Sleep Calculator
Wake up between sleep cycles, not in the middle of one. Tell the calculator when you need to wake up (or that you're going to bed now) and it suggests the best bed or wake times based on 90-minute sleep cycles.
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Result
To wake at your chosen time feeling refreshed, fall asleep at one of these times (allowing ~15 minutes to drift off).
How to use
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Save ⟶How to use the Sleep Calculator
- 1Choose whether you want to set a wake-up time or go to bed now.
- 2If setting a wake-up time, enter it.
- 3Read the suggested times, each a whole number of 90-minute cycles apart.
- 4Pick a time that gives you 5–6 cycles (about 7.5–9 hours) for most adults.
Why sleep cycles matter
Sleep runs in cycles of roughly 90 minutes, moving from light sleep into deep sleep and then REM. Waking at the end of a cycle — in light sleep — feels natural and refreshing, while waking mid-cycle from deep sleep causes grogginess (sleep inertia). Timing your night around whole cycles is why a 7.5-hour sleep can feel better than 8 hours that ends mid-cycle.
Most adults need five to six cycles (7.5–9 hours). The 90-minute figure and the ~15 minutes to fall asleep are averages — track how you feel and shift the times slightly to match your own rhythm.
Frequently Asked Questions
▸How many hours of sleep do I need?
Most adults need 7–9 hours, which is about 5–6 full 90-minute cycles. Teenagers need more; needs decline slightly with age.
▸Why do I feel groggy after 8 hours?
You probably woke in the middle of a deep-sleep cycle. Waking at the end of a cycle — even after slightly less total sleep — usually feels more refreshing.