What is the Sleep Calculator based on?

The Sleep Calculator is based on 90-minute sleep cycles, the average time it takes to move through light, deep and REM sleep once. Waking up at the end of a cycle, rather than in the middle of one, tends to feel less groggy.

How does the Sleep Calculator handle the time it takes to fall asleep?

It adds a 15-minute buffer on top of the sleep cycles by default, since most people do not fall asleep the instant they lie down. That buffer is included in every suggested bedtime and wake-up time.

Why does the Sleep Calculator show more than one bedtime option?

It shows options for 6, 5, 4 and 3 full sleep cycles so you can trade off total sleep time against how late you can stay up or how early you need to wake, rather than forcing a single fixed answer.

Can the Sleep Calculator suggest a wake-up time instead of a bedtime?

Yes. Switch to "I am going to sleep now," enter the time you plan to fall asleep, and it lists wake-up times for 6, 5, 4 and 3 full cycles from that moment.

Is the Sleep Calculator a substitute for medical sleep advice?

No. It is a general guide based on average 90-minute sleep cycles, not a medical tool. Individual cycle length varies, so treat the results as a starting point, not a diagnosis.

Does the Sleep Calculator store or upload the times I enter?

No. All the sleep-cycle math happens locally in your browser, and the times you enter are not sent anywhere or saved to an account.

Sleep Calculator Online

Find the best bedtime or wake-up time using 90-minute sleep cycles, calculated locally in your browser.

The Sleep Calculator runs entirely in your browser. The times you enter are used only to compute bedtimes and wake-up times on your device and are never uploaded.

Open the Time Calculator

About Sleep Calculator

Sleep Calculator works out the best bedtimes or wake-up times based on 90-minute sleep cycles, the average length of one full cycle of light, deep and REM sleep. Choose "I want to wake up at" and enter a wake time to see a handful of bedtime options — 6, 5, 4 or 3 full cycles back from that alarm, each including a 15-minute buffer to actually fall asleep. Or choose "I am going to sleep now" and enter the time you plan to close your eyes to see the wake-up times that let you finish a full cycle instead of waking up mid-cycle groggy. It is useful for planning an early flight, working around a late night, or just building a more consistent sleep schedule. Every calculation runs locally in your browser.

Features

Two modes: pick a wake-up time to see suggested bedtimes, or a bedtime to see wake-up times
Suggests 6, 5, 4 and 3 full 90-minute sleep cycles for each direction
Adds a configurable buffer for the time it takes to actually fall asleep
Shows the total sleep duration next to each suggested time
One-tap "Now" button to fill in the current clock time
Copy any single suggested time or the whole list at once
Handles times that cross midnight automatically
Runs entirely on your device — nothing about your schedule is uploaded

How to use the Sleep Calculator

Choose whether you are setting a wake-up time or falling asleep right now
Enter the time in the field, or tap Now to use the current clock time
Review the list of suggested bedtimes or wake-up times, each labeled with its cycle count
Copy the time that fits your schedule

Example

Input

wake at 6:30 AM

Output

Bedtime: 9:15 PM (6 cycles)

6 full 90-minute cycles plus a 15-minute buffer to fall asleep, counted back from the wake-up time.

Common errors & troubleshooting

The suggested bedtime feels too early or too late for a normal night. — Each row is a different number of full sleep cycles (6 down to 3) — pick whichever row matches how many hours of sleep you are aiming for, not just the first one.
Entered a wake-up time but got bedtimes for the previous evening instead of that same day. — That is expected: working backward 6 cycles from an early-morning wake time lands the night before, since 90-minute cycles add up to several hours.
Unsure why the times are not exact multiples of 90 minutes from the input. — Each result also includes a 15-minute buffer for the time it typically takes to fall asleep, on top of the sleep cycles themselves.
Time field will not accept a typed value. — Use the built-in time picker (or type HH:MM in 24-hour or the browser's local format) instead of an unstructured entry.

Frequently asked questions

What is the Sleep Calculator based on?: The Sleep Calculator is based on 90-minute sleep cycles, the average time it takes to move through light, deep and REM sleep once. Waking up at the end of a cycle, rather than in the middle of one, tends to feel less groggy.
How does the Sleep Calculator handle the time it takes to fall asleep?: It adds a 15-minute buffer on top of the sleep cycles by default, since most people do not fall asleep the instant they lie down. That buffer is included in every suggested bedtime and wake-up time.
Why does the Sleep Calculator show more than one bedtime option?: It shows options for 6, 5, 4 and 3 full sleep cycles so you can trade off total sleep time against how late you can stay up or how early you need to wake, rather than forcing a single fixed answer.
Can the Sleep Calculator suggest a wake-up time instead of a bedtime?: Yes. Switch to "I am going to sleep now," enter the time you plan to fall asleep, and it lists wake-up times for 6, 5, 4 and 3 full cycles from that moment.
Is the Sleep Calculator a substitute for medical sleep advice?: No. It is a general guide based on average 90-minute sleep cycles, not a medical tool. Individual cycle length varies, so treat the results as a starting point, not a diagnosis.
Does the Sleep Calculator store or upload the times I enter?: No. All the sleep-cycle math happens locally in your browser, and the times you enter are not sent anywhere or saved to an account.

Related tools

Time Calculator — Add or subtract times and durations, and sum multiple time spans.
GPA Calculator — Calculate your grade point average from course grades and credit hours.
Grade Calculator — Compute a weighted course grade and the score needed on the final exam.
Unit Conversion Calculator — Convert length, weight, temperature, volume, area, speed and more.
Age Calculator — Calculate exact age in years, months and days as of any date, plus totals in weeks, days, hours and next birthday.
Target Heart Rate Calculator — Find your target heart-rate training zones from age and resting rate.