Sleep Time Calculator

Sleep Time Calculator is a free web tool that helps you plan sleep around natural 90-minute sleep cycles, so you wake up at the end of a cycle instead of in the middle of deep sleep. Waking mid-cycle is a common reason people feel groggy even after a full night in bed; timing your alarm to cycle boundaries can make mornings noticeably easier.
Using it is simple. Enter the time you need to wake up, and the calculator suggests bedtimes that give you 3, 4, 5, or 6 complete sleep cycles. It automatically adds about 14 minutes, the typical time it takes to fall asleep, so the suggested times are realistic. You can also work forward: enter when you plan to go to bed and see the best times to set your alarm. A nap mode helps you choose a nap length that refreshes you without leaving you foggy.
The page goes beyond the numbers. It explains how the stages of sleep work, from light sleep to deep sleep and REM, and what sleep inertia is and why it happens. A cycle reference table lets you see bedtime and wake-up combinations at a glance, an FAQ answers common questions, and the content cites its sources so you can read further.
The site also offers related tools for naps, sleep debt, and jet lag, which are useful for shift workers, frequent travelers, students preparing for exams, and parents trying to fit rest into a busy schedule.
There is no sign-up, no account, and no cost. The calculator runs in your browser and works well on phones, so you can check your ideal bedtime right before you turn off the lights.
Sleep Time Calculator is for anyone who wants a quick, practical way to time their sleep better: pick a wake-up time, get your bedtime options, and wake up feeling more rested.