How to use this sleep calculator
- Choose whether you want to wake up at a set time, or go to bed at a set time.
- Enter the time using the time picker.
- Read the suggested bedtimes or wake-up times, they update instantly as you change the inputs.
Nothing is submitted or stored: everything is worked out in your browser, so your routine stays private.
How the sleep calculator works
Your night is not one long block of sleep. It runs in repeating cycles of about 90 minutes, each moving from light sleep into deep sleep and then back toward light sleep and dreaming. Waking up at the end of a cycle, while you are in light sleep, tends to feel far easier than being pulled out of deep sleep partway through one.
To make the times realistic, the calculator also adds about 15 minutes to fall asleep before the first cycle begins. From there it counts whole cycles. Aiming for 5 to 6 cycles gives roughly7.5 to 9 hours of sleep, which suits most adults, while 4 cycles (about 6 hours) is a lighter night for when time is short.
In wake-up mode it counts backward from your alarm to suggest bedtimes; in bedtime mode it counts forward from when you turn in to suggest the best moments to set your alarm. Pick whichever row fits your schedule and the number of hours you have available.
A note on accuracy
The 90-minute cycle is an average. Real cycles drift a little from night to night and person to person, and factors like caffeine, screens, stress and room temperature all affect how quickly you drop off and how well you sleep. Treat these times as a general guide to plan a more consistent routine, not a precise prescription, and speak to a doctor about ongoing sleep trouble.
Frequently asked questions
How does the sleep calculator work?
Sleep happens in cycles of about 90 minutes. The calculator steps backward or forward in 90-minute blocks and adds about 15 minutes to fall asleep, so each time lands at the end of a full cycle.
Why is waking at the end of a cycle better?
At the end of a cycle you are in light sleep, so waking feels easier and more refreshing. Waking during deep sleep can leave you groggy even after a long night.
How many cycles do I need?
Most adults do well with 5 to 6 cycles, roughly 7.5 to 9 hours. The tool also shows 4 cycles (about 6 hours) for shorter nights.
Why does it add 15 minutes?
Hardly anyone falls asleep instantly. Adding about 15 minutes of wind-down time gives a more realistic bedtime so your cycles still line up with your wake time.
Is my data uploaded?
No, everything is calculated on your device and nothing is sent anywhere.