Free tool
Sleep calculator
Sleep runs in cycles of about 90 minutes. Waking at the end of a cycle, rather than in the middle of one, tends to feel easier than waking groggy mid-cycle. This tool counts whole cycles, plus about 15 minutes to fall asleep, so you can aim for a natural wake point.
Medically reviewed by Shariq Refai, MD, MBA, FAPA, board certified psychiatrist · Last reviewed July 7, 2026 · Editorial policy
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How to read it
Whole cycles, not exact minutes
Each result is a whole number of 90-minute cycles. Aiming for five or six cycles, roughly 7.5 to 9 hours, suits most adults; four cycles is a shorter night. There's nothing magic about hitting an exact minute, since sleep need varies from person to person, but lining up with cycle ends is a simple way to wake feeling less foggy.
The science, briefly
Why 90-minute cycles
Across the night you move through light sleep, deep sleep, and REM in repeating cycles that average about 90 minutes. Waking during deep sleep is what produces that heavy, disoriented feeling; waking near the lighter end of a cycle is gentler. The 90-minute figure is an average, so treat these times as a target rather than a rule.
What matters more than timing tricks is consistency: a steady wake time, morning light, limiting late caffeine and alcohol, and a wind-down routine. Persistent trouble sleeping, loud snoring with pauses in breathing, or daytime sleepiness deserves a professional look.
Frequently asked questions
Good questions, clear answers
Is more sleep always better?
Most adults do best with 7 to 9 hours. Consistently far below or above that's worth discussing with a clinician, especially if you wake unrefreshed no matter how long you sleep.
Why does the tool add 15 minutes?
Most people take roughly 10 to 20 minutes to fall asleep. The tool adds about 15 minutes so the cycle math starts from when you're likely asleep, not when your head hits the pillow.
I still wake up tired. What helps?
Consistent wake times, morning light, limiting late caffeine and alcohol, and a wind-down routine matter more than timing tricks. Insomnia lasting weeks, or loud snoring with pauses, deserves a professional evaluation.
Does this store anything?
No. Everything is calculated in your browser; nothing is saved or sent.
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