About Sleep Calculator

What this tool does

Sleep happens in repeating cycles of roughly 90 minutes, moving through light sleep, deep sleep, and REM. Waking mid-cycle leaves you groggy (a state called sleep inertia), while waking at the end of a cycle tends to feel much easier. Our calculator counts in 90-minute blocks and adds an average 15 minutes to fall asleep, then gives you several good options:

• Wake up at… — enter your alarm time and see the best bedtimes (4, 5, or 6 full cycles).
• Sleep now — if you go to bed right now, see the best times to set your alarm.
• Nap now — get the ideal wake-up time for a 20-minute power nap or a full 90-minute cycle.

We also publish a recommended sleep-by-age guide, a deep-dive on sleep cycles and REM, and quick pages for common targets like waking at 6 AM.

Why we built it

Most people aim for a number of hours, but the timing of the alarm matters just as much for how rested you feel. We wanted a clean, no-login, ad-supported tool that works instantly on any phone or computer and explains the reasoning behind the numbers — in plain language and in several languages.

How it's funded

The site is free to use and supported by advertising (including Google AdSense). This lets us keep the calculator open to everyone with no account and no paywall. See our Privacy & Cookie Policy for details on cookies and your choices.

A note on accuracy — not medical advice

The 90-minute cycle is an average; real cycles range from about 70 to 110 minutes and change through the night. Treat the results as a helpful guide, not a strict rule. Sleep Calculator does not provide medical advice. If you have ongoing sleep problems or think you may have a sleep disorder, please talk to a qualified healthcare professional.

Contact

Questions, corrections, or feedback are welcome — reach us through the contact page.