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Sleep Calculator

Cycle-aligned bedtimes or wake times — multiple options, not one rigid answer.

Runs in your browserNo sign-up required

Based on 90-minute sleep cycles plus a 15-minute buffer to fall asleep — waking up between cycles (not mid-cycle) tends to feel less groggy.

Go to bed at one of these times:
6 cycles (~9.0h sleep)9:45 PM
5 cycles (~7.5h sleep)11:15 PM
4 cycles (~6.0h sleep)12:45 AM

This is a general estimate based on standard published formulas, not a medical assessment — it doesn’t account for your individual health history. Results are informational only and aren’t a substitute for advice from a doctor or qualified professional. Read the full disclaimer.

Built Around Full Cycles, Not a Flat 8 Hours

Rather than a single "sleep 8 hours" answer, this works backward or forward in real 90-minute sleep cycles, plus a realistic buffer for actually falling asleep — showing several cycle-aligned options so you can pick the one that fits your actual schedule rather than forcing one number.

How to Use This Tool

  1. 1Pick a directionWorking backward from a wake-up time, or forward from a bedtime.
  2. 2Enter the timeWhichever one you already know.
  3. 3Pick from the options shownSeveral cycle-aligned times, not a single forced answer.

Good to Know

  • Sleep happens in cycles of roughly 90 minutes each — waking up between cycles rather than in the middle of one is commonly reported as feeling less groggy, even with the same total sleep time.
  • The 15-minute buffer accounts for the time it typically takes to actually fall asleep after getting into bed, so the suggested time isn't just "hours before wake-up" but "time to actually be in bed by".
  • Most adults do best with 5-6 full cycles (7.5-9 hours) — the shorter options are shown for reference, not as a recommendation to regularly sleep less.
  • Cycle length varies somewhat by individual — treat these as a well-established starting point to experiment from, not an exact personal measurement.

Frequently Asked Questions

Why does the tool suggest multiple times instead of one?

Because there isn't one universally correct answer — different numbers of full sleep cycles suit different schedules, so seeing several cycle-aligned options lets you pick the one that actually fits your evening or morning.

Is waking up mid-cycle actually harmful?

Not harmful, but commonly reported as feeling groggier (sleep inertia) compared to waking up at a natural cycle boundary — this tool is built around that well-documented pattern.

Does this account for naps?

No — it's built around a single main sleep period. Nap timing has its own separate best-practice guidance (generally shorter, earlier in the day) that this tool doesn't cover.

Is this free to use?

Yes, completely free, with no account and no limit on how often you use it.