How sleep cycles work
You don't sleep at one steady depth — you move through cycles of light sleep, deep sleep and REM, each lasting roughly 90 minutes. Waking in the middle of a deep stage is what leaves you groggy; waking at the end of a cycle feels far more refreshing, even if you slept a little less.
Using the calculator
Tell it whether you want to wake at a set time or go to bed at a set time, enter the time, and it works backwards or forwards in 90-minute cycles — adding about 15 minutes for the time it takes to fall asleep. Aim for the options that give you five or six full cycles (7.5–9 hours), which suit most adults.
How much sleep you need
Most adults need 7–9 hours a night. Teenagers need more (8–10), and needs shift with age and activity. Consistency matters as much as duration: going to bed and waking at similar times every day keeps your body clock steady and your sleep deeper.
Sleeping better
Keep a regular schedule, dim screens and lights in the last hour before bed, keep the room cool and dark, and avoid caffeine late in the day. Daytime exercise deepens sleep — though training too close to bedtime can keep some people awake.
Helpful tools
Wake gently with simulated light at the end of a cycle.
View options →Measure your real cycles and sleep quality over time.
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Frequently asked questions
How long is one sleep cycle?
About 90 minutes on average. A full night is typically 5–6 cycles, which is why 7.5 and 9 hours are common sweet spots for waking refreshed.
Why do I feel groggy even after 8 hours?
You may be waking in the middle of a deep-sleep stage. Timing your wake-up to the end of a cycle — what this calculator does — usually helps you feel more alert.
How much sleep do I actually need?
Most adults need 7–9 hours. The right amount is what leaves you waking naturally and feeling rested, and it shifts with age, activity and health.
Does the 15-minute fall-asleep buffer matter?
It improves accuracy. Most people take 10–20 minutes to drift off, so the calculator adds 15 minutes when working out bedtimes and wake times.