Sleep Calculator
Work out the best time to go to bed or wake up, based on 90-minute sleep cycles and a buffer to fall asleep.
Sleep Calculator works out the best bedtime or wake-up time using 90-minute sleep cycles. Enter when you want to wake up to see matching bedtimes, or enter your bedtime to see matching wake-up times, each with the number of cycles and total hours of sleep. Runs entirely in your browser — no upload, no signup. Made by FreeToolHub.
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Recommended bedtimes
= 5 or 6 full sleep cycles (7.5–9 hours)
How it works
Choose whether you want to wake up at a certain time or you're going to bed now, then enter that time. The calculator counts backward or forward in 90-minute sleep cycles — the average time it takes to move through light sleep, deep sleep, and REM sleep — and adds a short buffer for the time it actually takes to fall asleep. The result is a set of bedtimes or wake-up times that land at the end of a full cycle, which tends to feel far less groggy than waking up in the middle of one.
Frequently Asked Questions
How does the sleep calculator work?
It works backward or forward from a time you choose in steps of 90 minutes — the average length of one full sleep cycle — plus a buffer to fall asleep, so you wake up at the end of a cycle instead of in the middle of one.
Why are sleep cycles 90 minutes long?
A full sleep cycle moves through light sleep, deep sleep, and REM sleep, and takes about 90 minutes on average for most adults. Waking up during light sleep, near the end of a cycle, feels more refreshing than waking up mid-cycle during deep sleep.
How many sleep cycles should I aim for?
Most adults feel best after 5 to 6 complete cycles, which is 7.5 to 9 hours of sleep. This calculator marks those as recommended, though individual needs vary.
What is the fall-asleep buffer?
Most people take about 10 to 20 minutes to actually fall asleep after getting into bed. The calculator adds this buffer so the cycle count lines up with your real wake-up or bedtime, not just the time you get into bed.
Should I set an alarm for the exact time shown?
Yes, for the best chance of waking up at the end of a cycle. Times are estimates based on averages — your own cycle length can vary by a few minutes from night to night.
Is this medical advice?
No. This tool is a general planning guide based on typical sleep cycle research, not a diagnosis or medical recommendation. If you have ongoing sleep problems, please talk to a healthcare professional.