles and falling-asleep time vary. The model cannot guarantee that a calculated time coincides with a particular sleep stage.
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
Going to bed at 23:00, with the model's 14-minute allowance and five 90-minute blocks, gives a wake time of 06:44 the following morning. This is a clock calculation, not a guarantee about sleep stages or how rested you will feel.