Sleep Cycle Calculator
Calculate the best bedtimes or wake-up times based on indicative 90-minute sleep cycles.
Possible bedtimes
For informational purposes only: sleep needs and actual cycle length vary from person to person. This calculator is not a personalized medical recommendation.
How is this result calculated?
The calculation applies the formula result time = target time ± (number of cycles × 90 min) ± 15 min, where 90 minutes corresponds to the average length of a complete sleep cycle and 15 minutes to the average time it takes to fall asleep.
In "I want to wake up at…" mode, the cycles and falling-asleep time are subtracted from the wake-up time to work back to the bedtime. In "I'm going to bed now" mode, they are added to the bedtime to obtain the wake-up time. All calculations are performed in minutes since midnight and then wrapped back onto a 24-hour clock, which handles crossing midnight without error, in either direction.
The 5- and 6-cycle ranges (7.5 to 9 hours of sleep) are highlighted because they correspond to the range most commonly recommended for adults, but these results remain general averages: actual needs vary from person to person and do not replace medical advice.
Frequently asked questions
Why does a sleep cycle last about 90 minutes?+
A night's sleep is organized into successive cycles that move through light sleep, deep sleep, and then REM sleep. In adults, a complete cycle lasts 90 minutes on average, although this can range from 70 to 110 minutes depending on the person and accumulated fatigue. Waking up at the end of a cycle rather than in the middle of deep sleep reduces the feeling of grogginess on waking.
Why is 15 minutes of falling-asleep time added?+
These 15 minutes represent the average time needed to fall asleep once in bed. This delay varies a lot from person to person: it can be shorter, or considerably longer in cases of stress, screen use before bed, or late caffeine. The calculator adds it by default to give a realistic estimate of bedtime or wake-up time.
What if I don't have a fixed bedtime or wake-up time?+
Simply use whichever time is closest to your actual situation: the "I'm going to bed now" mode defaults to the current time, which you can adjust if needed, and the "I want to wake up at…" mode accepts any target time, including for a nap or a shifted schedule. The calculation remains valid even if your schedule changes from one day to the next.
How many sleep cycles are recommended per night?+
General recommendations for an adult most often fall between 5 and 6 complete cycles, or roughly 7.5 to 9 hours of sleep. That's why these two ranges are highlighted in the results. This is a population average: some people feel rested with fewer cycles, others need more.
Do sleep needs vary with age?+
Yes, significantly. Infants and children need much more sleep than adults, while needs tend to slightly decrease with age in older adults. Cycle length itself also changes with age. This calculator applies an average adult formula, for informational purposes only, and does not replace personalized medical advice.