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ToolWrenWork out the best times to go to bed or wake up so you rise between sleep cycles feeling refreshed, not groggy.
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Sleep is a free, private online tool that lets you find the best bedtimes or wake-up times based on 90-minute sleep cycles. It runs entirely in your browser, so nothing you enter is uploaded to a server.
Privacy: this tool runs entirely in your browser. Your input is never sent to, received by, or stored on any server — there are no uploads and no tracking of what you enter.
Sleep runs in cycles of roughly 90 minutes, moving through light, deep and REM stages. Waking at the end of a cycle leaves you refreshed; waking mid-cycle leaves you groggy. Most people need 5–6 cycles (about 7.5–9 hours) a night.
Enter the time you must wake up to get ideal bedtimes, enter a bedtime to get ideal wake-up times, or choose 'Sleep now' to see when to set your alarm if you fall asleep right away.
Unlike basic bedtime calculators, this one lets you adjust the cycle length (85–100 minutes) and your personal fall-asleep time, and it labels each option with cycle count, total sleep hours and a quality rating.
Sleep is not uniform - it runs in repeating cycles of roughly 90 minutes, each moving through light sleep, deep sleep and REM (dreaming). Waking at the end of a cycle, in light sleep, leaves you refreshed; waking in the middle, during deep sleep, leaves you groggy even if you slept longer. This calculator times your sleep around those cycles.
Total hours matter, but where in a cycle you wake matters too. Most adults need 5 to 6 complete cycles - about 7.5 to 9 hours. Six hours of sleep ending cleanly at a cycle boundary can feel better than seven hours that cut a deep-sleep phase short. The tool suggests times that land your wake-up between cycles.
Work in whichever direction you need: enter the time you must wake up to get ideal bedtimes; enter a bedtime to get ideal wake-up times; or pick sleep now to see when to set your alarm if you fall asleep right away. Each suggested time is a whole number of cycles from when you actually fall asleep.
Most people take around 15 minutes to drift off, so a naive wake-minus-hours calculation is wrong. This tool adds your fall-asleep time so the cycles are counted from the moment you are actually asleep - and unlike basic bedtime calculators it lets you adjust both the cycle length (85 to 100 minutes) and that fall-asleep buffer to match your own pattern.
Sleep cycle length varies between people and even night to night, so treat the suggestions as a smart starting point rather than a guarantee. Consistent sleep and wake times, a dark cool room and limiting screens before bed all improve sleep quality. If you regularly wake unrefreshed, consider speaking to a doctor. Everything is calculated in your browser.