Online Tool Store
Sleep Cycle Calculator
Find the best bedtime for a target wake-up time, or the best wake-up time from right now, based on 90-minute sleep cycles, in your browser.
🔒 This tool runs entirely in your browser. Your files are never uploaded to a server.
We'll suggest bedtimes so you wake up between sleep cycles, not in the middle of one.
How the sleep cycle calculator works
- Choose whether you want to work backward from a wake-up time, or forward from right now.
- The calculator counts back or forward in whole 90-minute sleep cycles, adding 15 minutes for the time it typically takes to fall asleep.
- Three options are shown — 4, 5, or 6 full cycles — so you can pick whichever fits your schedule best.
The method
bedtime = wake-up time − (cycles × 90 minutes) − 15 minutes to fall asleep
FAQ
Why 90 minutes per cycle?
90 minutes is the commonly-cited average length of one full sleep cycle (moving through light sleep, deep sleep, and REM sleep). It varies somewhat person to person and night to night, so treat these times as a helpful estimate, not an exact science.
Why does it add 15 minutes before counting cycles?
Most people don't fall asleep the instant they lie down — 15 minutes is a typical estimate for the time it takes to actually fall asleep, so the suggested bedtime accounts for that gap rather than assuming instant sleep.
Why does waking up between cycles matter?
Waking up in the middle of a deep sleep phase (rather than at a natural transition between cycles) is commonly associated with feeling groggier, even after the same total amount of sleep — this is often called "sleep inertia."
Is my sleep data stored anywhere?
No. Every time is calculated instantly in your browser from the time you enter — nothing is sent to Online Tool Store or stored.
How we compare
| Feature | Online Tool Store | SleepCalculator.com | Sleep tracking apps |
|---|---|---|---|
| Both "wake up at" and "going to bed now" modes | ✓ | ✓ | Requires tracking data first |
| No app install | ✓ | ✓ | App install required |
| No account or wearable needed | ✓ | ✓ | Often needs a wearable |
For a quick bedtime or wake-up suggestion without a sleep tracker or app, the Online Tool Store Sleep Cycle Calculator gives you an instant answer.