Sleep Calculator
What time should you sleep? This free sleep cycle calculator finds the best bedtime or wake-up time so you rise between 90-minute cycles instead of in the middle of one.
Go to bed at one of these times
Times include a 15-minute buffer to fall asleep.
How the sleep calculator works
The calculator counts in whole 90-minute sleep cycles. If you enter the time you want to wake up, it subtracts complete cycles — plus a 15-minute buffer to fall asleep — to suggest bedtimes. If you enter a bedtime, it adds the 15-minute buffer and then whole cycles to suggest wake-up times. The option flagged “Recommended” is 5 to 6 cycles, which equals about 7.5 to 9 hours of sleep.
How to use the sleep calculator
- Choose whether you want to wake up at a set time or go to bed at a set time.
- Enter your target wake-up time or bedtime, or select “Use current time.”
- Select “Calculate” to see four options aligned to complete 90-minute sleep cycles, each including a 15-minute buffer to fall asleep.
- Choose one of the suggested times. The option marked “Recommended” gives 5 to 6 cycles, about 7.5 to 9 hours of sleep.
Frequently asked questions
How many hours of sleep do I need?
Most adults need 7 to 9 hours of sleep per night, which is the consensus range from sleep-science bodies such as the National Sleep Foundation. That works out to about 5 to 6 complete 90-minute sleep cycles.
How long does it take to fall asleep?
On average, it takes a healthy adult about 15 minutes to fall asleep. This sleep calculator automatically adds a 15-minute buffer, so the times it shows are lights-out targets, not the moment you are already asleep.
Why does waking up mid-cycle feel worse?
Sleep moves through roughly 90-minute cycles of light, deep, and REM sleep. Waking during deep sleep causes grogginess known as sleep inertia, while waking near the end of a cycle, when sleep is lighter, feels easier — even with the same total hours.
Is this a medical tool?
No. This is a planning tool based on the average 90-minute sleep cycle, not a medical or diagnostic device. If you regularly feel exhausted despite spending enough time in bed, talk to a doctor.