Sleep Cycle Calculator
Sleep cycles average about 90 minutes. Time your alarm for the end of a cycle, when sleep is lighter, and you're less likely to wake up groggy.
*Calculations include a standard 15-minute buffer representing the average latency to fall asleep.
90-Minute Sleep Cycles
Sleep runs in repeating cycles that average about 90 minutes (roughly 70-120). Each cycle moves from light sleep to deep sleep and ends with REM (dreaming) sleep.
Why Waking Mid-Cycle Feels Bad
If your alarm goes off during deep sleep, you can feel groggy and slow for 15-60 minutes. It passes, and light and movement help it pass faster.
Morning Light Sets Tonight
Get about 20 minutes of outdoor light soon after waking (never look directly at the sun). It sets tonight's sleep timing.
Plan the day around your sleep
ARC doesn't track sleep stages and needs no wearable. It works on the clock around your sleep: a caffeine cutoff from every drink you log, a 20-minute morning-light timer on your Lock Screen, and a wind-down reminder before the bedtime you set.
How the Sleep Cycle Calculator Works
Through the night you move through repeating cycles of light sleep, deep sleep and REM (dreaming) sleep. Each cycle lasts about 90 minutes on average. The calculator counts back (or forward) in 90-minute steps from the time you give it and adds 15 minutes, a typical time to fall asleep, so your alarm is more likely to land near the end of a cycle, when sleep is lighter.
Why the times are estimates
Cycles vary from about 70 to 120 minutes, differ between people, and change across the night: the first cycles hold most of your deep sleep, while REM stretches out towards morning. Stress, alcohol, caffeine and a late night all shift things too. Treat the suggested times as a good starting point, and adjust by 15 minutes either way if you still wake groggy.
How much sleep you need
Most adults need 7 to 9 hours a night, which is five or six cycles. Four cycles (6 hours) is fine for an occasional short night, but regularly sleeping less than you need builds sleep debt that no alarm timing can hide. If you want to see how far behind you are, try the sleep debt calculator.
If you wake up mid-cycle
Waking from deep sleep can leave you groggy for 15 to 60 minutes. This is sleep inertia, and it passes. Getting about 20 minutes of outdoor light soon after waking helps you feel alert and sets up tonight's sleep timing. Never look directly at the sun.
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Frequently asked questions
How long is a sleep cycle?
About 90 minutes on average, but cycles vary from roughly 70 to 120 minutes between people and across the night. Early cycles hold more deep sleep; later ones hold more REM sleep. That's why the times here are best estimates, not exact.
What time should I go to bed if I wake up at 7 AM?
For five cycles (7.5 hours of sleep) plus 15 minutes to fall asleep, aim for about 11:15 PM. For six cycles (9 hours), about 9:45 PM. For four cycles (6 hours), about 12:45 AM.
Is it better to sleep 6 or 7.5 hours?
7.5 hours, if you can. Most adults need 7 to 9 hours. Waking at the end of a cycle after 6 hours may feel easier than waking mid-cycle after 7, but it doesn't make up for the missing sleep over time.
What if I wake up mid-cycle and feel groggy?
Sleep inertia usually fades within 15 to 60 minutes. Get outdoor light, move around, and hold your first coffee a little if that suits you. If you regularly wake exhausted after a full night, it's worth talking to a doctor.