Sleep Calculator

Find the best times to wake up or go to sleep based on 90-minute sleep cycles.

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How Sleep Cycles Work

Sleep occurs in 90-minute cycles through NREM and REM stages. Waking between cycles (at the 6-hour or 7.5-hour mark) leaves you feeling more refreshed than waking mid-cycle.

Best wake-up times:

4:45 AM

Wake at 4:45 AM

6h sleep

4 cycles - Minimum - Groggy

6:15 AM

Wake at 6:15 AM

7.5h sleep

5 cycles - OK - Functional

7:45 AM

Wake at 7:45 AM

9h sleep

6 cycles - Optimal - Refreshed

How to Use the Sleep Calculator

  1. Choose your mode. "I am going to bed at" finds the best wake-up times based on your bedtime. "I need to wake up at" works backward to show you when to go to bed to complete full sleep cycles before your alarm.
  2. Set your time. Enter your planned bedtime or required wake-up time using the time picker.
  3. Adjust fall-asleep time. The average person takes 10-20 minutes to fall asleep. Change this if you typically fall asleep faster or slower. The calculator adds this to your bedtime before counting cycles.
  4. Read the optimal times. Three wake times or bedtimes are shown, based on completing 4 cycles (6 hours), 5 cycles (7.5 hours), or 6 cycles (9 hours). Waking at the end of a complete cycle reduces grogginess.

The goal is to wake up between REM stages, not during deep sleep. Even getting 7.5 hours aligned to cycle endpoints can feel more refreshing than 8 hours that interrupts a deep sleep phase.

Sleep Cycles and Recommended Sleep by Age

Each sleep cycle lasts approximately 90 minutes and progresses through four stages: N1 (light sleep), N2 (deeper light sleep), N3 (deep/slow-wave sleep), and REM (rapid eye movement, where most dreaming occurs). Early cycles have more deep sleep; later cycles have more REM. Waking during N3 causes the worst grogginess.

Wake time = Bedtime + Fall-asleep time + (Cycles × 90 minutes)

Example: Bedtime 10:30 PM, fall asleep in 15 min, 5 cycles:
10:30 PM + 0:15 + (5 × 1:30) = 10:30 + 7:45 = 6:15 AM

Recommended sleep duration by age (National Sleep Foundation):

Age GroupRecommended HoursAcceptable Range
Newborn (0-3 months)14-17 hours11-19 hours
Infant (4-11 months)12-15 hours10-18 hours
Toddler (1-2 years)11-14 hours9-16 hours
Preschool (3-5 years)10-13 hours8-14 hours
School age (6-13 years)9-11 hours7-12 hours
Teen (14-17 years)8-10 hours7-11 hours
Young adult (18-25)7-9 hours6-11 hours
Adult (26-64)7-9 hours6-10 hours
Older adult (65+)7-8 hours5-9 hours

Frequently Asked Questions

One complete sleep cycle lasts approximately 90 minutes. Each cycle includes light sleep (N1, N2), deep slow-wave sleep (N3), and REM sleep. A full night typically includes 4-6 cycles. The reason cycles matter is that waking mid-cycle, especially during deep N3 sleep, causes sleep inertia: intense grogginess that can last 30-60 minutes. Waking at the natural end of a cycle, during a lighter sleep phase, makes you feel alert immediately even with the same total hours.

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