How does the sleep calculator work?

It uses the 90-minute sleep cycle model: the average person moves through distinct sleep stages in roughly 90-minute cycles. Waking at the end of a cycle (during lighter sleep) feels easier than waking mid-cycle. The tool counts backwards from your target wake-up time (or forwards from now) in whole 90-minute blocks, adding ~15 minutes to account for falling asleep.

Why do you add 15 minutes to fall asleep?

Research suggests the average healthy adult takes about 14–17 minutes to fall asleep (sleep latency). Adding ~15 minutes means the 90-minute cycles start after you have actually drifted off, so the suggested wake time lands at the end of a cycle.

How many hours of sleep do I need?

Most adults need 7–9 hours, which corresponds to about 5–6 full sleep cycles. The calculator highlights the 5-cycle (~7.5h) and 6-cycle (~9h) options as recommended. Teenagers and younger children usually need more; older adults may need slightly less but should still aim for at least 5 cycles.

Is this sleep calculator accurate?

It is a helpful planning tool based on a well-established average, but sleep cycles vary between individuals and from night to night. Stress, caffeine, age, and sleep disorders can all shift your cycles. Use the suggested times as a starting point and adjust based on how you feel. It is not medical advice — for persistent sleep problems, consult a healthcare professional.

What is Sleep Calculator?

The Sleep Calculator is a free, browser-based tool that finds the best time to go to bed or wake up using the 90-minute sleep cycle model. Instead of simply adding eight hours, it counts in whole cycles so you wake at the end of a cycle — during lighter sleep — which leaves you feeling refreshed instead of groggy. Enter a target wake-up time to get recommended bedtimes, or choose "sleep now" to see optimal alarm times. It also adds about 15 minutes for the average time it takes to fall asleep. No downloads, no account, and nothing leaves your device.

Key Benefits

Wake at the end of a sleep cycle to feel refreshed, not groggy
Plan backwards from a fixed wake-up time or forwards from now
Recommended 5–6 cycle options for the best quality rest
Accounts for the ~15 minutes needed to fall asleep
Works offline and keeps your data private
Completely free with no registration

Common Use Cases

•Finding the right bedtime to wake up on time for work or school
•Setting an alarm when going to bed now so you wake rested
•Adjusting your schedule for an early meeting or flight
•Planning a power nap of one full cycle (~90 minutes)
•Building a consistent sleep routine that matches your natural cycles

How to Use the Sleep Calculator

Choose Your Mode: Pick "I want to wake up at" if you have a target alarm time, or "I'll sleep now" to find the best times to set your alarm.

Enter a Time: Type your desired wake-up time (e.g., 07:00) or confirm your current time if using sleep-now mode.

Read the Results: The tool shows four options at 3, 4, 5, and 6 sleep cycles. The highlighted 5–6 cycle options are recommended for the most refreshed wake-up.

Set Your Alarm: Choose the bedtime or alarm time that fits your schedule, accounting for the ~15 minutes it takes to fall asleep.

Key Features

Two modes: plan backwards from a target wake-up time, or forwards from "sleeping now"

Calculates 3, 4, 5, and 6 full 90-minute sleep cycles (4.5h to 9h)

Highlights the recommended cycles (5–6) for the most refreshed wake-up

Accounts for the ~15 minutes it takes to fall asleep

Editable time input with instant recalculation

100% client-side — no signup, no tracking, works offline

Sleep Calculator

Free sleep cycle calculator. Find the best bedtime for a target wake-up time, or see when to set your alarm if you sleep now. Based on 90-minute sleep cycles so you wake up refreshed and energized. 100% client-side.

What time do you want to wake up?

We calculate backwards using 90-minute sleep cycles plus ~15 min to fall asleep.

You should go to sleep at one of these times:

Recommended

21:45

6 cycles

9 hours of sleep

Recommended

23:15

5 cycles

7.5 hours of sleep

00:45

4 cycles

6 hours of sleep

02:15

3 cycles

4.5 hours of sleep

The highlighted times (6 or 5 full cycles) let you wake up at the end of a sleep cycle, so you feel more rested. Pick the one closest to your needs.

How 90-minute sleep cycles work

A full sleep cycle lasts about 90 minutes. Waking at the end of a cycle (rather than mid-cycle) reduces grogginess. Most adults need 4–6 cycles (6–9 hours). We also assume ~15 minutes to fall asleep.

About the Sleep Calculator

The Sleep Calculator helps you find the best time to go to bed or wake up based on the science of 90-minute sleep cycles. Instead of just counting 8 hours, it calculates backwards or forwards in full cycles so you wake up at the end of a cycle — the lightest phase of sleep — and feel refreshed rather than groggy. Choose a target wake-up time to see recommended bedtimes, or pick 'sleep now' to see when you should set your alarm. 100% client-side, free, and private.

Why Use Our Sleep Calculator?

•Based on the well-established 90-minute sleep cycle model used by sleep researchers
•Two modes: plan backwards from a wake-up time, or forwards from 'sleeping now'
•Highlights the cycles most likely to leave you feeling rested (5–6 cycles)
•Accounts for the ~15 minutes it takes the average person to fall asleep
•Completely free, works offline, and no data ever leaves your device

Features

✓Dual mode: target wake-up time or 'sleep now'
✓Calculates 3–6 full sleep cycles (4.5–9 hours)
✓Recommended cycle highlighting for best rest
✓Editable time input with instant recalculation
✓100% client-side, offline-ready, no tracking

Loading tool...

What is Sleep Calculator?

Key Benefits

Wake at the end of a sleep cycle to feel refreshed, not groggy
Plan backwards from a fixed wake-up time or forwards from now
Recommended 5–6 cycle options for the best quality rest
Accounts for the ~15 minutes needed to fall asleep
Works offline and keeps your data private
Completely free with no registration

Common Use Cases

•Finding the right bedtime to wake up on time for work or school
•Setting an alarm when going to bed now so you wake rested
•Adjusting your schedule for an early meeting or flight
•Planning a power nap of one full cycle (~90 minutes)
•Building a consistent sleep routine that matches your natural cycles

How to Use the Sleep Calculator

Choose Your Mode: Pick "I want to wake up at" if you have a target alarm time, or "I'll sleep now" to find the best times to set your alarm.
Enter a Time: Type your desired wake-up time (e.g., 07:00) or confirm your current time if using sleep-now mode.
Read the Results: The tool shows four options at 3, 4, 5, and 6 sleep cycles. The highlighted 5–6 cycle options are recommended for the most refreshed wake-up.
Set Your Alarm: Choose the bedtime or alarm time that fits your schedule, accounting for the ~15 minutes it takes to fall asleep.

Key Features

Two modes: plan backwards from a target wake-up time, or forwards from "sleeping now"
Calculates 3, 4, 5, and 6 full 90-minute sleep cycles (4.5h to 9h)
Highlights the recommended cycles (5–6) for the most refreshed wake-up
Accounts for the ~15 minutes it takes to fall asleep
Editable time input with instant recalculation
100% client-side — no signup, no tracking, works offline

Frequently Asked Questions

In-depth guide

Sleep Calculator Guide: Best Bedtimes From 90-Minute Sleep Cycles

Read the full guide →