Health & Sleep
Everyday health and sleep calculators that turn vague intentions into a specific clock time — planned locally in your browser.
1 tool
Health and sleep tools turn vague intentions — "I should go to bed earlier" — into a specific clock time you can actually act on. The Sleep Cycle Calculator works from the 90-minute sleep-cycle model: most people move through roughly 90-minute blocks of light, deep and REM sleep, and waking at the end of a block feels far better than waking mid-cycle.
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About Health & Sleep
These calculators are deliberately narrow. They do not diagnose, they do not prescribe, and they do not replace a clinician. What they do is take two numbers you already know — a wake-up time, a bedtime, or your age — and return the small set of times that line up with your body's natural rhythm, so the decision is made for you before you are tired and irrational at midnight.
As with every tool on the site, the math runs entirely in your browser. Nothing you type is uploaded or stored, which matters more for health inputs than for most categories: the times are personal, but they never leave the page.
Frequently asked questions
Is this a medical device or health advice?
No. The Sleep Cycle Calculator is an arithmetic planner based on the widely cited 90-minute sleep-cycle model. It is not a diagnosis, it is not medical advice, and it does not treat any condition. If you have persistent trouble sleeping, daytime fatigue, or a sleep disorder such as apnea, speak to a qualified clinician rather than relying on a timing calculator.
Why does the calculator add 15 minutes before counting cycles?
The 15 minutes is a sleep-onset latency — the typical time it takes to fall asleep after getting into bed. Your first 90-minute cycle does not start the moment you lie down; it starts once you are actually asleep. The tool therefore measures your time in bed from the moment you get in, then counts complete cycles from sleep onset, which is why the recommended bedtimes sit about 15 minutes earlier than a naive 90-minute subtraction would suggest.
Do I have to follow the suggested times exactly?
No, and you should not treat them as precise to the minute. Sleep is biological, not clockwork; individual cycle lengths vary from about 70 to 110 minutes. The suggestions are a good target to aim at, especially the one marked best, but shifting by 10 or 15 minutes in either direction is normal and will not ruin your night.
How is age used, and is it required?
Age is optional. If you provide it, the tool highlights the cycle counts that fall inside the sleep-duration range recommended for your age group by organisations such as the Sleep Foundation and the US National Sleep Foundation. Leaving it blank simply removes that highlight and shows all five options on equal footing.
Is my data uploaded to a server?
No. The calculation runs as JavaScript in your browser tab; the wake-up time, bedtime and age you enter are never sent anywhere or stored. You can disconnect your network and the calculator still works, because nothing it needs ever leaves the page.
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This space is reserved for a sponsor. Every tool on this site stays free and runs locally in your browser.