Sleep Calculator
Enter a wake-up time to see bedtimes that give you whole 90-minute sleep cycles, or enter a bedtime to see when each cycle count would land your alarm.
How to use it
- Pick a tab. "I want to wake up at" works backwards from your alarm. "I'm going to bed now" works forwards from the time you get into bed.
- Enter the time. The tool pre-fills the current time.
- Read the options from most to fewest cycles: 6 cycles is about 9 hours of sleep, 5 about 7.5, 4 about 6, and 3 about 4.5. Aim for 5 or 6 on a normal night.
How the times are calculated
Every bedtime is the wake-up time minus 15 minutes to fall asleep, minus 90 minutes per cycle. To wake at 7:00 after 6 cycles: 7:00 − 15 minutes − 6 × 90 minutes = 9:45 PM the previous evening. The wake-up direction runs the same math forwards: bedtime plus 15 minutes plus 90 minutes per cycle.
The idea behind the cycle count is that waking mid-cycle, in deep sleep, feels worse than waking between cycles. Real cycles vary between roughly 80 and 120 minutes, and the 15-minute fall-asleep figure is an average, so treat the listed times as targets to test, not guarantees. Sleep needs also vary by person: most adults need 7 to 9 hours, but some function well on less and others need more.
Frequently asked questions
Does this calculator keep a record of my sleep times?
No. The wake-up time or bedtime you enter is used inside your browser tab, and the cycle list is calculated there too. Nothing is sent to a server, nothing is saved on our side, and nothing is written to your browser's storage, so there is no history and nothing is remembered between visits. The site has no accounts and sets no cookies; if an ad is shown, the ad network follows its own policies and never sees the times you entered.
Why are sleep cycles 90 minutes?
Sleep moves through light, deep, and REM stages in cycles that average about 90 minutes in adults. Individual cycles run shorter and longer, so the 90-minute figure is a planning average, not a measurement.
Why does the calculator add 15 minutes?
Most people take around 15 minutes to fall asleep after getting into bed. If you usually fall asleep faster or slower, shift the listed times by the difference.
How many cycles should I aim for?
Most adults do best on 7 to 9 hours of sleep, which is about 5 to 6 cycles. The 3- and 4-cycle options are for occasional short nights, not a routine target. Sleep needs vary from person to person, so treat the options as a range to experiment with rather than a prescription.
What if I miss the listed bedtime?
Use the next option down instead. Consecutive options sit exactly one cycle, 90 minutes, apart, so going to bed one option later keeps you aligned with your alarm.
Is this medical advice?
No. It is a scheduling aid for healthy adults. If you regularly sleep poorly or feel tired despite enough hours in bed, talk to a doctor.