Sleep Calculator is a free, easy-to-use online tool that helps you wake up feeling refreshed instead of groggy. Most alarm clocks ignore how sleep actually works — but your body sleeps in repeating cycles of about 90 minutes, moving from light sleep into deep sleep and back again. Waking up in the middle of a deep-sleep phase is what leaves you feeling groggy and disoriented, even after a full night in bed.
Our calculator is built around this simple science. Tell it what time you need to wake up, and it counts backwards in complete 90-minute sleep cycles — plus about 15 minutes to fall asleep — to show you the ideal bedtimes. Prefer to plan the other way around? Enter your bedtime and it will tell you the best times to set your alarm. There is even an “I’ll sleep now” mode that starts from the current time.
Whether you are an early riser, a night owl, or just trying to fix your schedule, Sleep Calculator helps you time your rest so your alarm rings during light sleep, when waking feels natural. No sign-ups, no fees — just better mornings.