Sleep Calculator
Enter when you want to wake up (or when you're going to bed) and get the times that let you wake between sleep cycles instead of in the middle of one.
Go to bed at one of these times:
Sleep runs in cycles of about 90 minutes — waking between cycles feels far easier than waking mid-cycle. The times include 15 minutes to fall asleep. 5–6 cycles (7.5–9 hours) is ideal for most adults.
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How to use
- Pick a mode: set your wake-up time, or set the time you're going to bed.
- Enter the time.
- Pick one of the suggested times — the highlighted one gives you six full cycles (9 hours).
What is this tool?
Sleep progresses through cycles of roughly 90 minutes, from light sleep into deep sleep and REM. Waking at the end of a cycle feels refreshing; an alarm that fires mid-cycle — especially during deep sleep — causes that groggy, heavy feeling regardless of total hours slept. This calculator counts backwards (or forwards) in whole cycles and adds an average 15 minutes for falling asleep.
Common uses
- Setting an alarm so it lands between sleep cycles
- Deciding whether going to bed now is worth it before an early start
- Fixing a sleep schedule for shift work or early flights
- Explaining to teenagers why 'just one more episode' costs a whole cycle
FAQ
Are sleep cycles really exactly 90 minutes?
90 minutes is the average — individual cycles range from about 80 to 110 minutes and lengthen through the night. Treat the suggested times as good targets, not precision engineering.
How many cycles should I aim for?
Most adults do best with 5–6 cycles (7.5–9 hours). Four cycles (6 hours) works occasionally but isn't sustainable for most people.
Why add 15 minutes for falling asleep?
The average adult takes 10–20 minutes to fall asleep. If you routinely take longer or shorter, mentally shift the suggested times accordingly.