Sleep Calculator — best bedtimes & wake times
Find ideal bedtimes or wake times based on ~90-minute sleep cycles — in your browser.
🔒 Runs entirely in your browser. Your input and output are never sent to a server or stored — fully private.
Go to bed at one of these times
Based on ~90-min sleep cycles plus ~15 min to fall asleep.
Estimates only — not medical advice.
About this tool
Waking up groggy often means an alarm cut through deep sleep. This sleep calculator times your night around roughly 90-minute cycles: tell it when you must wake up and it suggests bedtimes that let you finish whole cycles, or tell it you are going to bed now and it lists good times to set the alarm. It is handy for shift workers, students and early risers. The times are worked out in your browser, so nothing you enter is uploaded.
Frequently asked questions
Why 90-minute cycles?
A full sleep cycle moves through light sleep, deep sleep and REM and lasts about 90 minutes on average. Waking at the end of a cycle, in light sleep, tends to feel easier than being jolted out of deep sleep mid-cycle. The calculator also adds about 15 minutes for the time it takes to fall asleep, so the suggested bedtimes line up with whole cycles.
How many cycles should I aim for?
Most adults do well on five or six cycles, which is about 7.5 to 9 hours of sleep. The tool highlights these as recommended, but the right number is personal — some people feel rested on four cycles now and then. Consistency matters as much as total time, so keeping similar bed and wake times each day helps you wake more naturally.
Is the 90-minute figure exact for everyone?
No. Cycle length varies from roughly 80 to 120 minutes between people, and even across one night, so think of the suggestions as helpful nudges rather than precise science. Ongoing tiredness, insomnia or snoring that disrupts sleep is worth raising with a doctor. The times are calculated entirely in your browser, and nothing you enter is uploaded or stored.