General tools
Sleep cycle calculator
Line your night up with 90-minute sleep cycles to wake up less groggy. Everything runs in your browser - nothing is uploaded.
How it works
Sleep runs in cycles of roughly 90 minutes, moving from light sleep down to deep sleep and back up. Waking at the end of a cycle - when sleep is lightest - tends to feel easier than being pulled out of deep sleep mid-cycle, which is the groggy feeling an alarm can cause. This calculator lines your night up with those cycles. Tell it when you need to wake and it counts back to give you the best bedtimes; tell it when you are going to bed and it counts forward to the best times to set your alarm.
It also adds about 15 minutes for the time it takes to actually fall asleep, and offers a few options - typically five or six cycles (about 7.5 to 9 hours), with shorter four-cycle nights for when that is all you have. The 90-minute cycle is an average, so treat the times as helpful targets rather than precise science, and use the option that fits how much sleep you can get. Everything runs in your browser and nothing is sent to a server.
Example. To wake at 7:00 AM, the calculator suggests going to bed around 9:45 PM (6 cycles), 11:15 PM (5 cycles), or 12:45 AM (4 cycles), each landing your wake-up at the end of a cycle. Choose "go to bed" and it does the reverse from your bedtime.
FAQ
How long is a sleep cycle?
About 90 minutes on average. A full night is usually five to six cycles, so roughly 7.5 to 9 hours. The exact length varies from person to person and night to night, so the times here are averages to aim for.
Why does waking at the end of a cycle matter?
At the end of a cycle sleep is at its lightest, so waking then generally feels less groggy than being woken from deep sleep partway through a cycle. Aligning your alarm with a cycle boundary is the idea behind this calculator.
Why does it add 15 minutes?
Most people do not fall asleep the instant their head hits the pillow. The calculator allows about 15 minutes to drift off, so the cycle count is measured from when you are actually asleep, not from when you get into bed.
Is this medical advice?
No. It is a simple planning aid based on the average 90-minute cycle. If you regularly struggle with sleep or wake unrefreshed despite enough hours, that is worth raising with a doctor.