Sleep Calculator
WEBFind the best times to sleep or wake up based on 90-minute sleep cycles.
- 9:45 PM6 cycles9 hrRecommended
- 11:15 PM5 cycles7 hr 30 minRecommended
- 12:45 AM4 cycles6 hr
- 2:15 AM3 cycles4 hr 30 min
Times allow ~15 min to fall asleep, then wake at the end of a full 90-minute cycle.
Based on 90-minute sleep cycles. A guide for better mornings, not medical advice.
What is the Sleep Calculator tool?
This sleep calculator works out the best times to go to bed or wake up so you finish on a natural boundary between sleep cycles instead of in the middle of deep sleep. Sleep runs in cycles of roughly 90 minutes, and it takes the average person about 15 minutes to actually drift off — waking at the end of a complete cycle is what leaves you feeling refreshed rather than groggy, even after the same number of hours in bed.
There are two ways to use it. Enter the time you need to wake up and it counts backwards to give you four bedtimes, one for each of 6, 5, 4 and 3 complete cycles, each already including the time it takes to fall asleep. Or choose "sleep now" and it counts forward from the current moment to show the ideal times to set your alarm. The 6-cycle (about nine hours) and 5-cycle (about seven and a half hours) options are marked as recommended, because most adults do best on seven to nine hours of sleep.
Everything is calculated instantly in your browser — nothing is uploaded and no sign-up is needed. It works on your phone, respects your privacy, and gives you a clear list of times you can act on tonight. Treat it as a practical guide for planning better mornings, not as medical advice: cycle length varies from person to person and night to night, so use the suggestions as a helpful starting point.
How to use the Sleep Calculator
- Pick a mode: "Wake up at" to plan bedtimes, or "Sleep now" to plan wake times.
- For wake mode, enter the time you need to wake up; for sleep-now mode, tap update time to use the current clock.
- Read the list of suggested times — each shows the number of sleep cycles and total hours of sleep.
- Choose a recommended option (5 or 6 cycles) for a full night, or a shorter one if you are short on time.
- Set your alarm or head to bed at the time you picked — the calculator already allows ~15 minutes to fall asleep.
Frequently asked questions
How does a sleep calculator work?
It counts in 90-minute sleep cycles and adds about 15 minutes to fall asleep. From a wake time it works backwards to bedtimes; from a bedtime it works forwards to wake times. Ending on a cycle boundary — rather than mid-cycle — is what helps you wake up feeling rested.
Why 90 minutes per cycle?
A full sleep cycle moves through light sleep, deep sleep and REM and averages around 90 minutes in adults. Waking at the end of a cycle, when sleep is lightest, tends to feel much easier than waking during deep sleep. Real cycles vary from 70 to 120 minutes, so 90 is a practical average.
What time should I go to bed to wake up at 6 or 7 am?
Enter your target wake time and the calculator lists the exact bedtimes for you. As a rough guide, to wake at 7:00 am refreshed, going to bed around 9:45 pm (6 cycles) or 11:15 pm (5 cycles) works for most people.
How many sleep cycles do I need?
Most adults do best with five to six complete cycles a night — roughly seven and a half to nine hours of sleep. The calculator marks those options as recommended. Four cycles (about six hours) is a workable minimum, and three (about four and a half hours) is best kept for the occasional short night.
Is the sleep calculator free and private?
Yes. It is free with no sign-up, and every calculation runs entirely in your browser — your times are never uploaded or stored on a server. Once the page has loaded it even works offline.
Is this medical advice?
No. It is a planning guide based on the 90-minute-cycle model, not a diagnosis or treatment. Sleep needs differ by age, health and lifestyle, so if you regularly struggle with sleep, speak to a health professional.