Calculate optimal sleep times and wake up refreshed
What is the Sleep Calculator?
The sleep works like this: it works backwards through sleep cycles (~90 minutes each plus ~15 minutes to fall asleep) to suggest bedtimes for a target wake time. Everything happens on your device - the input never leaves your tab unless the tool clearly states it performs a server-side lookup.
How to use the Sleep Calculator
Enter the values the calculator asks for.
Double-check the unit or period next to each field (per month vs per year, cm vs m).
Read the result together with the breakdown shown beneath it.
Adjust one input at a time to compare scenarios side by side.
Features
Calculate optimal sleep times and wake up refreshed
Instant result, no page reload
Works in any modern browser, desktop or mobile
Free with no signup
Common mistakes to avoid
Mixing units or periods between fields
Rounding inputs before the final calculation
Treating estimates as guarantees for major decisions
Examples
Input
Result
Wake at 06:30
Best bedtimes: 21:15 · 22:45 · 00:15 (6, 5 or 4 full cycles)
Best used for
Getting a reliable answer for sleep without installing an app
One-off tasks done in under a minute from any device
Checking results before committing them to something important
Working on a phone where desktop software is unavailable
Privacy
Calculations run in your browser. Enter only what the calculation needs.
Frequently asked questions
Why 90-minute cycles?
Waking mid-cycle causes sleep inertia - the groggy feeling. Waking between cycles, even after fewer total hours, often feels better.
How many cycles do I need?
Most adults need 5 cycles (7.5h); 4 is survivable short-term, 6 is a luxury.
What is the falling-asleep allowance?
The average person needs around 15 minutes to drop off, so the tool shifts every suggested bedtime that much earlier - if you know you take longer, subtract the difference yourself.