Health & Life

Sleep Cycle Calculator

Best wake-up and bedtimes based on 90-minute sleep cycles.

Calculation mode

Wake-up time

Time to fall asleep (minutes)

Recommended bedtime

권장 6주기 오후 9:45 9시간

권장 5주기 오후 11:15 7시간 30분

4주기 오전 12:45 6시간

3주기 오전 2:15 4시간 30분

Based on 90-minute sleep cycles; 7h 30m (5 cycles) and 9h (6 cycles) are marked "Recommended".

What is the Sleep Cycle Calculator?

The Sleep Cycle Calculator is based on sleep science showing that sleep alternates between light sleep and REM sleep in cycles that average about 90 minutes. It works backward from that cycle length to suggest bedtimes or wake-up times that land at a light-sleep boundary, so you wake up refreshed instead of mid-cycle. Enter the time you want to wake up to get suggested bedtimes, or enter (or use) a bedtime to get suggested wake-up times — each shown in several 90-minute increments.

How to use it

Choose a calculation mode at the top. Wake time → Bedtime is for when you know what time you need to wake up, Bedtime → Wake time is for when you know what time you'll fall asleep, and Sleep now shows good wake-up times if you fall asleep right away.

Enter the wake or bedtime, adjust the time it takes you to fall asleep if needed (15 minutes by default), and press Calculate to see four cards covering 3 to 6 sleep cycles. 7.5 hours (5 cycles) and 9 hours (6 cycles) are marked "Recommended".

How it's calculated — the 90-minute sleep cycle

Bedtime = Wake time − time to fall asleep − (90 min × number of cycles)

Wake time = Bedtime + time to fall asleep + (90 min × number of cycles)

For example, if you want to wake up at 7:00 AM and it takes you 15 minutes to fall asleep, a 6-cycle (9-hour) night means going to bed at 7:00 − 15 min − 9 hours = 9:45 PM, and a 5-cycle (7.5-hour) night means 11:15 PM.

Note: Actual sleep cycle length varies from person to person, and even day to day, typically reported in a range of about 75-110 minutes. 90 minutes is an average used for this calculation.

Examples

If you have a 7 AM flight tomorrow and absolutely need to be up by 6, enter 06:00 as your wake time to see what bedtime lets you wake up refreshed. If you expect to fall asleep around 1 AM tonight, enter 01:00 as your bedtime to see good alarm times in advance.

Recommended sleep by age

National Sleep Foundation recommended sleep duration
Age group	Recommended sleep
Teens (14-17)	8-10 hours
Adults (18-64)	7-9 hours
Older adults (65+)	7-8 hours

Note: This calculator gives a rough estimate based on general sleep-cycle theory. Actual sleep needs vary by age, health, and lifestyle. If you suspect a sleep disorder such as insomnia or excessive daytime sleepiness, please consult a sleep specialist or clinic.

FAQ

Why is a sleep cycle 90 minutes?

During sleep, the body alternates between light (non-REM) sleep and dream-filled REM sleep, and one full cycle averages about 90 minutes. Waking during the light-sleep part of a cycle feels refreshing, while waking during deep sleep or REM leaves you groggy from sleep inertia even after the alarm goes off. This calculator works backward from that 90-minute cycle to suggest good times to wake up.

How many cycles should I sleep for?

For adults, the National Sleep Foundation recommends 7-9 hours of sleep a night, which works out to 5-6 cycles of 90 minutes. That's why this calculator marks 5 cycles (7.5 hours) and 6 cycles (9 hours) as "Recommended". 3-4 cycles are a reference for when you're short on time, not a duration to rely on regularly.

Why do I enter how long it takes to fall asleep?

Getting into bed doesn't mean falling asleep instantly — it typically takes about 10 to 20 minutes to actually fall asleep, and adding that time makes the cycle calculation more accurate. If you know how long it usually takes you, enter that instead of the 15-minute default.

Is it okay to hit snooze several times when the alarm doesn't wake me up?

Repeatedly snoozing sends you back into short bursts of light sleep that get interrupted again within minutes, which can make sleep inertia worse rather than better. It's usually better to set a single alarm for a wake-up time aligned with your sleep cycle than to rely on snoozing.

Can the 90-minute sleep cycle apply to naps too?

Yes. For a short nap, a power nap of 20 minutes or less — while still in light sleep — tends to leave you refreshed, while a longer nap works best when it covers one full 90-minute cycle so you don't wake from deep sleep. Napping for something in between, like 40-60 minutes, risks waking you mid-deep-sleep and leaving you groggier than before.

Recommended sleep by age

FAQ

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