Sleep repeats in 90-minute cycles. Wake at the end of a cycle and you feel refreshed; wake in the middle of deep sleep and you feel groggy all day. Enter the time you need to wake up to get bedtimes counted back from it, or enter a bedtime to get recommended wake-up times.
How it works: 90-minute sleep cycles + an average of 14 minutes to fall asleep. The recommended times are the times you should get into bed. Total sleep is your actual sleep time after subtracting the 14 minutes it takes to fall asleep.
Sleep is a repeating structure: light sleep → deep sleep → REM sleep, the stage where you dream. One set is one cycle, and its length averages about 90 minutes. The end of a cycle is a period of lighter sleep, so even if an alarm goes off you can wake relatively refreshed, while waking in the middle of deep sleep leaves you heavy and groggy because of sleep inertia — the state of not having fully woken up. That's why how long you slept matters, but so does where in a cycle you woke up. The principles and how to use them in different situations are covered in the sleep cycle guide.
Cycle length is set to the commonly used average of 90 minutes, and the time it takes to fall asleep (sleep onset latency) to an average of 14 minutes. Recommendations are based on the number of cycles. Six cycles (9 hours) and five cycles (7 hours 30 minutes) fall within the recommended adult sleep range of 7–9 hours, so they're marked "Recommended," while four cycles (6 hours) is the minimum, marked "Short sleep." However, cycle length varies widely from person to person, from 80 to 120 minutes, and even for the same person it changes from day to day. Try it for a few days and use it to find the time that leaves you most refreshed.
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Q. Is the 90-minute sleep cycle the same for everyone?
No. 90 minutes is a commonly used average, and actual cycle length varies from 80 to 120 minutes from person to person. Try the recommended times for a few days and adjust gradually toward the one that leaves you most refreshed.
Q. How many cycles should I sleep per day?
The recommended sleep duration for adults is 7–9 hours, which corresponds to 5–6 cycles. Four cycles (6 hours) is the minimum amount of sleep, so use it only occasionally; repeating it every day makes sleep debt easy to build up.
Q. What if it takes me longer than 14 minutes to fall asleep?
This calculator assumes an average of 14 minutes to fall asleep. If it usually takes you about 30 minutes, adjust to your own sleep onset time — for example, get into bed about 15 minutes earlier than the recommended bedtime.
Q. Why do I feel more tired when an alarm wakes me mid-cycle?
Being forced awake during deep sleep causes pronounced sleep inertia, the groggy state of not having fully woken up. Waking during the light sleep at the end of a cycle lets you get up relatively refreshed.
Q. How long should a nap be?
A short nap of around 20 minutes or a full 90-minute one-cycle nap is best. A 30–60 minute nap makes it easy to wake in the middle of deep sleep, which can leave you even groggier.