Sleep Time Calculator

Sleep Time Calculator is a free web tool that plans sleep around 90-minute sleep cycles. Enter the time you need to wake up and it suggests bedtimes for 3 to 6 cycles, adding about 14 minutes for falling asleep; it can also work forward from a bedtime or plan a nap. The page explains how sleep stages and sleep inertia work, includes a cycle reference table, an FAQ, and cited sources. Related tools on the site cover naps, sleep debt, and jet lag. No account is needed.