Sleep occurs in cycles of approximately 90 minutes, each consisting of light sleep, deep sleep, and REM sleep. Waking up at the end of a complete cycle — rather than in the middle of one — leaves you feeling more refreshed and alert. Our sleep calculator works in two ways: enter your wake-up time to find the best bedtimes, or enter your bedtime to find the best times to set your alarm. The calculator accounts for the average 14 minutes it takes to fall asleep. Most adults need 5 to 6 complete sleep cycles (7.5 to 9 hours) per night for optimal health and cognitive function. The formula used is: Bedtime equals wake-up time minus the number of cycles multiplied by 90 minutes minus 14 minutes. For example, to wake at 7:00am after 5 cycles: 7:00am minus 450 minutes minus 14 minutes equals 11:16pm bedtime. Frequently Asked Questions: How long is a sleep cycle? The average sleep cycle lasts approximately 90 minutes, though this can range from 80 to 110 minutes depending on the individual. Earlier cycles tend to have more deep sleep; later cycles have more REM sleep. How many sleep cycles do I need? Most adults need 5 to 6 complete sleep cycles per night, which equals 7.5 to 9 hours of sleep. Some people function well on 4 cycles (6 hours), but consistently sleeping fewer than 5 cycles is associated with impaired cognitive function and health risks. What is sleep inertia? Sleep inertia is the groggy, disoriented feeling you experience when you wake up during deep sleep or REM sleep. Waking at the end of a complete sleep cycle minimises sleep inertia. What time should I go to bed to wake up at 7am? To wake at 7:00am feeling refreshed, aim to be in bed by 9:46pm for 6 cycles, 11:16pm for 5 cycles, or 12:46am for 4 cycles. Is this calculator suitable for children? This calculator is designed for adults. Children and teenagers require more sleep than adults, typically 9 to 11 hours for school-age children and 8 to 10 hours for teenagers.