Sleep Calculator — Best Time to Sleep or Wake Up

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Wake up fresh, not groggy — sleep in full 90-minute cycles and rise between them.

Wake-up time

A full sleep cycle averages 90 minutes; waking mid-cycle causes the groggy feeling. Falling asleep takes ~14 minutes on average — it is added automatically.

How to use this tool

1. Choose your mode: a fixed wake-up time, or I sleep now.
2. Press Show bedtimes — you get four options, from 6 sleep cycles down to 3.
3. Pick the one that fits your evening — 5–6 cycles is the healthy target for adults.

Example

To wake at 6:30 AM: 6 cycles → sleep at 9:16 PM; 5 cycles → 10:46 PM; 4 cycles → 12:16 AM. A 10:46 PM bedtime gives 7.5 hours — the sweet spot for most adults.

The formula

bedtime = wake time − (cycles × 90 min + 14 min to fall asleep)

What the result means

Sleep moves in ~90-minute cycles of light, deep and REM sleep. Waking at a cycle boundary (even after less total sleep) feels dramatically better than waking mid-deep-sleep. The four options let you trade sleep length for wake quality — but regularly below 5 cycles is borrowing from tomorrow.

Frequently asked questions

Is 90 minutes exact for everyone?

No — cycles run roughly 80–110 minutes and shift with age, alcohol and stress. Treat 90 as the average the science agrees on, and adjust by how you actually feel.

Why 14 minutes to fall asleep?

Studies put average sleep latency around 14 minutes. If you know you take longer, add the difference mentally.

Does it work for naps?

Loosely — one cycle (~90 min) or a short 20-minute power nap (before deep sleep starts) are the two nap lengths that avoid grogginess.

Is this medical advice?

No — it's cycle arithmetic for planning. For serious sleep problems, see a doctor.

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Explore the island world

• Clocktower Town — the island where this tool lives.
• The Main Island — drive the boat, explore the whole world.
• The 90s Kids Island — where most of us started.

Privacy & data: this tool runs entirely in your browser — nothing you type is uploaded, saved or sent anywhere. · Disclaimer: results are estimates for everyday planning — rates and rules change, so verify with official bills and documents where it matters. · Privacy Policy · Disclaimer