Sleep Time Calculator
Calculate the best time to go to sleep or wake up based on 90-minute sleep cycles. Wake up feeling refreshed and energized.
I want to wake up at...
To wake up refreshed, try going to bed at one of these times (includes 15 min to fall asleep):
09:45 PM
6 Cycles (9 hrs)11:15 PM
5 Cycles (7.5 hrs)12:45 AM
4 Cycles (6 hrs)02:15 AM
3 Cycles (4.5 hrs)I am going to sleep at...
If you go to sleep now, try waking up at one of these times to feel rested:
03:15 AM
3 Cycles (4.5 hrs)04:45 AM
4 Cycles (6 hrs)06:15 AM
5 Cycles (7.5 hrs)07:45 AM
6 Cycles (9 hrs)Sleep Cycle & Bedtime Calculator
A good night's sleep consists of 5-6 complete sleep cycles. Waking up in the middle of a sleep cycle can leave you feeling groggy and tired. Use this sleep clock calculator to find the perfect bedtime based on when you want to wake up, or find out when to set your alarm if you're going to sleep right now — it also works as a simple sleeping-hours calculator for any two times you enter.
How to Use It
- Choose whether you want to calculate a bedtime or a wake-up time.
- Enter your target time — the calculator adds ~15 minutes to fall asleep, then works in 90-minute cycles.
- Pick from the suggested times, each landing at the end of a complete sleep cycle.
Example — How Many Hours of Sleep Is That?
If you need to wake up at 7:00 AM, going to bed around 11:15 PM or 9:45 PM lets you complete 5 or 6 full 90-minute cycles, respectively, rather than waking mid-cycle. Beyond cycle planning, this bedtime calculator can also just answer the plain arithmetic question of "how many hours of sleep is that?" for a given bedtime and wake time:
- Sleep at 9:00 PM, wake at 5:00 AM → 8 hours of sleep.
- Sleep at 10:00 PM, wake at 5:00 AM → 7 hours of sleep.
- Sleep at 12:00 AM (midnight), wake at 8:00 AM → 8 hours of sleep.
- Sleep at 10:00 PM, wake at 7:00 AM → 9 hours of sleep.
Common Use Cases
- Planning bedtime around a fixed wake-up alarm (bedtime calculator).
- Figuring out the best time to nap or go to bed after a late night.
- Quickly checking how many hours of sleep you got (or will get) between two clock times.
FAQs
Why 90-minute cycles?
A full sleep cycle (light sleep, deep sleep, REM) averages about 90 minutes. Waking up at the end of a cycle, rather than in the middle of deep sleep, generally feels less groggy.
How many hours of sleep is 9pm to 5am?
Sleeping from 9:00 PM to 5:00 AM is 8 hours of sleep — that's 5 complete 90-minute sleep cycles plus a little extra.
If I sleep at 10 and wake up at 5, how many hours is that?
Sleeping from 10:00 PM to 5:00 AM is 7 hours of sleep.
If I sleep at 12 and wake up at 8, how many hours is that?
Sleeping from 12:00 AM (midnight) to 8:00 AM is 8 hours of sleep.
How many hours of sleep is 10 to 7?
Sleeping from 10:00 PM to 7:00 AM is 9 hours of sleep.