Free sleep calculator: get bedtimes from your wake-up time or wake-up times from your bedtime, using 90-minute sleep cycles and 7-9 hour age recommendations.
Sleep follows a natural rhythm of 90-minute cycles, alternating between light, deep, and REM sleep. Waking up in the middle of a cycle leaves you groggy, while waking at a cycle boundary feels more refreshing. This is why the same 7-8 hours can feel completely different depending on when you fall asleep.
The sleep calculator works in both directions. Enter the time you need to wake up and it works backwards — subtracting whole 90-minute cycles plus a 15-minute buffer for falling asleep — to show a 5-cycle and a 6-cycle bedtime. Or enter your bedtime and it works forwards to show wake-up times that land on a cycle boundary. Pick whichever fits your schedule.
Sleep needs change with age. Adults aged 18-64 typically need 7-9 hours, while adults 65 and older usually need 7-8 hours. The calculator adjusts the recommended range automatically based on your age.
Choose a direction, enter a time and your age — the calculator shows the best bedtimes or wake-up times at a cycle boundary.
Depending on the mode, it works backwards from your wake-up time to find bedtimes, or forwards from your bedtime to find wake-up times. Either way it uses whole 90-minute sleep cycles plus a 15-minute buffer for falling asleep, so you wake up at a cycle boundary rather than in the middle of one.
15 minutes is the average time it takes to fall asleep (sleep latency). If you usually take longer, add your own estimate; the math works the same way.
Most adults need 7-9 hours per night (National Sleep Foundation). Adults 65 and older typically need 7-8 hours. Sleep needs also vary from person to person.
Waking at a cycle boundary helps you feel more refreshed because you don't interrupt deep or REM sleep. That's why the calculator shows whole-cycle bedtimes.
Yes. Use your actual wake-up time — for example 15:00 — or, in reverse mode, your bedtime, and the calculator will find whole-cycle times for your schedule. Keep the timing consistent on days off too.
A short nap of about 20 minutes can boost alertness, and a full 90-minute cycle can substitute for part of the night. Avoid long naps late in the day, as they can delay your bedtime.