Sleep Calculator
Find the best times to wake up or go to sleep based on 90-minute sleep cycles.
Go to sleep at:
9:45 PMRecommended
9.0h sleep
6 cycles
11:15 PM
7.5h sleep
5 cycles
12:45 AM
6.0h sleep
4 cycles
2:15 AM
4.5h sleep
3 cycles
These times account for ~15 minutes to fall asleep. Each sleep cycle is 90 minutes.
How it works
Enter the time you need to wake up (or the time you plan to sleep) and the sleep calculator generates a list of optimal bedtimes or wake-up times based on 90-minute sleep cycles plus an average 15-minute time to fall asleep.
Why use this tool?
Waking up at the end of a sleep cycle — rather than in the middle — leaves you feeling refreshed instead of groggy. This free sleep cycle calculator is used by students, shift workers, and anyone optimizing their sleep quality.
Frequently Asked Questions
What is a sleep cycle?
A complete sleep cycle lasts approximately 90 minutes and includes light sleep, deep sleep, and REM (Rapid Eye Movement) sleep. Adults need 4–6 complete cycles per night (6–9 hours total).
Why do I feel groggy even after 8 hours of sleep?
Waking during deep sleep (N3 stage) causes sleep inertia — grogginess that can last 30–60 minutes. The sleep calculator shows wake times that align with the end of a cycle, when you naturally sleep lightest.
How much sleep do adults actually need?
The CDC recommends 7–9 hours per night for adults (18–64). Less than 6 hours consistently is linked to impaired cognition, weakened immunity, and increased cardiovascular risk.
Is this free?
Yes, free to use with no sign-up. Enter your wake-up time to get optimal bedtimes.
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