About this tool
A sleep calculator suggests bedtimes or wake-up times that fall between 90-minute sleep cycles, so you rise near the end of a cycle instead of in the middle of one. Enter a target wake-up time and it lists the bedtimes that fit whole cycles; switch to the go-to-sleep-now mode and it reads your device's current clock to list the best times to wake up.
Each result covers a whole number of cycles — 6, 5, or 4, equal to 9, 7.5, or 6 hours of sleep. Every calculation adds a fixed 14 minutes for the average time it takes to fall asleep, results that cross midnight roll over to the next day, and times are shown in your locale's clock format (12- or 24-hour).
The math runs entirely in your browser with no uploads, no accounts, and no tracking, and it keeps working offline once the page has loaded. Nothing about your schedule leaves the page.