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§ · free tool

Sleep calculator. Wake between cycles, not mid-dream.

Tell it when you want to wake up — or when you're heading to bed — and get bedtimes and alarm times aligned to 90-minute sleep cycles. Runs entirely in your browser; nothing is uploaded.

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A free online tool by Digital Heroes

To wake up at 7:00 AM, head to bed at one of these times. Each option is a whole number of sleep cycles — the highlighted ones are the sweet spot.

Runs entirely in your browser — nothing is uploaded. Only your mode and fall-asleep preference are saved locally.

Why 90 minutes?

Sleep moves through repeating cycles of roughly 90 minutes, each running from light sleep down into deep sleep and REM, then back toward light sleep. Waking at the end of a cycle — when you're already near the surface — tends to feel far easier than being jolted awake from deep sleep, even if you've slept slightly less overall.

Most adults do best on 5–6 cycles (about 7.5–9 hours). We also add ~15 minutes to fall asleep, since you don't drift off the instant your head hits the pillow. These are friendly estimates — cycle length varies from person to person, so treat the highlighted picks as a starting point and adjust to what leaves you feeling rested.

§ 02 · what you can do

Better mornings, by arithmetic.

  • Find the ideal bedtime by entering your target wake-up time — four cycle-aligned options appear at once.
  • Find the best wake-up time by telling the tool when you're going to sleep, including a “right now” mode.
  • See cycles and total sleep for every option — from 3 cycles (minimum) to 6 cycles (optimal), each with a visual cycle bar.
  • Adjust your fall-asleep delay from instantly to ~30 minutes so the math reflects how long you actually take to drift off.
  • Spot the sweet spot at a glance: the 5- and 6-cycle options carry a Recommended tag; very short nights are flagged in amber.
  • Stay current automatically: in sleep-now mode the times refresh every 30 seconds, and your preferences persist locally and sync across open tabs.
§ 03 · how to use it

Four steps, then lights out.

  1. Pick a mode: I want to wake up at… or I’m going to sleep…
  2. Enter your alarm time — or, in sleep mode, choose Right now or a specific bedtime.
  3. Set your time to fall asleep with the drop-down (the default is ~15 minutes).
  4. Read the result cards and go with a Recommended one — 5 or 6 full cycles suits most adults.
§ 04 · faq

Frequently asked questions.

Is the Sleep Calculator free?
Yes — completely free for everyone. No sign-up, no account and no usage limits.
Is it private? Is anything uploaded?
No upload, ever. All the math runs in your browser on your own device. The only thing saved is your mode and fall-asleep preference, stored locally so the tool remembers them next time.
Why does it use 90-minute cycles?
Sleep runs in repeating cycles of roughly 90 minutes, moving from light sleep into deep sleep and REM and back. Waking at the end of a cycle, when you're already near the surface, usually feels much easier than being pulled out of deep sleep mid-cycle.
How many sleep cycles do I need?
Most adults do best on 5–6 cycles per night — about 7.5 to 9 hours of sleep. The tool shows options from 3 to 6 cycles and highlights the 5- and 6-cycle times as Recommended. Cycle length varies from person to person, so treat them as a starting point.
What does “time to fall asleep” change?
It's the buffer between getting into bed and actually being asleep. The default is ~15 minutes, adjustable from instantly to ~30 minutes; every bedtime and alarm suggestion shifts by that amount so the cycles line up with real sleep, not lights-out.
Does it work offline?
Yes — once the page has loaded, all calculations run locally in your browser, so switching modes, changing times and reading results keeps working without a connection.

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