Sleep Calculator (Free & Easy to Use)
Use this free sleep calculator to find the best time to go to sleep or wake up. Based on 90-minute sleep cycles, this tool helps you wake up feeling refreshed and avoid morning grogginess.
Sleep Calculator
⏰ Based on 90-minute sleep cycles and 15 minutes to fall asleep. Waking at the end of a cycle helps you feel more refreshed.
How to Use the Sleep Calculator
Select one of the three modes — wake up time, bedtime, or sleep right now — enter your time if needed, and click Calculate. The tool will show you the best times based on your natural sleep cycles.
How Sleep Cycles Work
Your sleep is made up of 90-minute cycles. Each cycle moves through light sleep, deep sleep, and REM sleep. Waking up at the end of a complete cycle — rather than in the middle — is what makes the difference between feeling refreshed and feeling groggy.
The formula used is: Bedtime = Wake-up time − (90 min × number of cycles) − 15 min to fall asleep
Most adults need 5 to 6 complete cycles per night, which equals 7.5 to 9 hours of sleep.
FAQs
How many hours of sleep do I need?
Most adults need between 7.5 and 9 hours, which equals 5 to 6 complete sleep cycles of 90 minutes each.
Why do I feel tired even after 8 hours of sleep?
You likely woke up in the middle of a sleep cycle. Use this calculator to time your alarm to the end of a cycle instead.
Is this calculator accurate?
It uses the standard 90-minute cycle model used by sleep researchers. Individual sleep times vary slightly but this gives a very reliable guideline.
Can I use this on my phone?
Yes, this calculator works on all devices including mobile and tablet.