Sleep Cycle Calculator

Find the best time to wake up or go to sleep — wake up refreshed, not groggy.

I need to wake up at

hour

min

Sleep cycles last approximately 90 minutes. Waking mid-cycle causes grogginess. This calculator adds 14 minutes to account for falling asleep.

Sleep stages per cycle

Stage	Type	What happens
1–2	Light sleep	Heart rate slows, body relaxes
3	Deep sleep	Physical restoration, immune support
4	REM	Dreaming, memory consolidation

If your alarm wakes you mid-cycle, you'll feel groggy regardless of total sleep time. Sleep cycles last around 90 minutes, and waking during deep sleep is particularly disorienting. Aim to wake at the end of a complete cycle — this calculator shows you exactly when that is.

Most adults need 5–6 complete sleep cycles per night, totalling 7.5–9 hours. The sweet spot for most people is 5 cycles — 7 hours 30 minutes of actual sleep. Fewer than 4 cycles consistently leads to cognitive impairment.

To wake at 6:00 AM at the end of a sleep cycle: for 6 cycles (9h) sleep at 8:46 PM; for 5 cycles (7.5h) sleep at 10:16 PM; for 4 cycles (6h) sleep at 11:46 PM. These account for the average 14 minutes to fall asleep. Use the calculator above for any wake-up time.

Yes. A 20-minute power nap keeps you in light sleep and avoids grogginess. A 90-minute nap completes one full cycle. Naps of 30–60 minutes often leave you feeling worse due to waking during deep sleep.

About sleepcalc.co.uk

sleepcalc.co.uk is a free sleep cycle calculator designed to help you wake up feeling refreshed by timing your sleep around the natural 90-minute sleep cycle. The calculator works in two modes: enter your required wake-up time to find the best bedtimes, or calculate from now to find the best times to wake up.

Sleep science shows that waking at the end of a complete 90-minute cycle results in significantly less grogginess than waking mid-cycle — even if total sleep time is longer. This tool helps you align your alarm with your body's natural rhythm.

No account or sign-up is required. This tool is for general guidance and does not constitute medical advice. If you experience persistent sleep problems, please consult your GP.

Contact

Have a question, spotted an error, or want to suggest an improvement? We'd love to hear from you.

Email: hello@sleepcalc.co.uk — we aim to respond within 2 business days.

For sleep health advice please consult your GP or visit the Sleep Charity.

Privacy Policy

Last updated: May 2025

Who we are

sleepcalc.co.uk is a free sleep cycle calculator. "We", "us", or "our" refers to the operators of this site.

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No personal information is required to use this tool. Standard server logs may be collected automatically including anonymised IP address, browser type, and pages visited. We do not sell this data.

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Changes

We may update this policy. The "last updated" date reflects the most recent revision. Continued use of this site constitutes acceptance.