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Sleep cycle calculator

Pick a wake-up or bedtime to see the cycle-aligned suggestions.

How Sleep cycle calculator works

Sleep calculator recommends optimal bedtimes or wake-up times based on 90-minute sleep cycles. Enter when you want to wake up and it shows you the best times to fall asleep; or enter when you plan to go to bed and it lists the ideal times to set your alarm, all timed so you wake at the end of a cycle, not mid-cycle when you feel groggiest.

The science is straightforward: sleep runs in roughly 90-minute cycles, and waking mid-cycle produces grogginess. The tool counts back or forward in multiples of 90 minutes from your target time and adds a standard 15-minute allowance to fall asleep.

Frequently asked questions

Is the time I enter, bedtime or wake-up, sent to Sunasty or stored?

No. The calculation is pure arithmetic done in your browser tab. The times you enter are never transmitted to Sunasty or any third party and are not stored in any database or cookie.

Why 90-minute cycles?

A full sleep cycle, light sleep, deep sleep and REM, takes approximately 90 minutes on average. Waking at the end of a cycle, rather than partway through deep or REM sleep, typically results in feeling more refreshed. The tool calculates multiples of 90 minutes from your target time.

Does it account for time to fall asleep?

Yes. It adds a standard 15-minute allowance for sleep onset, so the suggested bedtimes are already offset to account for the time it typically takes to drift off.

Does it work offline?

Yes, once the page has loaded. Because the processing happens on your device, you can disconnect from the network and the tool keeps working. This is the clearest proof that your data never leaves your machine.

Is it free? Do I need an account?

It is completely free and requires no sign-up. There are no watermarks, no daily limits and no tracking cookies needed to use the tool.