Sleep Calculator

What a sleep calculator does

A sleep calculator answers one of two everyday questions: "I need to wake up at 7am — when should I go to bed?" or "I'm heading to bed now — when should I set the alarm?" Instead of guessing, it counts in 90-minute sleep cycles so your alarm lands when you're in lighter sleep and more likely to wake up feeling clear rather than groggy.

It runs entirely in your browser and takes two inputs: a time and how long you usually take to fall asleep. Everything updates the moment you type.

How sleep cycles work

During the night your brain moves through a repeating pattern — light sleep, deep (slow-wave) sleep, then REM — and back again. One full loop takes roughly 90 minutes, so a good night is usually five or six complete cycles:

• 6 cycles ≈ 9 hours of sleep
• 5 cycles ≈ 7.5 hours
• 4 cycles ≈ 6 hours
• 3 cycles ≈ 4.5 hours

Being woken in the middle of deep sleep is what produces that heavy, disoriented feeling (sleep inertia). Aiming to wake at the end of a cycle sidesteps the worst of it.

How to use it

1. Choose your mode: wake up at a time or go to bed at a time.
2. Enter the time using the 24-hour picker (each result is shown in both 24-hour and 12-hour form).
3. Adjust minutes to fall asleep if 15 isn't right for you.
4. Read the suggested times. The six- and five-cycle options are highlighted as recommended; the shorter ones are there for tight nights.
5. Hit Copy to grab the list, or Clear to start over.

A worked example

Say you must be up at 07:00 and you usually take about 15 minutes to fall asleep. Working backwards:

• 6 cycles: 07:00 − 9h − 15m = 21:45 bedtime
• 5 cycles: 07:00 − 7.5h − 15m = 23:15
• 4 cycles: 07:00 − 6h − 15m = 00:45
• 3 cycles: 07:00 − 4.5h − 15m = 02:15

So going to bed around 21:45 or 23:15 gives you a full, cycle-aligned night. Heading to bed at 00:45 still works if the evening ran long — you'll just get fewer cycles.

A note on accuracy

Ninety minutes is an average, not a law. Real cycles vary from person to person and night to night (roughly 80–110 minutes), and factors like caffeine, alcohol, screens and stress shift them further. Use the results as a gentle guide toward better-timed wake-ups, and lean on the biggest lever of all: a consistent sleep and wake schedule.

If you're tracking other health numbers, the BMI Calculator and the TDEE Calculator pair well with a good sleep routine, and you can browse the full set on the all tools page.

Private by design

Nothing you enter here is uploaded or stored — the cycle maths runs locally in your browser with plain JavaScript, so your bedtimes stay on your device.